Carbohydrate Availability and Training Adaptation

It is well known that nutrition can influence the acute and chronic response to exercise, including athletic performance and training-induced adaptations in skeletal muscle. One classic example is the positive relationship between carbohydrate intake, muscle glycogen content, and endurance exercise capacity. Leading sports nutrition experts continue to advocate that athletes who partake in activities that are heavily reliant on glycogen for energy ingest sufficient amounts of carbohydrate before, during, and after exercise (1). The general premise for this recommendation is that habitual training in a chronic high-carbohydrate state will maximize the training impulse and optimize performance. However, a long-standing unresolved question is whether it is a lack or a surplus of substrate that triggers the adaptive response to exercise. In this issue of Exercise and Sport Sciences Reviews, Drs. John A. Hawley and Louise M. Burke review the evidence from recent studies that have investigated whether selected markers of endurance training adaptation are enhanced to a greater extent when individuals commence periodic training sessions with low compared with normal or high carbohydrate availability (4). Several years ago, Danish researchers (3) published an intriguing study that reported greater increases in the maximal activities of oxidative enzymes and exercise time to fatigue when one leg was trained twice per day (with restricted carbohydrate intake between sessions) as compared with the contralateral leg that trained once daily. Given the unique experimental design, the original study authors were careful to note the limitations of their work, cautioning, "Coaches and athletes should be careful not to draw practical consequences of the present study with regard to training regimens." Nonetheless, the article coined the term "train low, compete high," a catchphrase borrowed from altitude physiology that Hawley and Burke (4) argue has become widely used in athletic circles (and scientific literature) to describe a range of practices other than the original protocol, with the potential for confusion owing to misunderstood terminology. For example, it is often overlooked that in the original study by Hansen et al. (3), only half of the training sessions were initiated with low muscle glycogen. Hawley and Burke (4) observe that there are many ways of manipulating carbohydrate availability, and their considerate review highlights subtle differences in various experimental designs that are important to properly evaluate the evidence for and against "training low." A periodic train-low approach may indeed offer a time-efficient method to augment adaptations, even in athletes who are already highly trained (5). Using a nutritional manipulation similar to Hansen et al. (3), but with a more applied research design that simulated the usual practice of competitive athletes, Yeo et al. (5) showed that cyclists who trained twice a day on alternate days experienced greater increases in mitochondrial enzymes and whole-body fat oxidation compared with a group that trained once daily. The mechanisms responsible for the enhanced skeletal muscle oxidative capacity under conditions of restricted carbohydrate intake remain largely elusive, but recent evidence points to several nutrient-sensitive signaling molecules that may be involved in the adaptive response (2). A conundrum highlighted by Hawley and Burke is that, despite the changes in "mechanistic" variables that should in theory enhance exercise performance (e.g., increased phosphorylation state of signaling molecules and/or the expression of proteins involved in mitochondrial biogenesis), there is a mismatch in many studies in that whole-body functional outcomes (e.g., changes in training capacity or measures of performance) remain unchanged. The authors propose several explanations for this disconnect, noting we have inadequate knowledge of underlying adaptive mechanisms as well as surprisingly little knowledge about glycogen utilization during the training sessions typically undertaken by competitive athletes or how their current real-world training and dietary practices interact to influence performance.

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