Glucose ingestion before and during intense exercise

Glucose and insulin responses were compared in glycogen depleted subjects when glucose (1.5 g/kg) was ingested 15 min before (n = 7) or during (min 3-5) intense exercise (80% VO2 max; n = 8). A nonexercise group (n = 8) and a no-glucose group (n = 8) were also included for comparisons. A 36- to 44-h fast, combined with exhaustive exercise to deplete muscle glycogen, (congruent to 80% in pilot studies) suggested that the subjects initiated exercise with substantially depleted hepatic and muscle glycogen reserves. With no glucose ingestion, blood glucose decreased during exercise (P less than 0.05) and blood lactate (HLa congruent to 3.8 mM) and the respiratory exchange ratio (R) remained low (0.83); with glucose ingestion before or during exercise, HLa concentrations were doubled (7.3 mM) and R was greater (0.90-0.92; P less than 0.05). Although insulin concentrations decreased rapidly to basal levels within 10 min after the onset of exercise in the preexercise glucose group (P less than 0.05), blood glucose continued to decrease throughout exercise. No such decrease occurred in the subjects who ingested glucose during exercise, nor did insulin concentrations change markedly in this group (P greater than 0.05). The HLa and R data indicated that a considerable portion of glucose was metabolized during exercise. Differences in the preexercise insulin environment appear to exert a persistent effect on glucose uptake throughout exercise.