SUBSTRATE UTILIZATION DURING EXERCISE IN ACTIVE PEOPLE

When people walk at low intensity after fasting, the energy needed is provided mostly by oxidation of plasma fatty acids. As exercise intens ity increases (eg, to moderate running), plasma fatty acid turnover do es not increase and the additional energy is obtained by utilization o f muscle glycogen, blood glucose, and intramuscular triglyceride. Furt her increases in exercise intensity are fueled mostly by increases in muscle glycogen utilization with some additional increase in blood glu cose oxidation. Muscle glycogen and blood glucose contribute equally t o carbohydrate energy production over 2-3 h of moderate-intensity exer cise; fatigue develops when these substrates are depleted. Active peop le can deplete muscle glycogen with 30-60 min of high intensity, inter mittent exercise. When the ingestion of dietary carbohydrate is optima l, it is possible to resynthesize muscle glycogen to high concentratio ns in approximate to 24 h, which is the major factor in recovery of ex ercise tolerance. However, this requires that a 70-kg person eat at le ast 50 g carbohydrate per every 2 h, beginning soon after exercise, an d ingest 500-600 g in 24 h (ie; approximate to 7-9 g/kg body wt). Carb ohydrate foods eliciting high glycemic and insulinemic responses promo te more rapid glycogen resynthesis than do foods eliciting lower glyce mic responses. Therefore, foods ingested for energy before, during, or after exercise should be classified according to their glycemic index . Although carbohydrate ingestion before and during exercise adds exog enous substrate to the body, it usually attenuates plasma fatty acid m obilization and oxidation.