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.
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.