Fuel substrate kinetics of carbohydrate loading differs from that of carbohydrate ingestion during prolonged exercise

This study compared fuel substrate kinetics in trained cyclists who ingested a 10% carbohydrate (CHO) drink without prior CHO-loading ([NLC] n = 9) with those in cyclists who ingested a water placebo after CHO-loading ([CLP] n = 7) during 180 minutes of cycling at 70% maximum oxygen consumption ((V over dot O-2 max). Muscle glycogen at the start of exercise was 194 +/- 4 and 124 +/- 8 mmol/kg wet weight (mean +/- SEM) in CLP and NLC subjects, respectively. Total CHO oxidation was similar. Total rate of appearance of glucose from endogenous (Ra-end) and exogenous (Ra-exog) origin and plasma glucose oxidation increased significantly (P < .05), with NLC subjects ending significantly higher than CLP subjects (104 +/- 17 v 79 +/- 9 and 115 +/- 16 v 74 +/- 11 mu mol/min/kg fat-free mass [FFM], respectively). However, Ra-end was lower (P <.05) in NLC than in CLP subjects (40 +/- 10 v 79 +/- 9 mu mol/min/kg FFM), as was endogenous plasma glucose oxidation (42 +/- 13 v 75 +/- 11 mu mol/min/kg FFM). Muscle glycogen disappearance was identical in the first hour, but declined thereafter in NLC subjects. Two NLC subjects with the lowest muscle glycogen content were unable to complete the trial despite CHO ingestion. We conclude that with respect to the groups studied (1) CHO loading before exercise reduces the relative contribution of plasma glucose oxidation to total CHO oxidation, but may prolong time to exhaustion as a function of higher muscle glycogen concentration; (2) CHO ingestion has a liver glycogen-sparing effect, causes a reduction in gluconeogenesis, or both, that should delay the onset of hypoglycemia; (3) the progressive increase in plasma glucose oxidation that occurs during prolonged exercise is related to muscle glycogen status and occurs irrespective of whether CHO is ingested: and (4) the effects of CHO ingestion and CHO-loading on fuel substrate kinetics are different. (C) 1996 by W.B. Saunders Company