Energy utilization and repletion during endurance exercise: an historical perspective

Abstract

Some of the first studies of the effects of diet on exercise performance were reported in the late 1920s. These studies established that eating a high carbohydrate diet before, and ingesting glucose during prolonged exercise delayed the onset of fatigue and enhanced performance, in part by preventing the development of hypoglycaemia.The next significant advance came with the introduction of the muscle biopsy needle which allowed muscle carbohydrate (glycogen) content to be measured for the first time. This technique focused attention on the role of muscle glycogen as possibly the more important factor explaining fatigue during prolonged exercise. Subsequent research inevitably focused on the identification of techniques that would either increase muscle glycogen storage before exercise or decrease the rate of muscle glycogen utilization during subsequent exercise, or both.Coincidentally, studies of fluid balance and of the control of gastric emptying during exercise suggested (a) that dehydration posed a serious health risk especially to marathon runners and (b) that dehydration could only be prevented during prolonged exercise if water alone was ingested. Thus the ‘state of the art’ knowledge in 1970 was that a high carbohydrate diet should be eaten before competition; that muscle glycogen depletion was the main cause of fatigue during prolonged exercise; and that water alone should be ingested in large volumes during exercise. The value of carbohydrate ingestion during exercise was systematically ignored despite the evidence from the classical studies of the 1920s.Fortunately commercial interests came to the rescue of exercise physiology in the mid‐1980s. Whereas water is without commercial value, carbohydrate added to the water and ingested by millions of athletes throughout the world has attractive commercial potential. Thus research funded and orchestrated largely by the commercial sector has established that adequate fluid delivery can be provided from drinks containing carbohydrate and that the ingestion of carbohydrate during prolonged exercise can prevent the onset of premature fatigue caused by liver glycogen depletion, hypoglycaemia and a low rate of carbohydrate oxidation.Thus the ‘state of the art’ knowledge in 1990 is that there is no proven value of ingesting water alone during prolonged exercise: that liver glycogen depletion is an important and preventable cause of fatigue during prolonged exercise: and that although muscle glycogen depletion is an important contributor, the real biochemical explanation for fatigue during prolonged exercise is still not clearly established. Nevertheless, there is clear evidence that the provision of exogenous carbohydrate at high rates is essential to delay fatigue and optimize performance during prolonged exercise. It has also become apparent that high rates of both carbohydrate and fluid delivery can be achieved by the adoption of appropriate drinking techniques during exercise.