Training in a fasted state

Nutrition for the runner is almost as important as training, but it is a topic that many athletes shy away from because it is bedevilled by crankiness.   In recent years, nutritional science has yielded a huge amount of potentially useful evidence about what nutrition is likely to work best in particular circumstances.  But the clamour of enthusiasts who seize upon a particular nutritional notion and assert that it is best for everyone makes it difficult to identify might be useful to a particular individual in particular circumstances.  Endurance athletes, sprinters and sumo wrestlers all practice the art of applying optimum muscular force at the right time, for the right duration, and in the right place.  There are similarities in the training that all must do, but also differences. Likewise, there are similarities in their nutritional requirements but also differences.   Just as with differences in training requirements, the differences in nutritional requirements depend not only on the type of sport but also on the individual’s genes, life history and the particular goal at the present time.  It is a complex topic.

In recent times there has been a great interest in the Paleo diet, a diet supposed to reflect the diet of our primitive ancestors. It is heavily biased towards the protein and fat available in meat, and biased away from carbohydrates, especially from cereals that have only been a staple since humans developed agriculture.   The Paleo diet has been given a little added spice by Tim Noakes’ endorsement of a similar diet, in a rather dramatic reversal of his prior recommendation of carbohydrates in ‘Lore of Running’, the book that has perhaps done more than any other to shape the opinions of runners since its first publication in 1991. It is of interest to note in his statement published in Runner’s World in 2012, Noakes was careful to state that on current evidence he only recommends the diet for individuals suffering what he described as carbohydrate resistance, a metabolic condition predisposing them to diabetes. Furthermore he emphasises that the proposed diet requires a long-term commitment for a life-time

In this post I will not focus on the issue of long term diet. That is an important topic for runners and much good data is now available, which I will return to in future posts.  Meanwhile I want to focus on the question of training in the fasted state, a question that both Robert Osfield and Eternal Fury have raised in their comments on my recent blog posted two weeks ago.   Both Robert and EF are advocates of training in the fasted state, and both are currently running very well.  Their accounts are of course only anecdotal evidence. Nonetheless, there is quite substantial body of relevant scientific research that is worth examining.

The question of training in a fasted state does overlap with the issue of long term diet and in particular, with the issue of the proportion of carbohydrates to fats.  It also raises the issue of differences between different types of carbohydrate: high glycaemic index (GI) carbohydrates which produce a rapid spike in blood glucose levels, and low GI carbohydrates which are absorbed more gradually; and also between different types of fats: especially the difference between omega-6 fatty acids which predominate in the typical Western diet, and omega-3 fatty acids, more abundant in both the putatively healthier Mediterranean and Japanese diets.     So the topic is already complex and the interpretation of the evidence must take account of this complexity

The metabolic requirements of endurance athletes

Our bodies store two main types of fuel: glycogen, which is stored in liver and muscle, and is the precursor of glucose; and fats which are stored in adipose tissue.  Glycogen stores are relatively limited and are typically exhausted by about 2 hours of running. Fat stores, even in the most slender of runners are virtually inexhaustible except in extreme starvation.   Muscle can utilise either fat or glucose when fuel is burned aerobically, but only glucose can be used to provide energy within consuming oxygen.  Hence muscles mainly utilise glucose at high intensities, when the demand for energy exceeds the available supply of oxygen.    For ultra marathon runners, running at aerobic intensities for many hours, fat is the preferable muscle fuel, not only because it is abundant but also because the brain requires glucose, making it crucial to conserve glycogen.   The mitochondrial enzymes that catalyse the combustion of fat are the same enzymes as catalyse the combustion of glucose.  It is therefore important to maximise the development of these enzymes for the effective use of either type of fuel.  However the special requirement for the ultra-marathoner is the ability to promote the mobilization of fats and the transport of fat into muscle cells, thereby promoting preferential utilization of fat.

For shorter endurance event (such as 5K or 10K) the efficient use of oxygen is paramount.  At the pace of these events, adrenalin levels promote mobilization of glucose from the glycogen stores, and glucose contributes a greater proportion to the fuel mix.   It is noteworthy that a little less oxygen is required to produce a given amount of energy from glucose than from fat, and hence the greater mobilization of glucose promotes more efficient utilization of oxygen.   Although the majority of the energy required at 5K or 10K pace is provided by aerobic metabolism, a small amount of anaerobic metabolism occurs, and the accumulation of the lactic acid produced by anaerobic metabolism is a limiting factor, so the 5K or 10 K runner also requires a well developed ability to metabolise lactate to minimise its accumulation.

In summary, both the ultra runner and the 5K/10K runner require well developed mitochondrial enzymes to enable muscle to burn either glucose or fat efficiently, ultra-marathoners also need to enhance the ability to mobilise fats and transport them into muscle cells, while 5K/10K runners requires well developed ability to metabolise lactate.    The marathon presents a unique combination of challenges. Typical race pace is not far below the level where anaerobic metabolism is appreciable, yet race duration is long enough to deplete glycogen supplies,  Therefore the marathoner requires well developed ability to mobilise and transport fats, highly developed mitochondria enzymes and also the capacity to metabolise lactate.

What role does the nature and timing of nutrition play in the development of these various metabolic capacities, and in particular, what role might training in a carbohydrate depleted state play?

The different goals of training and racing

It should be emphasized that optimum nutrition for training is likely to be different from that for racing.  There is no doubt that for endurance racing whatever the distance, it is crucial to ensure that glycogen stores do not become depleted, both to ensure adequate energy supply for the brain and to fuel increased muscle output required for hills or surges of increased pace.  Glycogen depletion is not likely to be a problem in short races, but beyond 30 Km, it becomes a major issue.  Many studies (reviewed by Burke) demonstrate that both adequate carbohydrate loading before the event and administration of carbohydrate during the event enhance performance, though these studies have not explicitly addressed the question of the timing and amount of carbohydrate that is optimal for an athlete who is well adapted to preferential use of fats.  It is probably best for the fat-adapted long distance runner to augment carbohydrates on race day sparingly to avoid undermining the advantage of preferential utilisation of fats.

Training in a glycogen-deleted state.

The potential benefits of training in a glycogen depleted state with the goal of enhancing fat mobilization, and also possibly enhancing mitochondrial enzymes that are involved in metabolism of either fat or glucose, have been debated for many years.  Various strategies for inducing carbohydrate depletion have been considered.  One strategy is to train twice a day without refuelling between sessions, thereby ensuring that glycogen stores are depleted at the start of the second session.  Perhaps the most impressive study using this strategy was performed by Hansen and colleagues from Copenhagen.  They trained the athletes’ knee extensors, applying different regimes for the two legs.  One leg was trained twice for one hour with a two hour rest between sessions, on alternate days, while the other leg was trained for one hour daily.  Thus both legs did the same total volume of training, but for one leg, half of the sessions were performed in a glycogen depleted state.   After 10 weeks of training, time until exhaustion when working at 90% of VO2 max exhibited a markedly greater increase in the leg that had trained in the glycogen depleted state compared with the other leg.  Mitochondrial enzymes also exhibited a greater increase in the leg trained during glycogen depletion.   Hansen concluded that training twice every second day might be superior to daily training.   Of course, the benefit of training twice on alternate days was not necessarily due to the glycogen depletion. It might have reflected other benefits of thorough recovery between sessions.

The findings from studies in which glycogen deletion is achieved by training after an overnight fast are less convincing.  On balance there is little consistent evidence for improved endurance performance.   There is however consistent evidence for the enhancement of enzymes involved in fat mobilization, and consistent evidence for a decrease in respiratory exchange ratio (relatively less carbon dioxide is produced compared with oxygen consumed) which is characteristic of fat metabolism (see the review by Burke).

The effects on mitochondrial enzymes are less consistent and might depend on the composition of the diet.  For example, Van Proeyen and colleagues from Leuven in Belgium demonstrated that in individuals consuming a carbohydrate rich diet, training in a fasting state improved both ability to utilise fats and also mitochondrial oxidative enzymes compared with training fuelled by carbohydrates before and during training.  However, in individuals consuming a high fat diet, the advantages of training in a fasted state on fat utilization were abolished.  Furthermore, fasted and non-fasted training produced similar improvements in time to exhaustion, while only non-fasted training produced a significant increase in VO2max.  Thus, for individuals on a high fat diet, the advantages of fasted training on ability to utilise fats and on aerobic development are lost; while non-fasted training actually produces a greater increase in VO2max.  It should be noted that the training intensity in the fasted and non-fasted groups was matched.  In light of the strong expectation that fasting would diminish the capacity for high intensity training, even greater gains in aerobic capacity might be expected from non-fasted training if exercise intensity was not controlled.

Potential disadvantages

At least in the absence of a fat rich diet, training in the fasted state does enhance both the development of fat utilization and the development of mitochondrial oxidative enzymes, but might it have disadvantages?   As already implied, it might impair the ability to perform well and thereby gain maximum benefit from a high intensity training session.  Perhaps even more important is the risk of excessive cortisol release.   Cortisol mobilizes helpful adaptations to stress, but excessive and prolonged cortisol release has adverse effects on many body tissues, and on the immune system.  Furthermore, as discussed in my post two weeks ago, there is evidence that endurance athletes tend to have sustained and potentially harmful increases in cortisol that are proportion to training volume. Hence, it is probably desirable to minimise elevation of cortisol during training.  Gleeson and colleagues demonstrated that exercising at 70% of maximal oxygen uptake for 60 minutes produced greater elevation of cortisol and potential disadvantageous changes in the immune system in individuals in whom glycogen had been depleted by three days of low carbohydrate intake.   Thus, the evidence is indirect, but suggests that training in a glycogen depleted state does create an increased risk of harmful elevation of cortisol.


Training in the fasted, glycogen depleted state is likely to enhance capacity to utilise fats, which is advantageous for an ultra-marathoner and perhaps also for marathoners.  Under some circumstances, it might also produce enhancement of aerobic enzymes.  However, a high fat diet abolishes these advantages of training in the fasted state.  Furthermore, training in a glycogen depleted state increases the risk of excessive elevation of cortisol during either intense or prolonged training sessions.  In addition, training in a glycogen depleted state would be expected to diminish performance in a high intensity training session and might thereby limit the benefits obtained from high intensity sessions.

Overall, I think that training in a fasted state has a very limited role to play, though might be advantageous for ultra-marathoners who wish to maximise capacity to utilise fats.  Although the alternative strategy of a high fat diet might nullify this advantage, it is necessary to evaluate the risks and benefits of a high fat diet.  I will address this issue in detail in a future post, but here, I will merely note that a substantial body of evidence does indicate that moderately high fat diets can be healthy, provided there is an approximately equal balance of omega-3 and omega-6 fatty acids, typical of the classic Mediterranean diet. In contrast, in most Western diets, there is an unhealthy preponderance of omega-6 fatty acids.  Furthermore, Venkatraman and colleagues have shown that increasing fats up to 40% of energy requirements, leads to increased endurance in healthy trained runners.

While there is strong evidence that avoiding glycogen depletion during races is crucial for peak performance, I am inclined to think that in many situations, the mild glycogen depletion achieved by avoiding carbohydrate consumption during training might be advantageous, because it is likely to enhance the capacity for fat utilization and might also promote mitochondrial enzyme development, especially in athletes who consume only moderate amounts of fat in their diet.   Therefore I do not consume carbohydrate during training (except to test strategies for refuelling during a race).  Furthermore, if training in a non-fasted state, it is worth considering what type of pre-training nutrition is optimal.  Sun and colleagues have demonstrated that a low glycaemic index (low GI) breakfast promotes utilization of fat rather than glucose during low and moderate intensity exercise.  A low GI meal minimises the spike in blood glucose but instead produces sustained release of glucose into the blood stream.  Therefore, I consider that a low GI breakfast is optimal for both training and also before a long race.

50 Responses to “Training in a fasted state”

  1. Joe Says:

    You wrote:
    “Muscle can utilise either fat or glucose when fuel is burned aerobically, but only glucose can be used to provide energy in the absence of oxygen. Hence muscles mainly utilise glucose at high intensities, when the demand for energy exceed the available supply of oxygen. ”

    However, my understanding is that this is not quite accurate. My understanding (based on Noakes) is that there is no internal lack of oxygen while exercising (even at high intensity). High intensity exercise does burn a lot of glucose, but it’s not due to a lack of oxygen. The term anaerobic is misleading here. Noakes argues (I believe) that if there was truly a lack of oxygen while exercising, then the heart would become damaged (due to lack of oxygen) every time we engaged in high intensity exercise, but this obviously does not happen. This is because the heart does not posess the ability to utilize glucose in the absence of oxygen. However, it is still true that the higher the intensity, the more glucose (vs. fat) we tend to burn. It’s just that this mechanism (lack of oxygen) is probably not the correct explanation for that.

    • canute1 Says:

      Joe, thanks for your comment.

      I think what you and Tim Noakes are saying is that muscle switches first to use of anaerobic metabolism and then contraction ceases entirely before the arterial oxygen content has fallen to a level which is dangerously low. I agree that this is the case. Otherwise brain and heart would be at risk of being deprived of oxygen. However, when you refer to internal level of oxygen, I think it is crucial to ask what you mean. If you mean arterial oxygen level I agree with you. However, I was implicitly referring to the availability of enough oxygen to meet requirements for energy production in the muscle mitochondria. The key role of oxygen in enabling aerobic metabolism in the mitochondria is known as the Pasteur effect. When inadequate oxygen reaches the mitochondria, mitochondrial respiration decreases and lactate production increases. Lactate is produced by the reduction of pyruvate, which is one of the intermediate substances produced in the process of glucose metabolism.

      To understand the mechanism in greater detail we need to consider the regulation of the process by which pyruvate (which is a metabolite of glucose) is shunted away from the mitochondria where it would be metabolised aerobically, to the process of reduction to lactate via the enzyme lactate dehydrogenase. The reduction of pyruvate by lactate dehydrogenase also requires availability of a substance called NADH to provide hydrogen ions. Because the reduction of pyrvate to lactate dehyrogenase releases energy it is in effect a downhill process that will occur readily whenever spare pyruvate and NADH are available. When there is adequate oxygen the respiratory process occurring in mitochondria mops up all the available NADH, so pyruvate cannot be reduced to lactate. However, when mitochondrial respiration slows down due to a relative lack of oxygen, NADH accumulates and reduction of pyruvate to lactate can occur.

      So I would still propose that the crucial factor in the transition to anaerobic metabolism is the failure to deliver enough oxygen to mitochondria to maintain the required rate of energy output, though the molecular signally processes are complex.

      Note also that I understand that in his well known article in Scandinavian Journal of Medicine and Science in Sports ( Tim Noakes was addressing the question of what leads to cessation of exercise in a fatigued athlete. Exercise ceases well before serious reduction in either arterial or muscle levels of oxygen. Cessation of exercise is not the same thing as the onset of lactate production.

  2. Robert Osfield Says:

    Hi Canute,

    It was great to meet and run with you last Tuesday, and now have to say thanks for another interesting tour of the possible benefits/risks associated with training fasted. Pulling the various bits of evidence form the studies together it sounds like keep the insulin levels low prior to training is useful.

    In the case of high fat diet it’s intrinsically low GI so eating prior to exercise doesn’t risk increasing insulin levels significantly above fasted levels so training fasted doesn’t provide further benefits w.r.t studies reviewed.

    For a high carb diet then one either has to train fasted, or work to make sure that the carbs you do consume are part of an overall low GI meal.

    For myself, I’m probably somewhere in the middle, I’ve reduced my carb intake, increased fat intake but wouldn’t yet describe myself as low carb, more moderate carb, moderate fat, modest protein. I wonder if in my case the one might choose to run fasted depending on the type of training run – for long or intense session eat a low GI meal, while for easy and medium distance runs run fasted.

    Do you have any details on what the balance of carb/fat/protein was for the studies you’ve looked at? I’m curious what they classify as high fat?

    For the anecdotal evidence record, here’s my experiences with runs this week.

    Tuesday: 14.5 miles (with Canute :-), Average HR 137, HR 78% of LT-HR, 10:46min/mile pace.

    Two hours before run ate a big breakfast of french toast (fried bread soaked in egg). I never intended to eat but I cooked the breakfast for my three girls, but only one ate any leaving me to polish off the rest!

    Had good energy levels throughout the run, didn’t feel in glycogen depleted at the end of the run. The relaxed pace will have help with sparing glycogen stores.

    Wednesday: No running, just 7 hours in the car. Hip flexors didn’t like sitting the car so long.

    Thursday: 6.5 mile hard tempo run, ran fasted, second fastest time for route, 6:56 min/mile pace (it’s a hilly route equivalent to about 6:40 on flat). Lactate threshold/muscle strength endurance seemed to be the main limiter. Didn’t feel glycogen depleted in any way. Curiously could only manage an average HR of 168, while my LT-HR is usually up around 175, the max HR on this run was 172, which suggests that either I was a bit fatigued or that simply my fitness right now is such that my HR at all paces has reduced. Hip flexors were a bit sore in the last couple of miles run.

    Friday: 21.3 miles long run at marathon PB pace (7:55 min/mile), Average HR 156, 78% of LT-HR, ran fasted, with no water or food. Good energy levels throughout, no signs of glycogen depletion. A PB for route by a couple of minutes, and done at 6bpm lower HR, previous runs on route done after breakfast and with water and consuming 100 to 200 cals per hour. Only problem during run was hip flexors ached in the last five miles, a hang up of driving too much in the last week?

    After run feasted well – big lunch and snacks in afternoon. Felt like I had run my two hardest workouts this year back to back, which I had… My digestion wasn’t quite as robust as it has been of later after longish runs, a bit similar to how it used to be after running 20+ milers before my changes to diet/training fasted. My guess would be that while I didn’t feel glycogen depleted my bodies hormone levels were probably affecting my digestion suggesting that I was actually reasonably glycogen depleted. However, the feeling was less than it used to be when running this route after breakfast with eating and drinking on route.

    Sunday: 4 mile recovery run, run fasted, Average HR 136. Hip flexors mostly settled down, little fatigue or stiffness otherwise. Previously two days after a tough long run I’d be struggling to walk down stairs etc. Is this down to accumulation of training or the new diet regime?

    While I have no direct evidence that my cortisol levels aren’t high, it doesn’t feel like my body is struggling to cope with and recover from training whilst fasted, to me I feel nearer to homoeostasis more of the time. After Friday’s long hard run was the only time after training that I’ve felt at all exhausted. Perhaps in future I should look at eating a low carb breakfast before the 20+ miler runs?

    For the rest of this week I’m tapering for next Sunday’s Jedburgh 3 peaks race so won’t do any more long or hard runs. I will probably keep up the intermittent fasting (missing breakfast) and run fasted on the days that I run. The morning of the race I’ll eating low GI breakfast and will be planning on consuming around 200 Calories of carbs, fat and protein during the race.

    • canute1 Says:


      Thank you for your company on our run last week. It was a pleasure to chat while running along river banks and over the hills. Even on the occasions when pace slowed, heart rate climbed and conversation ceased as we slogged across ploughed land, it was satisfying to know that we were reclaiming the right of way where the farmer had sown across the public path.

      With regard to the content of the diet in the studies, prior to the high fat diet study by Van Proeyen, subjects, on average, ingested 3,000 kcal/day, of which, 50% was carbohydrates, 35% fat, and 15% protein. During the study, the participants received one of four different menus containing 3,000 kcal, 3,500kcal, 4,000 kcal, or 4,500 kcal/day. Each of the four diets contained 50% fat, 40% carbohydrates, and 10% protein. This high fat diet abolished the benefit of fasted training compared with non-fasted training on both the ability to mobilise fat and on increase in aerobic capacity. It is noteworthy that the diet contained not only a high proportion of fat, but also in at least two of the menus, the total energy content was somewhat greater than I would consume even during heavy training.

      I hope all goes well in the Jedburgh three peaks race.

      • Robert Osfield Says:

        Hi Canute,

        For our muddy run my efficiency was around 10% lower than for my other long runs of similar duration. Part of this would be down to running at slightly lower pace than I usually do, but I expect most of it will have been down to the muddy sections. My efficiency on the muddy sections must have been really low to see such a big impact overall efficiency -something we could both observe with slowed pace and higher HR’s.

        Take away the mud I would have thought we could have averaged 10 minute/mile pace and covered around 16 miles in around the same duration and effort level. Fatigue due to eccentric loads would have been higher so would have been somewhat of different type of workout.

        Thanks for the details on the studies diet composition. I don’t know what my calories mix is but I guess I must be getting up to around 50% calories from fat. Low carb diets tend to be up around 70+% calories from fat so it’s interesting to see that differences were observed without going to the extreme end of diet. The studies broad finding suggest that my running fasted is probably not making a huge difference and I could safely add back in breakfast and see the same benefits of improvements in my fat burning and aerobic fitness.

        Looking at my HR/pace/calories reported consumed for runs in the last week I am now at the peak of where I have been over the last few years. Whether it’s a short recovery run, long run or tempo run by figures are all best I’ve seen or equal to the best I’ve seen after previous periods of good training.

        I doubt I’m yet close to how fit I got when I was a young teenager but I can’t be too far off, especially w.r.t fat burning capacity and endurance, it’s only really the speed that I’m missing, but then I don’t do much high intensity work thanks to my focus on ultra marathon training.

        My feeling is at present I’m not suffering from problems with my cortisol levels being elevated. I’m recovering well, able to cope with tough training runs and seeing improvements in fitness as a result.

        Could it be that now I’m more fat adapted that running fasted doesn’t result in elevated cortisol levels any more than training non fasted before my changes to diet? That’s how it feels subjectively, if anything I’d say I’m feel less glycogen depleted after runs now, especially long runs.

        It would be really interesting to do a study of how much glycogen stores runners on different diets have as they conduct training. Especially interesting if your tracked the period of them changing diet, then follow up months later to see how adaptation progress.

        My feeling is that a higher fat and low carb diet doesn’t immediately map to a glycogen depleted state once adaptations are complete. Instead it’s quite possible to keep glycogen stores topped up, one just need to drip feed the carbs in, in a low GI way, and when training avoid dipping too deep into glycogen stores without topping up.

  3. Robert Osfield Says:

    One aspect of fasting that “Perfect Health Diet” book discusses is the positive health benefits of a short fast each day to bring about Autophagy. They suggest too long a fast is counter-productive as Autophagy is switched off too much when you return to eating after a long fast. They propose 18 hour fast, and 6 hour eating window from lunch through to dinner, and eating during daylight.

    I have read that training runs also can fire up Autophagy, although I’m guessing this won’t last long if you feast directly after training like I do.

    I’m curious to your thoughts on Autophagy and it’s effect on training effects and general health.

    • canute1 Says:

      With regard to my views on autophagy, I think there are several well established facts. Debris collects in cells, probably ultimately limiting their function. This debris can be removed by lysomes (autophagy), and this process almost certainly prolongs the useful life of cells. Many different stimuli, including fasting and exercise can promote autophagy, though some of these stimuli (eg anything that enhances production of reactive oxygen species) have the potential to be harmful.

      The optimum combination of stimuli to induce the optimum amount of autophagy is likely to depend on individual circumstances. I suspect that for a person leading a sedentary western lifestyle, some strategies for increasing autophagy (eg intermittent fasting) are likely to promote effective cleaning-up of cellular debris and promote improved health.

      However, on the basis of my current understanding, I would not be inclined to practice both strict fasting and a demanding endurance exercise program concurrently, on account of the risk of inducing too much stress. However I think it is likely that eating low GI carbs and avoiding high GI carbs is a gentle way of promoting autophagy that is compatible with endurance training. I do sometimes drink green tea which is probably another fairly gentle way to activate autophagy, though I probably do not drink it regulatory enough to get much benefit. I watch the emerging evidence with interest.

      • Robert Osfield Says:

        Thanks for your thoughts, it’s helpful in getting my head around the topic.

        W.r.t production of reactive oxygen species (ROS), in the “Art and Science of Low Carbohydrate Performance” they suggest a link between a low carb diet and lower oxidative stress, and go on to say that this lower oxidative stress is the reason why low carb athletes are finding their recovery quicker after training.

        I presume they associate a low carb diet with lower oxidative stress based on the principle that metabolising fat is aerobic process and doesn’t produce ROS, while metabolising glycogen can be aerobic or anaerobic with the later producing more ROS. They don’t specify this in the book though, I’m just trying to infer what might be going on. Thoughts?

        W.r.t fasting + training resulting in too much stress, from my experience I haven’t yet hit an issue with over training. I have been able to shrug of colds that are going around quite well in the past six months, enough that that I have been surprised on a couple of occasions when I’ve only had a short sniffle, expected it to turn into a full cold, then bounced back right away. This is purely anecdotal evidence, but it would seem at least that the stress has not been too much in my case. If anything my body feels less stressed as I’m recovering from tough training quicker.

        I would caution reading too much into some of the studies in terms of long term detrimental effects of running fasted and low carb. The Gleen at. al paper only uses three days of low carb diet before the physical test. This approach to testing is only valid for looking at short term response before any meaningful adaptation to far burning metabolism have taken place. It’s now well established that you need at least two weeks before the body shifts across, and for an athlete I feel that full adaptation take much longer. Once adapted you aren’t permanently glycogen depleted, you are in fact glycogen sparing so it’s perfectly possible to maintain good reserves of glycogen.

        Perhaps this might go some way to explaining why I don’t seem to fit the profile of one who is glycogen depleted and flirting with over stressing my body.

        Before I started experimenting with a high fat/lower carb diet, and before I started doing intermittent fasting I was convinced that I *had* to have a big breakfast everyday, and running fasted would result in me hitting the wall on my longer runs. Now I’ve switched over it’s not at all scary and problem fraught. I’m fit, healthy and running better than ever.

    • canute1 Says:

      I agree that it is plausible that after long term adaptation to a low carbohydrate diet that the risk of adverse effects from combining of fasting and heavy training will be reduced. However, until this is proven, I would be very cautious about the combination of fasting and heavy training.

      As far as I understand, you introduced your changes in the timing and content of food intake during a period when you were doing a reduced volume of training due to injury. I think it is likely that this was a sensible time to introduce the changes. I have little doubt that during my recent period of half marathon preparation in which I set out to establish what was the upper limit of training volume that my body could cope with, it would have been unwise to for me to have trained in a fasted state. On the other hand, I never consumed calories during training runs.

      Overall I think that the evidence suggests that some if not all of the main benefits of fasting can be achieved by less risky strategies. In particular, minimising high GI carbohydrates and also increasing total fat content while ensuring approximately equal amounts of omega-3 and omega-6 fats, appears to produces some or all of the advantages of fasting.

      With regard to reactive oxygen species (ROS), I am not sure that you are right to suggest that the culprit is anaerobic metabolism. Production of ROS is an almost inevitable consequence of aerobic metabolism in mitochondria. The aerobic metabolism of both fat and of glucose are carried out by the same mitochondrial enzymes and leads to engagement of the mitochondrial electron transport chain, which is a major site of ROS generation. However the circumstances under which generation of ROS is harmful are poorly understood. Transient elevation of ROS might trigger healthy autophagy. It is plausible the chronic inflammation due to poor recovery from excessive training creates the greatest risk of harm from ROS. Therefore, my approach is to look for ways of achieving the benefits of fat adaptation (and perhaps enhanced autophagy) in the least stressful way possible. At this stage, I think that the preferred strategies are

      1) good recovery from training. Achieving this while training near one’s limit is tricky – that is why I developed the submaximal tests that I described in recently on my blog.

      2) increase one’s capacity for preferential metabolism of fats rather than glucose during running by a diet in which high GI carbs are replaced by low GI carbs.

      3) increase preferential mobilization of fats by increasing proportion of fat in the diet, while ensuring good balance of omega-3 and omega-6 fats. At this stage I am not sure what is the preferred proportion of total calories from fat, but estimate that it is probably somewhere in the range 40-50%.

      As stated before I am cautious about fasting, and would only consider introducing it during a period of low or moderate training load. I also recognise that individuals are likely to differ in metabolism not only on account of recent diet and training but also on account of genes and perhaps early life nutrition.

      • Robert Osfield Says:

        Thanks for your thoughts on ROS. I am still left curious why runners moving to high fat diets report better recovery after training and racing. Is fat metabolism protective? Might the fat content of the diet help with maintenance of cells? Does far metabolism result in less ROS than glycogen metabolism? All questions that I might hope you’d be able to answer, but expect that it’d require quite a bit of research to find out…

        Your three point plan looks sound, changing diet during a period of lowered physical and mental stress is something I probably benefited from.

        The only thing I’d add is that achieving a good balance of Omega-3 and Omega-6 fats is important for itself, but not for upping overall intake of fats. I would not recommend using polyunsaturated fats as a source of fuel, rather they should be their for maintenance of the body, a bit like making sure you get the correct amounts of different vitamins and minerals. For fuel I would recommend focusing on mono-unsaturated and saturated fats, so as you lower your carb intake I would keep the polyunsaturated fats balanced but modest, and push up the consumption of mono-unsaturated and saturated fats.

        If you look towards the Mediterranean diets as inspiration you’ll find that they are much higher in mono-unsaturated and saturated fats than recent “western diet”, the balancing of polyunsaturated fats is important but it’s not foundation of the diet.

  4. Robert Osfield Says:

    A further area to consider w.r.t training fasted is mental preparation and confidence building. I could easily see that this area would be one to difficult to study on a scientific basis but from coaching/training perspective it’s easier to see how it might help.

    An example of the confidence building side was provided by EternalFury in the comment section of the previous blog entry where encouraged by my own experience he ran a long run fasted and was amazed to see how his body was able to cope.

    If you are full of anxiety about hitting the wall in a long race like a marathon then being able to do complete long runs fasted will help address this mental block to performance. When actually racing it will also give you the confidence about just drinking water, or just getting by with a modest energy intake.

    The risk of gastric stress when consuming food when racing is significant, I know it’s adversely affected me on several occasions. It’s only since I’ve started running fasted that I’ve had the confidence in my bodies ability to function at a high level without needing to consume lots of energy.

    On the mental preparation side actually experiencing low glycogen levels in training may help you cope when you see signs of it when racing. If you see the signs you might have the confidence to continue on and not panic, or you spot it early you can do something about like take on some food. The only sticking point in this is that now I’ve changed my diet to higher in fat, and training fasted I hardly ever get any signs of low glycogen levels when training 😉

    Adding fasted runs in for mental preparation and confidence building is something that could be done sparingly, such as after a good period of base building when your aerobic fitness is well established, or perhaps near to a race when you need to check your bodies ability to handling getting near glycogen exhaustion. Doing short fasted test runs before the longer fasted runs would probably be sensible.

    • canute1 Says:

      I agree that gastric upset when running is a serious issue, and hence I am strongly in agreement with maximising one’s ability to utilise fats, thereby minimising reliance on intake of calories when running. As described above, I think that there are safer ways to achieve this than by training in a fasted state, but also accept that for both yourself and EF, training is a fasted state is working well at present.

      • Robert Osfield Says:

        Hi Canute,

        The key point I was trying to make was that there are psychological benefits from having done fasted training runs. Once you know you are capable of running fasted without problems your whole perspective on fuelling can be turned upside down.

        To gain this benefit one doesn’t need to regularly train fasted, an occasional fasted run once well adapted to burning fat could be all it takes to cement your confidence in your bodies ability to burn fat efficiently and spare glycogen.

        For me this aspect has been quite profound. The actual physical benefits were gained mostly from the change in diet, but the psychological benefits were made once I completed a few longer fasted runs.

    • canute1 Says:

      Confidence is important, though the issue of what builds your confidence depends on what you believe. If you are confident that you are well adapted to fat utiization by whatever means (eg high training volume; high fats; low GI carbohydrates; or even fasting) and are confident in your fuelling strategy before the race (eg low GI breakfast) you have good grounds for confidence that you will not be heavily dependent on intake of carbs during the race. When I ran marathons I probably suffered from the opposite problem: it never occurred to me I even needed any carbs during the race because I had never been aware of needing refuelling during training. However the flaw in my belief was that at race pace I almost certainly depleted my glycogen stores more rapidly than in training. A similar flaw applies to relying on a few fasted training runs to demonstrate that you do not need refuelling. I personally feel more confident with a consistent strategy (e.g. the intake of a low GI breakfast before both training and racing), but acknowledge that almost nothing you do in training will exactly mimic your experience during a marathon. A marathoner also requires mental resilience. However I do not wish to undermine your confidence or that of EF. It sounds as if you are both in great shape for either a marathon or ultra at present.

  5. Robert Osfield Says:

    Another topic that isn’t explored by the cited studies is that of Ketosis. I don’t personally have experience yet with being in Nutritional Ketosis, my diet is too high in carbs it seems for that, even when testing after a training in a fasted state. However, I still find it in interesting topic so worthy of bringing up in the context of training fasted.

    Most people following a Ketogenic diet will aim to stay in Nutritional Ketosis throughout the day, but I came across an account of one athlete that practices intermittent Ketosis. If I recall correctly he would have a big evening meal and drop out of Nutritional Ketosis but would return to Ketosis when fasting in the morning and would then train in Nutritional Ketosis.

    I don’t have any plans with dropping my carb intake low enough to go into Nutritional Ketosis, although I’m tempted to from a purely experimental point of view. Personally I can’t be bothered with the effort of carefully tracking my Ketones every day, so can’t ever imagine doing it as a regular diet – I enjoy all my food too much 🙂

    • canute1 Says:

      Ketogenic diet is an effective in promoting fat metabolism and in producing weight loss, though there are potential risks and I think these risks are increased for an athlete. Unless the drastic reduction in carbs is accompanied by the appropriately increased intake of protein or amino acids, exercise is likely to produce increase in blood ammonia which is toxic. Furthermore, aerobic metabolism of ketone bodies is unlikely to provide enough energy to support sustained high intensity exercise (eg faster than 5K pace or running up hills), so it is necessary to alternate periods of ketosis induced by low carbohydrate intake with periods of carbohydrate ingestion to restock glycogen stores. I suspect that getting the balance right would be very tricky, especially as the precise mechanisms are still speculative.

      • Robert Osfield Says:

        I think your overplay the downsides and difficulties rather too much, quite a bit has now been learnt and applied that allows one to manage a ketoginc diet and train successfully. You really need to get yourself a couple of “Art and Science of Low Carbohydrate Performance” 🙂

        An doctor/athlete in the US, Peter Attia, who blog “The Eating Academy” chronicles various of thoughts on Ketosis+Diet and analysis of his own training under Ketosis. From Peter’s exercises it’s clear that one does need to be careful about how you manage your ketogenic diet and exercise hard, but it’s also very clear it perfectly possible to avoid the pitfalls. Peter is doing sustained high intensity training successfully.

        A couple of articles on his website of interest:

        There are plenty of other interesting articles on his website. One thing that Peter does use and promote is “Super Starch”, which is a carb that engineered to not produce a insulin spike and can be consumed without stopping ketosis.

        The other approach, as you suggest, is Cyclic Ketosis where one goes very low carb for the 5-6 days a week and then carbo load for a day or two.

        Personally I’m not about to dive into a Ketoginc diet, but I sure want to learn what I can from those who have used it. For me my current balance of fat/carbs/protiens seems to have provided the benefits I was after – to be able to recover quicker during and to be able to burn enough fat during ultras to avoid gastric stress and running out of glycogen stores.

    • canute1 Says:

      I am familiar with Pete Attia’s blog, and consider that it contains lots of interesting information. However there are several features that confirm my reservations about ketogenic diet for endurance athletes.

      He performs frequent measurements of many physiological variables, some of which require complex techniques. He also measures ketone levels daily when he is preparing for a long cycle ride. He very carefully calculates how much carbohydrate he needs to consume to fuel his rides. Too little and he will have inadequate energy for the ride; too much and he will undermine his ketotic condition. As far as I can see he needs data not only regarding ketone levels but also respiratory exchange ratio, which is not accessible to most recreational runners. He quite frequently changes his diet as he discovers new things about how his body reacts. He very openly and honestly acknowledges there are many things he does not understand. Also, as Tim Noakes points out, there are important difference between individuals in how our bodies handle energy metabolism. Peter Attia has several metabolic oddities. Therefore I regard Peter Attia’s experience as evidence that it is possible to combine ketogenic diet with demanding endurance exercise provided one performs very meticulous measurements and calculations, but also would advise against assuming that what works for Peter Attia will work for anyone else.

      I also note that following long rides he observes appreciable increase in body weight which he attributes to widespread inflammation (leading to accumulation of fluid in tissues).

      However one very positive lesson to learn from Peter Attia’s blog is that it is possible to substantially reduce one’s dependence on glucose during submaximal exercise, and this is almost certainly very advantageous for ultra runners and long distance cyclists. This provides justification for your focus on a similar goal by increasing fats, decreasing carbs and fasting. I too strive to achieve similar goals by potentially less stressful (ie potentially less inflammatory) methods. I avoid high GI carbs; have cautiously increased fats; and I minimize carbohydrate intake during exercise – in fact I almost never consume any carbs when running). At least for an elderly person, and perhaps for anyone who hopes to continue to run in old age, I would recommend a more gentle approach to increasing fat adaptation until the long term consequences of more dramatic approaches have been demonstrated.

      • Robert Osfield Says:

        I expect Peter Attia is doing far more testing of his physiological state than is required – he’s purposefully trying to learn and educate, the level of testing goes beyond what an average athlete will require. Testing ketone level is probably sufficient for most athletes, and once one is familiar with how the body responds to diet and exercise it’s like that even testing ketone levels daily could be dropped.

        W.r.t aging and retaining muscle strength, what are your opinions on testosterone? There have been some high profile older advocates of using testosterone injections to bolster the bodies ability to build muscle. I’m guessing you’ll be against hormone injections it on principle, but I wonder if there things one can do diet wise to avoid the normal tailing off of testosterone as well age?

    • canute1 Says:

      My own experience suggests that it is fairly easy for an elderly runner to maintain adequate strength by a modest amount of resistance training. I very much doubt that it is desirable for an elderly male runner to specially augment testosterone unless he has a clear-cut deficiency. I think the key goal is achieving a good balance between anabolic and catabolic processes. Strategies such as minimising stress, getting adequate sleep, adequate dietary fat, and resistance training all shift the balance in favour of anabolism.

      I believe all of these are worthwhile, though I was a little disappointed by the outcome of the resistance training I did a year ago. I did three weekly sessions with exercises that engaged a large muscle mass (eg squats, deadlift) at high load (5×5 reps at a load very near 5 rep max). Over a period of about three months this produced major gains in strength (eg 5 RM for squats increase to near twice body weight) with minimal evidence of benefit for either running performance or hopping performance. An episode of arthritis partially confounded the assessment of benefit, but nonetheless the available evidence indicated that this amount of strength building (and probable associated increase in anabolic hormones) was of remarkably little benefit to my running. I currently do less intense resistance training. I believe my current program is enough to preserve muscle strength adequate for running. At this stage, I think that improving power and elastic recoil is a more important goal.

      One of the major issues for the elderly runner is cardiac health. Overall there is a low correlation between serum testosterone level and cardiac health, but evidence suggest this is due to a J shaped relationship. While abnormally low testosterone is associated with morality from ischaemic heart disease, so too is high testosterone. This is consistent with the small and controversial body of evidence indicating that the elevated risk of ischaemic heart disease in marathon runners is a male problem.

      So overall, I see little point in additional measures to push the catabolic/anabolic balance any further towards anabolism than can be achieved by minimising stress, adequate sleep, adequate fats in the diet and modest amount of resistance exercise.

      • Robert Osfield Says:

        Thanks for your thoughts on testosterone.

        I have to say that I was very impressed by just how much weight your are able to train with when doing deep squats, when I picked it up I was reluctant, just before a long run, to do a deep squat with the weight you had on the bars. It’s a real shame that this strength doesn’t directly map to running performance.

        I do wonder if there might be a neuromuscular role at play, you’ve trained the strength of your muscles and you’ve trained you neuromuscular pathways to engage sufficient fibres to do the job. Is there any chance that when running+hopping you aren’t recruiting all the fibres you could, with your brain now conditioned to hold back and avoid risking injury?

        If this is the case then you might be able to train specifically to recruit more fibres when looking for explosive power rather than strength. I would guess that isometric exercises would be useful here. Sprints and specific drills might be useful too.

        I would understand your caution here though, personally I’m a bit paranoid about really high intensity work with injury risk. Perhaps it’s something I should work into my own training, I do lots of aerobic training and an occasional tempo run and hill sprint, but never have done more than a few days here and there of sprints and other really high intensity workouts. Rather a contrast to how I used train as kid.

        Truth be told, I like just running out in the hills and forest without my lungs bursting and my eyes popping out. High intensity training feels so unpleasant in comparison!

  6. canute1 Says:

    The question of optimum recruitment of muscles to gain the benefits of the strength developed by resistance training is crucial. Due to arthritis, I did not do the plyometrics and drills I had intended early this year. I am doing that right now and will blog about my conclusions soon.

    The question of the benefits of high intensity training v lower intensity & high volume training is one of the most tortuous debates within the domain of endurance training. One the whole I incline towards the Lydiard view: a foundation should be created by a substantial period of base-building that is predominantly of low or medium intensity (though not entirely), followed by a relatively short period of more intense training that brings the athlete into peak condition for competition. I am impressed by the evidence that high intensity interval training produces multiple gains including increased aerobic capacity, increased ability to pump potassium back into muscle cells, increased capacity to metabolise fat. In my view the evidence supports the view that once you have reached a plateau of aerobic fitness via high volume training, a few weeks of HIIT can lift you to a higher level. At present I am working on increasing eccentric strength via plyometrics, drills and hills, but after that will probably do about 4 weeks of HIIT to see if I can improve my 5K time. I will blog about this is greater detail in a few weeks time.

    • Robert Osfield Says:

      I am torn about what to do with my own training, I haven’t yet tried adding consistent high intensity work, most of my training probably fits quite well with Lydiard’s base building. Over the past couple of years I’ve completed base building and then got injured before I’ve been able to add high intensity work, or simply run out of run.

      One of the problems with running ultras and marathons is that it breaks your body down a bit so you have so you have have at least a week or two recovery before you can start back training properly. I have found that I have to go back to base building to make sure all the new fibre development and older repaired fibres develops aerobically. At least that’s the sense I get – my aerobic fitness normally suffers in the initial month after a hard run.

      The exception to this was the RAW, I only required a week to get back into meaningful training, and my aerobic fitness was back making progress within two weeks.

      It’ll be interesting to see what happens at Jedburgh and my recovery afterwards. My training has gone well so for the first time this year I’m going into a race firing on all cylinders. Hopefully recovery will be swift afterwards. Then I’ll need to decided what to do next.

      I may take a month off running, let my niggling left foot injury settle, or get back into gentle aerobic running and then work towards base building. Or perhaps if I recover quickly look at trying the high intensity work and some shorter races as a motivator.

      I can’t help but feel that adding a few more races into your training schedule would be helpful. 5k’s and 10k’s can make good high intensity and tempo work outs if you approach them as such.

    • canute1 Says:


      I think that if you are want to reach your best possible level of performance that you should add a period in which you do some higher intensity training, though as recreational runner we each have to makes the decision about what we value most. Nonetheless, Ed Whitlock demonstrates that at least some individuals can achieve spectacular performance based on high volume of low intensity training with only a moderate number of fairly intense races – though he also states (in his rather dry manner) that he finds his training rather boring.

      With regard to my own training, I agree that including more races would be one way to add some high quality sessions. In fact I do intend to do a few more races. However, most of the time I am trying to push the limits of what I can include in my training. I struggle to achieve the planned key sessions, so I am reluctant to spend additional time travelling to races when I can use my limited available time more efficiently by doing the key sessions near to home. At the moment I am trying to find time to fit in more hill sessions.

      • Robert Osfield Says:

        I agree that just doing aerobic sessions won’t be sufficient to getting into peak shape, even for one training for ultras.

        With anaerobic workouts I do wonder about which ones to use and when. Intervals of short 10-20 second sprints/hill sprints is likely to provide a different stimulus/stress to an interval sessions comprising of 2-3 minutes of fast running. I guess the answer is probably to do both types.

        For now I’m in fully recovery mode, having just run the Jedburgh Ultra, I’d be lucky to be able to do a 10min/mile interval session let alone sprint 🙂

    • canute1 Says:

      Robert, With regard to optimum duration of high intensity efforts, I am aware on one study that found that for cyclists doing 6 HIIT session over a period of three weeks, 12x30sec min and 8x4min were equally effective, and both produced greater enhancement of 40Km time trial performance than 12×1 min, 12×2 min and 8×8 min. As far as I know this has not been replicated, but if robust, it might indicate that 30sec is the longest that a cyclist can maintain an ‘all-out effort’ while 4 min is a convenient duration for a VO2max effort, but at intermediate durations the cyclists find it harder to adjust to the optimum speed. (Stepto NK, Hawley JA, Dennis SC & Hopkins WG (1999). Effects of different interval-training programs on cycling time-trial performance. Med Sci Sports Exerc 31, 736–741.)
      Well done in the Jedburgh Ultra. Your present training and diet appears to be working well.
      Good luck, with the ballot for WHW – and then with the running. (I tried to post a comment on your blog site but it would not post- I think due to a problem with my software)

      • Robert Osfield Says:

        Interesting study. What were the rest periods between the intervals?

        I wonder if a one trying to optimize fat metabolism might need to adjust the rest periods and duration of the high intensity intervals. 12x30secs is just 6 minutes of high intensity work so unlikely to cause any problems with exhausting glycogen stores. While 8x4min would be 32 minutes of VOmax effort, might be more likely to see the body drain the glycogen stores, especially if the cori cycle isn’t yet well developed to convert back all the generated lactate.

        W.r.t my blog, I recently had problems posting a comment to another based blog, not sure if it’s my recent update to the Chromium browser I use, or something that’s happened with Logging into blogger seemed to address the problem for me and was able to post.

    • canute1 Says:

      My statement that 8x4min was at VO2 max in the Stepto study was misleading. Power output was only at 85% of power at VO2max, but clearly above lactate threshold. I believe the recovery intervals were 60 sec, but unfortunately I no longer have a copy of the paper, so I am basing this on memory. (see edit below)

      The same group reported a study of 8x5min at 86% of power at VO2 max with 1 min recovery. Lactate rose from baseline around 1 mMol to 5 mMol during 1st work bout and 6 mMol in 3rd bout but then remained fairly constant around 6 mMol is subsequent bouts, so it appears that the 1 min recovery was adequate to prevent lactate going to extreme levels. Also arterial oxygen saturation recovered to about 97% in all recovery intervals. Respiratory exchange ratio fell from 0.97 in the first bout to 0.92 in the 7th. Fat oxidation rate rose from 16 micromol/Kg/min in 1st bout to 25 in the 7th . Muscle glycogen concentration fell from 501 mmol/kg at baseline to 243 after the session, so about 50% of the muscle glycogen store was used. Your estimate that 8x4min would cause appreciable glycogen depletion is likely to be correct.

      Edit added 30 Oct: I was intrigued to note that Alex Hutchinson reviewed the study by Stepto today in his Sweat Science Column at Runners World, today, a day after I mentioned it. An amazing coincidence as the study was done a decade ago. Martin Giballa (HIIT expert from McMaster University) recently discussed it at a lecture attended by Alex. Alex published some additional details about duration of recovery that are worth noting. First with regard to recovery period, my memory was not quite correct: recovery duration in the 8×4 min session was 1:30 min. Even more interesting, the recovery in the 12x30sec session was 4:30. Thus, the two sessions are likely to achieve quite different things. In the 8x4min session, recovery is long enough to allow arterial oxygen saturation to return to 97% while lactate level is likely to remain moderately elvated (perhaps around 5- 6 mM, and appreciable glycogen depletion will occur. The long rest in the 12x30sec session probably allows replenishment of the creatine phosphate (phoshagen) system between work bouts, so it is likely that an appreciable fraction of the energy during the work bout comes from the phosphagen system and a modest amount via anaerobic glycolysis. Therefore lactate accumulation will be relatively low, and depletion of glycogen stores minimal. However, the work bouts would be expected to produce a massive flux of potassium ions out of muscle fibres, and the pump that returns the potassium back into the fibres will act vigorously during the recovery period. Enhanced ability to pump potassium is likely to delay fatigue during endurance events. Although Stepto did not discuss this, I would speculate that enhanced ability to pump potassium made a major contribution to improved 40K time trial performance.

  7. Ewen Says:

    I’d be worried about the stress of training in a fasted state.

    After watching Catalyst: ‘Heart of the Matter’ and reading Maffetone, I’m convinced about the benefits of a diet of natural foods, low in sugar and processed carbohydrates, higher in protein and ‘good’ fats.

  8. canute1 Says:

    Thanks for the link to the Catalyst discussion. I agree. I think the evidence that high GI carbs are more harmful than saturated fat is now very strong. There was a crazy announcement today that Nestle and several of the supermarket chains in the UK will reduce saturated fat in their products (eg less saturated fat Nestle’s KitKat) but no mention of reducing sugar content. The UK Minister for Health seemed to think this was a good step forward!

    I also agree with you that there are safer ways to improve fat metabolism than training in a fasted state.

  9. Robert Osfield Says:

    I couldn’t believe it when they announced this initiative. What is our government thinking? “Lets just ignore the science, cherry pick the studies that cherry pick the data that might suggest a relationship between dietary fat and heart disease and turn it into “fact” to dictate what the population should eat. Totally ignore all the studies and experts that don’t support the witch hunt against saturated fats.”?

    Not only will this terrible advice harm the UK population’s health, with it it’ll cause more stress on over burdened NHS. Not to mention the economic harm associated with lost health of the workforce and the tax burden required to support an unhealthy population.

    Me I’m very happy eating plenty of mono-unsaturated fat, saturated fat and modest amount of omega-3 and omega-6 unsaturated fats. I’ll guide my family in this direction too, but can’t help but worry about the consequences of bad government initiatives on the rest of our population.

    I guess it’s time to write to one’s MP.

  10. EternalFury Says:

    I missed a big discussion here! Sorry!

    I am not sure how much nutrition matters. I tend to believe that eating too much is at the source of most modern diseases.
    Obviously, eating too little can cause problems too.

    Otherwise, your body probably doesn’t care where the nutrients it needs come from.
    Sure, it’s probably best to stick with natural things, which have centuries of consumption data going for them.

    As for running in a fasted state…well, intuition would make me believe it’s a bad idea, but Kenyan runners seem to be doing it every morning (from 5 AM for some) and they don’t seem concerned by it.
    Based on their experience, it seems much more important to feed correctly after the effort has been accumulated, than to “top off the tank” prior to accumulating the effort.

    It stands to reason that initially some biomarkers would indicate that running in a fasted state is a very bad idea. For instance, you could panic at the sight of elevated cortisol levels.
    But, it seems the body always reacts with some unusual biomarker when you submit it to a training stimulus it has not experienced before. Then, invariably, adaptation occurs and the biomarkers that signalled something unusual recede.

    To go back to Kenyan runners, I believe some physiologist measured their cortisol levels after their morning run and throughout their training cycles and didn’t notice anything alarming at all.
    Maybe their bodies are at homeostasis with their training regimen because they have had years to adapt to it.

    To push that idea further, maybe a training intervention that doesn’t cause the body to react in some extraordinary manner (by releasing some hormonal or neuromuscular response) is in fact wasted effort. That’s where intuition starts and science needs to catch up. I do not hold this as fact.


    Here is another article related to this discussion:

    • canute1 Says:

      Thanks for your comment. I agree that over-indulgence in any type of food is unhealthy, but think there is good evidence that some foods, especially high GI carbohydrates, are harmful in the proportions in a typical western diet, even when total calories are modest.

      With regard to the nutrition to fuel training, one important issue is optimising the capacity to utilize fats and spare glycogen/glucose (especially for marathon runners). Various types of training session, but especially long runs, contribute to this goal, but various nutritional strategies can also contribute. Of these I think that a diet that is moderately high in fat and low GI carbs, with a modest amount of protein, is the best for this purpose. Training in a fasted state will also promote fat utilization, but as discussed above, there is a risk of excessive cortisol release. I agree that this risk can be minimised by gradual adaptation. Furthermore, I agree that most of the benefits of training come from adaptation to appropriate stress. However, at least for me in my present circumstances I have been pushing volume and intensity of training to the limit that my body can cope with so I do not think that the additional stress of training n a fasted state is worth the risk. Furthermore, the growing evidence that male marathoners are at risk of atherosclerosis, possibly due to chronic inflammation, makes me very cautious about any training strategies that might lead to chronic inflammation.

      There is moderately convincing evidence that a high ratio of omega 6 to omega 3 fatty acids promotes inflammation and atheroma. Magnesium deficiency is another pro-inflammatory factor The role of cortisol is questionable. Acute elevation of cortisol is both catabolic (i.e. breaks down tissues including muscle) and also anti-inflammatory. In the short term this might antagonise the adaptations of muscle to training. But I am even more concerned about longer term effects of training stress. There is intriguing evidence that long term exposure to stress decreases the receptiveness of the receptors that mediate the effects of cortisol thereby promoting chronic inflammation. See The evidence , discussed in my recent blog post, that even recreational endurance athletes exhibit chronic elevation of cortisol suggests that endurance training has the potential to decrease the receptiveness of the cortisol receptors. Hence, I think that it is important to minimise an unnecessary stress associated with training.

      With regard to the experience of Kenyan athletes, it is noteworthy that the re-establishment of normal cortisol levels after exercise is strongly influenced by the combination of mental and physical stress. Thus, if one wants to emulate the life style of a the select Kenyan athletes who participate in training camps, it is probably necessary to spend the time between training sessions eating, relaxing and sleeping.

      • EternalFury Says:

        If you subscribe to the dose-response theory, it’s important to remember you do not have to swing your personal pendulum all the way to the other side.
        Doing a 5-kilometer fasted run a week could maybe have a long-term positive effect without overstressing your body.
        Just food for thought.

    • canute1 Says:


      I agree.

  11. EternalFury Says:

    Another link to share:

    Pardon the source, but consider that even they fund actual science.

    • canute1 Says:

      Thanks for that interesting article by Keith Baar from the Gatorade Sports Science Institute. I agree with most of the science presented. I note particularly that he points out that the nutritional strategy he proposes (including training in a glycogen-depleted state) is designed to maximize metabolic stress. I agree that this is likely to produce desirable adaptations such as increase in mitochondria and capillaries, but it will also create a risk decreased immune function. He does not discuss the possible reduction in cortisol receptor responsiveness, and the associated risk of chronic inflammation. Until these issues have been addressed adequately, I personally would be reluctant to follow his recommendations.

      Some of the effects of this nutritional strategy can also be achieved with HIIT. I am intrigued by the possible benefits of HIIT, but I am also cautious about risks.

      It seems to me that a desirable thing for an athlete wishing to adopt any of these ‘high stress’ strategies is an adequate way of monitoring the return of cortisol and adrenaline to baseline in the hours after the session. However, there are two practical problems. One is the fact that we would not expect a fixed baseline, but rather a baseline that varies during the day. The second is the accessibility of the relevant measurement. Salivary cortisol can be measured but I think a lot of measurements would be required to yield reliable information. Adrenaline activity can be assessed with minimum cost of time and money, using HR, but it is probable that many factors confound interpretation of HR. Nonetheless, I do use the rate of recovery of HR as a crude estimate of the enduring stress produced by a particular session.

      • Robert Osfield Says:

        Thanks EF for posting the link to the article.

        I have been using intermittent fasting and training in fasted for almost all my recent training runs without getting ill or suffering from over training.

        While my volume of training was initially reduced due to injury for part of the last six months, in the last three months I put my body through three ultras and squeezed in as much training as I could given recovery and taper periods had to be fitted in between these Ultras.

        These past three months have been the most intense period of ultra racing I’ve ever done but quite a long way. Previously I was managing two ultras a year, this year I’ve done four in total, three recently. Being able to cope with everything I’ve thrown at my body these past months suggests that rather than increasing risk of over training I have at least kept it stable. I don’t think it’s much of stretch to suggest that my recovery and ability to avoid over training has been improved.

        I’m usually one who starts breaking down and getting injured when I put too much volume of training and racing in, but this hasn’t happened. I really doubt that I’m the exception to the rule, I can’t help but feel that the potentially increases in risk of over training are being over played.

        Sometimes I think one can spend too much time reading, pontificating and not enough doing. Occasionally you just have to try something before dismissing it. Things that you might worry about might just turn out to be nothing to worry about after all.

    • canute1 Says:


      I think that your recent excellent performance in three ultras demonstrates that you are doing something right. I think it is very likely that the increased proportion of fat in your diet is beneficial. I also think that in light of your relatively low volume of training in recent months (as indicated in your own blog), that doing some of that training in a fasted state migth well have been more beneficial than harmful. It appears that the major stress your body has faced in recent months has been the three ultras. You did not run these in a fasted state. I note that at least for the Devil and the RAW you consumed a breakfast that was relatively low in carbs, though you had done a modest amount of carb loading in the days preceding the RAW. You also consumed food duirng these races.

      Therefore, while I think that your experience confirms the value of increasing proportion of dietary fat, I do not think you have yet demonstrated that heavy training in a fasted state achieves a net benefit. Furthermore, my major concern about heavy training in a fasted state is the long term consequence of chronic inflammation – especially atherosclerosis. The magnitude of this risk is difficult to estimate, but I will require a lot more evidence for lack of long term harm before I take this risk, when there are probably safer ways of achieving the same goals.

      • Robert Osfield Says:

        Hi Canute,

        I agree that it’s likely that my overall diet is probably key to my success over the last few months.

        However, I wouldn’t characterize my training since the River Ayr Way ultra, from mid September, as relatively low volume. My training for Jedburgh Three Peaks was up near the maximum I usually can manage and wasn’t compromised by injury worries. I don’t believe I could have coped with putting in so much training so soon after the RAW had my bodies ability to cope with the load have been compromised by running fasted.

        Food consumption wise, I wouldn’t race fasted and see no benefit in running without food. Breakfast was a low GI, and the food I consumed during the races contained easily digestible carbs and fats, and modest amount of protein. I was able to consume a lot of food during my last three ultras, far more than I had been able to previous due to gastric stress. I was averaging around 200 calories per hour in each on my races – during the Jedburgh race I consumed 1400 calories in the six and half hours that I was running.

        Being able to consume food consistently is something new to me as I always used to struggle to eat from four hour point onwards. This ability to keep consuming food is likely a factor in being able to run strongly through to the end. I believe this is secondary benefit of becoming better at metabolising fat. I burn less glycogen so my muscles and liver doesn’t exhaust so quickly, so my hormone levels don’t need to go to any extremes to compensate, so I stay nearer homoeostasis and able to keep digesting food.

        Whether I could have achieved this fat burning capacity without training fasted is a big question. I suspect it’s just the icing on the cake and not crucial as you suggest, the actual overall diet is more important. I would say thought that once adapted to fat burning, training fasted become less stressful on your body, I believe this is certainly why I’ve not struggled in these last months.

  12. canute1 Says:

    According to your blog, you did relatively little training prior to either the Devil or the RAW (and some of that was evening training) yet you ran very well in both of these races. Therefore, I think the main benefit is likely to have come from the increase in fat in your diet, though I think it is plausible that training fasted when you were doing a low volume of training might have added to the benefit from the increase proportion of fat. As you imply, it might have been some sugar-free icing on the cake.
    I also agree that the ability to consume nutrition during an ultra is probably one of the benefits of the increased proportion of fat in your diet.

    • Robert Osfield Says:

      For the Devil and RAW my training volume was low, but after the RAW the real test of running fasted came – my volume and intensity was back up to around the upper end of what I can normally tolerate. I passed this test, with no signs of over training or lack of response to training. This is why I’m asserting that running fasted hasn’t had the detrimental effects that you’ve been suggesting.

      You can’t just ignore this finding by keeping looking back to when I had a low volume of training and making conclusions from that. One has to look at all the data, including the bits that don’t quite fit with the hypothesis that running fasted is likely to result in high risk of overload.

    • canute1 Says:

      My point is that training in a fasted state is unnecessarily risky in the long term. I myself have trained in a fasted state for a period of several weeks in the past, with no immediate adverse effects. In the end the pressure of a busy job curtailed that particular phase of training At that time, I considered that in order to continue with a moderate volume of training, it was wisest to minimise unnecessary stress, and to avoid the inevitable sleep loss due to working late and training in the early morning.

      The long term risks of fasted training are not clearly established, but neither is there convincing evidence of benefit sufficient to justify the risk. The formal research studies have yielded mixed results. In your own case, the impressive evidence of improvement occurred following a change in your diet at a time when your training was relatively modest. Nonetheless I do not entirely discount the possibility that fasted training has contributed to your improvement. If a runner wants to experiment with fasted training, it is almost certainly best to introduce it at a time when training load is not very heavy, as you have. However, until there is clearer evidence regarding the causes and long term consequences of chronic inflammation in runners, I consider that the risk is not worth taking.

      • Robert Osfield Says:

        From my own experience and how my body responds when running fasted I would say that my cortisol levels are no higher after a fasted run than they were previous to doing fasted runs/changing my diet. This is after a period of adaptation to diet and easing in the fasted runs.

        Long term I don’t think there is particular risk, it’s more short term if you are pushing your too hard before your body is sufficiently fat adapted. Initially if one does try fasted runs I’d recommend keeping fasted runs at gentle pace and relatively short. Then add distance or pace as the months progress. For 6 months now I’ve skipped breakfast most days and have been training fasted on most runs, so far haven’t noticed any issues with over training, in fact recover quicker now than before. Would six months qualify as long term?

        Whether fasted runs are beneficial will probably depend upon whether it adds the types of stress on the body that you seek and are able to adapt to. Like with any training too much training volume and intensity will lead to over training. Could it be that you can get the same training stimulus from less training volume by running fasted?

        The fact that the study, by Van Proeyen and colleagues, didn’t find an advantage in a high fat diet athlete running fasted suggested either that that running fasted didn’t add any additional useful stress, or added too much stress and led to over training, or just caused a reduction in the intensity of the training so resulted in not enough stress of the desired type. I haven’t read the whole paper so can’t tease apart which of these or what combination is most likely the case for this subjects in that study.

        From my own diet and training going forward I haven’t experienced any problems with running fasted and have rather got out of the habit of eating in the morning so will happily continue. For runs less than two hours, includes fast ones, I really detect no problems with blood sugar even when running after an 18 hours fast, my metabolism really doesn’t feel under stress at all. It’s hard to see that I’m struggling with high cortisol levels given just how easy I’m finding these runs and how quickly I’m recovering.

        For run longer than two hours I will start taking food with me to avoid sugar levels getting too low too often. I don’t actually run much longer than two hours too often so it shouldn’t effect most of my runs.

        I’ve now completed all my races for this year so I’m taking it easy this November. I plan to run twice a week or so just to keep the legs loose and enjoy the sunshine when it comes out – like it did today, had a beautiful 11 miler, fasted of course 🙂

    • canute1 Says:

      By long term consequences, I mean the consequences such as atherosclerosis that apparently develops over a period of years of running. However the causes, mechanism and prevalence of atherosclerosis in runners is a topic of speculation based on scant evidence. The facts are that in on average runners live longer healthier lives that people who do not exercise, but some, especially males, develop cardiac rhythm abnormalities, and some develop atherosclerosis. There is strong evidence that the prevalence of rhythm disturbances is substantially higher in committed endurance athletes than in age matched controls, and moderate evidence that atherosclerosis has an increased prevalence in males who have run many marathons over a period of years.

      The most likely mechanism of the atherosclerosis is chronic inflammation. The most likely cause of chronic inflammation in runners is inadequate recovery from stressful races and training. My own speculative conclusion is that male runners who plan to run a relatively high mileage on a regular basis over a period of many years, should take great care to ensure adequate recovery from the inevitable stresses that are necessary for improvement, while also avoiding any unnecessary stress.

      Until there is clear evidence that the benefits of fasted training outweigh the risk, I personally will not make it a regular part of my training. At this stage I think that minimising high GI carbohydrates and consuming a moderately high proportion of fat, with a good balance of omega 3 and omega 6 fats, is the safest way to get the major benefits of nutritional adjustment. However I accept that if a runner chooses to the try fasted running, building up gradually is the right way to start. Once you are well adapted to utilizing fats, fasted running is likely to be less stressful. Furthermore, your overall program that includes a few easy months of undemanding fasted training at some point in the year, is a sensible approach.

      • Robert Osfield Says:

        I would agree that balancing recovery with stress is crucial for both training adaptations and long term health.

        However, I don’t believe there any particular about running fasted that adds a special risk, it’s just another training stress that needs to be balanced. If a runner can’t manage large training volume and intensity, such as when rehabilitating injuries, that it may be easier to run fasted to get closer to the benefits of high training volume/intensity.

        I would agree that eating a diet moderately high in healthy fats is likely to be protective for general health, and specifically beneficial for training and recovery – I expect this is the main reason for my quicker recovery from training and better racing this year. If done properly It’s a no risk win-win and easy to recommend.

        It might well be that the changes I made and the order I made them in this year just happened to be a sweet spot for an athlete looking to benefit from fasted runs. My training volume was initially low, so I needed all the stimulus I could get to encourage beneficial adaptations. My dietary changes also helped move by body across to metabolising fats and sparing glycogen stores so that running fasted was relatively low risk.

        Going forward I’d love to have tools to track my hormone levels so I monitor the stress levels that my body is under. All this speculation about risks and benefits is done on limited scientific trials and subjective feelings – i.e. I feel fine during after fasted runs, so would perceive this as an indicator of no undue stress. What is really going on inside and in the long term I can only guess until we have these tools.

  13. Cortisol and the stress response | Canute's Efficient Running Site Says:

    […] utilise fat as fuel, but overall the studies have yielded mixed results, as I have discussed in a previous post. I suspect this is because training in a fasted state also promotes increased cortisol levels. I […]

  14. Getting maximum benefit from low intensity training | Canute's Efficient Running Site Says:

    […] but care should be taken to avoid excessive stress. As I suggested in my previous discussion of training in the fasted state, I suspect that the inconsistent results reported by different studies of training in the fasted […]

  15. Testosterone Supplements And Products Says:

    Hi, I read on another forum that testosterone therapy could be detrimental to health because of the dangerous side effects found in them. Have you know anything about this?

    • canute1 Says:

      I do not know anything about contaminants is testosterone supplements. Nonetheless, in my opinion it is risky to take testosterone supplements unless it is to treat an identifiable deficit in testosterone, in which case the supplementation should be under medical supervision.
      In individual who do not have an identified deficit in testosterone, it is nonetheless important to achieve a good balance between the catabolic hormones (eg cortisol) required to mobilise the resources required to cope with acute stress and anabolic hormones required to strengthen the body after stressful experiences, to prepare it to cope with future stress (ie the training effect). The older you get, the more important it is to ensure that catabolism does not overwhelm anabolism. My referred way to promote anabolism is adequate protein intake and lifting weights to promote the release of anabolic hormones.

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