PREEXERCISE DIET AND PERFORMANCE
Exercisess 3.5 and 3.8 show that a high-carbohydrate (75 to 90%) diet eaten for the last 3 days before exercise causes maximum filling of muscle and liver glycogen stores. Exercises 3.7 also shows that the carbohydrate content of the diet needs to approach 500 g a day for optimum filling of these stores. (post 15 provides practical dietary advice for achieving these goals.)
The length of time for which prolonged submaximal exercise can be sustained is closely linked to the size of the preexercise muscle glycogen stores. Karlsson and Saltin (1971) found that time in a 30-km race was best when the prerace muscle glycogen levels were 21.9 g/kg wet muscle (high-carbohydrate diet) but that the time was 12 minutes slower when prerace muscle glycogen levels were only 11.2 g/kg (high-fat, high-protein diet). Extrapolated to the marathon distance, these data suggest that an athlete who eats a low-carbohydrate diet will run about 30 to 40 minutes slower than one who eats a high-carbohydrate diet.
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In a more recent laboratory study, Bebb et al. (1984) showed that subjects who carbohydrate loaded before exercise were able to run 12% longer at 70% V02max than those who ate only their normal diets for the 3 days before the exercise.
If a high-carbohydrate diet is so beneficial for prolonged submaximal exercise, does it have any role in exercise at 100% or more of V02max? At present, the answer seems to be affirmative.
Maughan and Poole (1981) and Greenhaff et al. (1987a, 1987b) showed that athletes could exercise longer at higher intensities after ingesting high-carbohydrate diets, and I. Jacobs (1987) found that the work performed during 50 maximal muscle contractions improved 10 to 20% with carbohydrate loading. Greenhaff et al. (1987a, 1987b, 1988a, 1988b) found that blood pH, blood bicarbonate levels, and blood-base excess before, during, and after maximal exercise were all higher after carbohydrate loading, indicating that this diet favorably affects blood and muscle acid-base status by reducing the decrease in pH during exercise. The authors suggested that the improved exercise performance during maximal exercise after carbohydrate loading may be due to an enhanced buffering capacity in muscle and blood. They also noted that those subjects who ate low-carbohy-drate diets had increased dietary acid intakes.