Debates about diet for general health and specifically about the optimum nutrition for athletes have raged for many decades, but in the past decade this debate has largely been dominated by the polarization of opinions for or against the Paleo diet, the presumed diet rich in fat and protein consumed by our distant ancestors who hunted on the African savannah. In recent years, Tim Noakes’ rejection of the high carbohydrate diet that he had advocated in his authoritative ‘Lore of Running’ added momentum to the shift away from carbohydrate towards fat, but on the whole professional dieticians have remained sceptical of fat. The debate is far from over, but I think that there has been substantial progress in assembling evidence regarding optimal nutrition for endurance athletes. As with virtually all studies of something as complex as human physiology, no single study is definitive. Staunch advocates of either side of the argument can point to the limitations of any single study, but the overall body of evidence does provide a fairly consistent picture. Perhaps it even allows us to speculate on how difference between individuals might account for some for the contradictory findings from studies.
There have also been numerous debates about specific micronutrients, food supplements and ‘super-foods’, ranging from beetroot to chocolate, which I will scarcely touch on here to avoid the post being excessively long. My overall conclusion is that while taking a particular additive might have beneficial effects in some individuals in some circumstances, the unforeseen consequences of many additives often nullify the potential benefits, because the body is a homeostatic system that acts to compensate for any abrupt changes. This is perhaps best illustrated by the antioxidants which have the potential to avert the tissue damage due to free radicals that are a by-product of energy metabolism. Nonetheless, many studies of anti-oxidant supplementation do not show a net benefit. In general, I aim to achieve the required intake of micronutrients via a fairly diverse diet, including a diverse range of fatty acids, rather than by taking supplements. Despite my general scepticism, I am moderately convinced by the evidence that green tea does have an overall beneficial effect on well-being. It contains moderate amounts of antioxidants along with many other compounds with potential health benefits, including catechins with beneficial effects on cardiovascular health. I myself drink modest amount so green tea as I enjoy it as a beverage.
Nutrition during training and while racing.
There are two aspects of the physiology of running that are established beyond debate and provide the starting point when planning nutrition during preparation for a race and during the race itself. The first is that the metabolism of glucose (via glycolysis and subsequent oxidative metabolism via the Krebs cycle) produces energy at a faster rate than metabolism of either fat or protein. The second is that the supply of the glucose precursor, glycogen, is limited, whereas even lean runners carry a virtually inexhaustible supply of fat.
Up to the half marathon
The fact that metabolism of glucose generates energy more rapidly makes it essential to burn glucose for events run at speeds near or above lactate threshold pace. Provided we consume enough carbohydrate in the pre-race period to ensure that glycogen stocks are full before the race, the stored glycogen will last for events lasting up to 90 minutes or even longer. Consistent with this, in a meta-analysis of 20 studies comparing the effects of high carbohydrate with high fat with diet on endurance exercise, Erlenbusch and colleagues found that overall, subjects consuming a high-carbohydrate diet exercised significantly longer until exhaustion, but there was substantial variation in finding between different studies. I myself do a modest amount of carbohydrate loading before a half marathon but consume nothing apart from water during the event itself.
In a marathon, the fact that the store of glycogen is limited comes into play. To avoid running out of glucose we need to do two things: increase our capacity to utilise fat during the preparatory period and augment our glucose supply by ingestion of carbohydrate during the race. Irrespective of specific nutritional strategies, training itself – both high volume, low intensity training and also high intensity interval training, enhance the production of the enzymes involved in fat metabolism and thereby increase the ability to utilise fat.
Two other strategies have been studied fairly intensively but with inconclusive overall outcome. First, training in a fasted state might be expected to enhance the ability to utilise fats. Indeed it does, and at least in some studies it enhances endurance performance. However, another factor comes into play. Training in a carbohydrate depleted state encourages the adrenal glands to release cortisol, which acts to ensure that blood glucose is reserved for use by the brain. In the short term cortisol promotes an effective response to stress but if the elevation of cortisol is sustained, it has a damaging effect on many tissues. A study by Skoluda, using measurement of cortisol in hair to assess sustained cortisol levels revealed that many endurance athletes have excessive sustained levels of cortisol. I believe that elevation of cortisol might be one reason why studies of training in a carbohydrate depleted state yield inconsistent findings. My personal conclusion is that as an elderly runner, for me the risks of tissue damage due to sustained cortisol levels are too high. However, for a younger athlete, training in a carbohydrate depleted state might be beneficial provided care is taken to minimize any unnecessary stresses. It is probably useful to monitor for signs of excessive cortisol – though direct measurement is impractical for the recreational athlete. Perhaps assessment of mood via the Profile of Mood States questionnaire provides the most practical proxy measurement.
The other widely studied strategy is consumption of a high fat diet to promote preferential use of fats until a few days before the race and then topping up the glycogen supply via carbohydrate loading. A few studies have found this to be beneficial in endurance events. For example a study from Noakes’ lab using the nutritional periodization strategy found that high-fat consumption for 10 days prior to carbohydrate loading was associated with an increased utilization of fat, a decreased reliance on muscle glycogen, and improved time trial performance in a 20 Km time-trial following 150 minutes of medium intensity cycling. However, other studies, such as that by Carey and colleagues, have not shown improved endurance performance and overall the results are inconclusive. I suspect that this is because the body is a homeostatic system that adjusts to compensate for any abrupt change in circumstances. Therefore the body is likely to react to thwart any strategy that entails abrupt changes.
My own approach to marathon training is a balanced diet during the period of heavy training (for reasons discussed in the section on healthy nutrition below), moderate carbohydrate loading immediately preceding the event and the consumption of carbohydrate in small amount during the event – though I have yet more experimenting to do to identify the within-race fuelling schedule that suits me best
Developing a high capacity to utilise fats is a high priority in training for an ultra-marathon. Nonetheless, as in the case of marathon training, I would be inclined to recommend a balanced diet during high volume training, and rely on the high volume of training, augmented by a small amount of high intensity interval training, to maximise the capacity to utilise fat. But it is pre-race and within race nutrition that raises the big issues. An ultra challenges virtually all systems within the body including the brain. The first issue is ensuring an adequate supply of glucose for the brain. Thus in the pre-race period it is important to ensure that the liver is well stocked with glycogen. Furthermore, because pace is below threshold pace, metabolism of fat is fast enough to provide a large proportion of the energy required by muscles, but at least some glucose metabolism is required. Fat metabolism leads to energy production via the Krebs cycle, but unlike glucose metabolism, fat metabolism cannot restock the pool of Krebs cycle metabolites. This pool gets depleted due to the production of glutamine – an amino acid produced in muscle by an offshoot of the Krebs cycle. Glutamine is transported from the muscle to other organs, most importantly to the gut where it plays a key role in keeping the gut functioning well. So an ultra-runner relies on a modest amount of glucose metabolism within muscle. What does this tell us about nutrition during an ultra? Clearly a supply of carbs is required but the stomach rejects simple sugars after a few hours. In part this might be a matter of the consistency of the food, but probably even more importantly, the body craves additional things – not only amino acids including glutamine but other things as well. The several possible mechanisms by which augmentation of carbohydrate ingestion with protein might enhance endurance performance has been reviewed by Saunders. In my limited experience of 24 hour events, I have relied on solid food with a fairly high carbohydrate content augmented by protein.
Nutrition for long term health
For the athlete, heart health is of special importance. Not only is heart disease the major cause of mortality in the general population but in addition there is some evidence that extensive endurance training and racing might in fact increase the risk of cardiovascular disease in athletes. Furthermore, most evidence suggests that a healthy diet for the heart minimises cancer. For many years, public health professionals have expressed concern about the unhealthiness of the typical Western diet. Concerns have focussed on the excessive total calories, saturated fats, high salt content, and more recently, high sugar content.
Foods with high sugar content produce a rapid rise in blood glucose that stimulates release of insulin thereby promoting increased resistance to the effects of insulin, while also producing an associated increase in arachidonic acid, an omega-6 fatty acid which is pro-inflammatory. This exacerbates the problems arising for the fact that the typical Western diet is already unbalanced by an excess of omega-6 fatty acids. However, the effects of arachidonic acid and inflammation are not all bad. Acute inflammation is probably crucial for recovery and strengthening after training. The crucial issue is achieving the right balance between omega-6 fats and omega-3 fats that are much less inflammatory and reduce the inflammatory effect of omega-6 fats.
The recent comprehensive review of nutritional recommendations for heart health, Eilat-Adar and colleagues reported that both low fat and low carbohydrate diets are a healthy alternative to the typical Western diet. They found that low carbohydrate diets, which typically derive 30%–40% of calories from carbohydrates and are low in saturated fat but higher in monounsaturated fat, are associated with a healthy balance of fats in the blood, with lower levels of potentially harmful tryglycerides and with higher levels of beneficial high density lipoprotein (HDL). Eliat-Adar also found good evidence that Mediterranean diets, which include high consumption of fruit, vegetables and legumes, together with moderately large amounts of fish but less red meat may improve quality and life expectancy in healthy people, as well as in patients with diabetes, and heart disease. Mediterranean diets are preferable to a low-fat diet in reducing triglyceride levels, increasing HDL cholesterol, and improving insulin sensitivity.
A rigorous meta-analysis of trials by the Cochrane Collaboration also concluded that the evidence suggests favourable effects of the Mediterranean diet on cardiovascular risk factors, though with their usual caution, they stated that more trials are needed.
Because of many confounding effects in studies of self-selected diet, there is special value in large studies in which people are randomly allocated to different diets. One such study is the Spanish Prevención con Dieta Mediterránea (PREDIMED) trial, in which 7,216 men and women aged 55 to 80 years were randomized to 1 of 3 interventions: Mediterranean diets supplemented with nuts or olive oil or a control diet, and followed for a period of approximately 5 years. The Mediterranean diets were healther than the control diet. Nut supplementation was especially protective. Subjects on the Mediterranean diet consuming more than 3 servings/week of nuts had a 39% lower mortality risk than those on the control diet, due to protective effects against both cardiovascular and cancer mortality.
The debate about the merits of saturated versus mono-unsaturated fats has thrown up some surprising evidence contrary to the prominent advice to substitute polyunsaturated fats for saturated fats in worldwide dietary guidelines for reducing risk of coronary heart disease. Recent re-analysis of the large West Sydney Heart study found that replacing dietary saturated fat with omega- 6 linoleic acid, for subjects with known cardiovascular disease, actually led to higher all-cause death rate, and to higher death rate from cardiovascular disease. The most plausible explanation is that the increased death rate was due to the pro-inflammatory effects of omega-6 fatty acids. Since the typical Western diet contains a high proportion of omega-6 fats, at least a modest intake of omega-3 fats, typically found in oily fish, is likely to be more healthy than increasing omega-6 fats.
There is overwhelming evidence that diet plays a large role in health and longevity, and after many years of confusing debate, there is emerging clarity that the healthiest diet is neither a high fat/low carbohydrate Paleo diet nor a low fat/high carbohydrate diet. Rather, the evidence suggests that a Mediterranean diet is preferable. Augmentation with extra nuts is probably worthwhile. It is also important to achieve a good balance between the pro-inflammatory omega-6 fats and the less inflammatory omega-3 fats, typically found in oily fish. Such a diet is likely to be optimal for athletes during periods of sustained heavy training. For longer endurance events, increased carbohydrate consumption in the immediate pre-race period will ensure that glycogen stores are replenished. During a marathon, regular intake of a small amount of carbohydrate will help maintain the supply of glucose to both brain and muscle, while in an ultra, more complex and palatable food including both carbohydrate and protein is better able to meet the more complex metabolic demands.