EFFECT OF FLUID INGESTION ON MUSCLE METABOLISM DURING PROLONGED EXERCISE

Five trained men were studied during 2 h of cycling exercise at 67% pe ak oxygen uptake at 20-22 degrees C to examine the effect of fluid ing estion on muscle metabolism. On one occasion, the subjects completed t his exercise without fluid ingestion (NF) while on the other they inge sted a volume of distilled deionized water that prevented loss of body mass (FR). No differences in oxygen uptake during exercise were obser ved between the two trials. Heart rate was lower (P < 0.01) throughout exercise when fluid was ingested, and rectal temperature after 2 h of exercise was lower (38.0 +/- 0.2 and 38.6 +/- 0.2 degrees C for FR an d NF, respectively; P < 0.01), as was muscle (vastus lateralis) temper ature (38.5 +/- 0.4 and 39.1 +/- 0.5 degrees C for FR and NF, respecti vely; P < 0.05). Resting muscle ATP, creatine phosphate, creatine, gly cogen, and lactate levels were similar in the two trials, as were the postexercise ATP, creatine phosphate, and creatine levels. In contrast , muscle glycogen was higher (P < 0.05) and muscle lactate was lower ( P < 0.05) after 2 h of exercise in FR compared with NF. Net muscle gly cogen utilization during exercise was reduced by 16% when fluid was in gested (318 +/- 46 and 380 +/- 53 mmol/kg dry weight for FR and NF, re spectively; P < 0.05). These results indicate that fluid ingestion red uces muscle glycogen use during prolonged exercise, which may account, in part, for the improved performance previously observed with fluid ingestion.