Carbohydrate nutrition and exercise

Extract: Carbohydrates are not required in man's diet. Nevertheless, dietary carbohydrates constitute 40% or more of the American diet. Although most if not all body tissues have the capacity to store intracellular glucose as glycogen, the largest stores are found in liver and muscle. When exercise begins, muscle fuel utilization is optimized by an orderly sequence of events. Complex hormonal and neural glucoregulatory mechanisms ensure that the simultaneous fuel demands of different tissues will be met. Regardless of the nature of the exercise that follows, the initial period of muscular contraction is fueled by endougenously stored (local) energy substrates. Although local (intramuscular) triacylglycerol stores are used during this period, glycolysis from glycogen provides most of the needed energy. After this initial period of reliance on local stores, the intensity of subsequent exercise determines what fuels will be used. Diet also has profound influence on the selection of fuels during exercise. This is true for the athlete's habitual diet as well as what he or she consumes immediately before and during competition. In a sense the body is metabolically programmed by one's customary diet. Both carbohydrates and fats are metabolized in greater quantities when the regular daily intake of these substrates is high. This is true both at rest and during exercise. Training status is yet another important determinant of fuel utilization. Inherited and acquired physiologic needs also influence the choice of fuels during exercise. Thus the amounts of carbohydrate fuel used during exercise are determined by a complex interplay between the many previously identified variables. On balance, however, exercise duration and intensity emerge as the most important determinants thereof. Present thinking holds that performance in events of less than 1 to 1.5 hours' duration is not limited by glycogen availability; however, endurance performance is limited by muscle gyycogen stores when competition extends beyond this time. (author)