Nutrition: Diet Makes a Difference in Fuels for Exercise
Muscle glycogen is the preferred fuel for high intensity exercise. Because carbohydrate stores are limited, maintaining optimal muscle glycogen levels during periods of long duration or intense training requires consuming an adequate level of carbohydrate intake on a daily basis. For both strength and power athletes and endurance athletes, the energy and carbohydrate demands of hard training are often greater than those required for competition. Fatigue, caused by inadequate recovery from one training session to the next, is a problem common to many high striving athletes. Nutrition support that complements the demands made by training is part of the solution.
This graph (Costill, 1980) is based on a study where muscle glycogen levels were measured in trained cyclists. The cyclists were matched as to their energy requirements. They each consumed a 3,500 calorie diet that was either low in carbohydrate (40% of total calories, 300-350 grams of carbohydrate daily) or high in carbohydrate (70% of total calories, 500-600 grams of carbohydrate daily.) Subjects engaged in two hours of strenuous cycling exercise each day over three consecutive days. On the low carbohydrate regimen, muscle glycogen levels dropped lower each succeeding day. By the third day, these subjects were unable to complete their workout at the required intensity. Alternatively, athletes on the high carbohydrate diet experienced near maximal repletion of muscle glycogen on successive training days and completed all workouts. Further studies have indicated that for a 150 pound male endurance athlete, a diet consisting of 8-10 grams of carbohydrate per kilogram of body weight is needed to restore depleted muscle glycogen levels within a 24-hour period.
Additional studies have shown that the rate of muscle glycogen synthesis is most rapid when carbohydrate is consumed immediately (or within two hours) after cessation of either intense endurance or resistance exercise. Several studies indicate that co-ingestion of carbohydrate and protein soon after exercise ends promotes both muscle glycogen and amino acid resynthesis.
