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Exercise scientists teach that since muscle glycogen utilisation occurs at high rates (during high-intensity exercise in carbohydrate-adapted athletes), all athletes must be advised to ingest large amounts of carbohydrate before and during exercise.1 ,2 But this does not seem entirely logical.
Why, for example, should athletes involved in prolonged submaximal exercise—probably the most common form of exercise performed by most elite and recreational athletes in training and competition—need always to eat high-carbohydrate diets in which at least 40–60% of calories are derived from carbohydrates? Surely our abundant body fat stores could provide most if not all the energy necessary to fuel activities of a submaximal intensity? Might our ability to use fat as a fuel during most activities explain the opinion that ‘a conclusive endorsement of a high-carbohydrate diet (for improved athletic performance) based on the literature is difficult to make’.3
Long-term health risks of high-carbohydrate diets
What of the long-term health consequences of habitually eating a high-carbohydrate diet especially for that increasing number of recreational athletes who may be unaware that they are insulin resistant (IR)4–⇓6 and for whom carbohydrates pose a major health hazard including the risk of developing type 2 diabetes mellitus? The recent finding that habitual marathon runners can have marked coronary artery disease,7–⇓9 worse than that present in sedentary individuals, despite similar or lower coronary risk factors, invites sober reflection.10 Is it possible that there is a dietary component to this? Could a high-carbohydrate diet in marathon runners with IR induce an inflammatory state promoting coronary atherosclerosis? And is the poor dental and overall health of many Olympic athletes11 the result of eating a high-carbohydrate diet and frequently ingesting sugary sports drinks? Perhaps the time has come to question the popular advice that all athletes must only ingest high-carbohydrate diets.
Assessing the current state of knowledge
What is the current state of knowledge of carbohydrate-restricted diets for athletes?12 We note that the survival of humans in the frozen Arctic eating diets that contain little or no carbohydrate shows that modern humans have no essential requirement for carbohydrate—indeed a human carbohydrate deficiency disease has yet to be described. Historic reports that human explorers in the Arctic and Antarctic could adapt to a carbohydrate-free diet inspired the first modern study of human exercise adaptations to a low-carbohydrate diet in 1983.13 That study found that humans can adapt to this diet without any impairment or improvement in submaximal exercise performance.
We could trace just 10 other studies of the performance effects of low-carbohydrate diets in humans published in the past 31 years. Of the total of 11 studies, 3 found that exercise performance improved with adoption of a low-carbohydrate diet; another 4 showed equivocal results favouring the low-carbohydrate diet but limited by small sample sizes; 2 found no beneficial effect and 2 reported an adverse outcome.
However, none of the studies evaluated chronic (6–12 months) adaptations to the diet; only one involved very prolonged exercise (a 200 km cycling time trial); none compared the effects of dietary change in athletes with normal carbohydrate metabolism to those with known IR; and none was properly placebo-controlled (if that is indeed possible). Missing are studies addressing the effects of low-carbohydrate diets on the ease of weight control in athletes, on their capacity to train and their ability to recover, on their immune function and injury risk, or on their hand-eye coordination or capacity to concentrate in sports like golf and cricket, to name but a few obvious research questions. Clearly there is still much to be done.
However, studies of elite athletes chronically adapted to low-carbohydrate diets has uncovered one unexpected finding—their extraordinary ability to produce energy at very high rates purely from the oxidation of fat. Thus some highly adapted runners consuming less than 10% of energy from carbohydrate are able to oxidise fat at greater than 1.5 g/min during progressive intensity exercise and consistently sustain rates of fat oxidation exceeding 1.2 g/min during exercise at ∼65% VO2max,12 ,13 thereby providing 56 kJ/min during prolonged exercise.14 The remaining energy would comfortably be covered by the oxidation of blood lactate, ketone bodies and glucose derived from gluconeogenesis.14 Thus a fully fat-adapted athlete able to oxidise fat at 1.5 g/min would cover his or her energy cost during an Ironman Triathlon without needing to ingest exogenous fuels especially carbohydrate. This contrasts with the need of carbohydrate-adapted athletes to ingest 90–105 g/h during prolonged exercise if they wish to maintain their performance.1 ,2
Indeed this simple calculation identifies the key difference in the low and high-carbohydrate dietary approach for endurance athletes. Once they deplete their endogenous carbohydrate reserves, athletes chronically adapted to high-carbohydrate diets likely become entirely dependent on exogenous carbohydrate for their performance. In contrast, athletes adapted to a low-carbohydrate diet carry all the energy they need in their abundant fat reserves. And because they live and train with chronically low blood insulin concentrations, they have instantaneous access to those fat reserves at all times. Just as should occur in a metabolism crafted by our evolutionary history as predatory hunters.15
The recent study by Shimazu et al 16 has established another unexpected potential benefit of the low-carbohydrate diet. They showed that chronic ketosis in mice downregulates the expression of the class-1 histone deacetylase (HDAC) enzymes. Reduced HDAC activity lowers oxidative stress. Nutritional ketosis is also associated with a reduced generation of reactive oxygen species by mitochondria.17 Together these ketosis-induced adaptations should reduce oxidative stress and might influence rates of recovery from demanding exercise, an advantage anecdotally reported by athletes chronically adapted to a low-carbohydrate diet.12
All this evidence suggests that there should be more to dietary prescription for athletes than an exclusive high-carbohydrate diet. There is a proven need for research of low-carbohydrate diets in all sports, not just those involving endurance. Now is the time to determine whether the conclusion that ‘there is very little or no evidence to support the use of high-fat diets’ by athletes18 is an eternal truth.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.