Effects of moderate and high glycemic index meals on metabolism and exercise performance

The purpose of this study was to determine whether pre-exercise ingestion o f meals with moderate and high glycemic indexes (GI) affects glucose availa bility during exercise and exercise performance time. Six male volunteers ( 22 +/- 1 years; 80.4 +/- 3.7 kg; Vo(2peak), 54.3 +/- 1.2 ml . kg(-1). min(- 1)) ingested 75 g of carbohydrate in the form of 2 different breakfast cere als, rolled oats (moderate GI, similar to 61; MOD-GI) or puffed rice (high GI, similar to 82; HI-GI), combined with 300 mt of water; or water alone (c ontrol). The trials were randomized,and the meals were ingested 45 minutes before the subjects performed cycling exercise (60% Vo(2peak)) to exhaustio n. Venous blood samples were drawn to measure glucose, free fatty acids (FF As), glycerol, insulin (INS), epinephrine (EPI) and norepinephrine (NE) con centrations. A muscle biopsy specimen was obtained from the vastus laterali s before the meal and immediately after exercise for glycogen determination . Before exercise, both test meals elicited significant (P < .05) hyperglyc emia and hyperinsulinemia compared with control. The glycemic response was higher (P < .05) at the start of exercise after the HI-GI meal than after t he control. During exercise, plasma glucose levels were higher (P < .05) at 60 (5.2 +/- 0.1, 4.2 +/- 0.2, and 4.6 +/- 0.1 mmol L-1) and 90 (4.8 +/- 0. 1, 4.1 +/- 0.1, and 4.3 +/- 0.1 mmol L-1) minutes after the MOD-GI meal tha n after either the HI-GI or control. Total carbohydrate oxidation was great er (P < .05) during the MOD-GI trial than in control and was directly corre lated with exercise performance time (r = .95, P < .0001). Pre-exercise pla sma FFA levels were suppressed (P < .05) 30 and 45 minutes after ingestion of the HI-GI meal and 45 minutes after the MOD-GI meal compared with contro l. At 30, 60, and 120 minutes of exercise, FFAs remained suppressed (P < .0 5) for both test meals compared with control. At exhaustion, plasma glucose , INS, FFA, glycerol, EPI, and NE levels and muscle glycogen use were not d ifferent for all trials. Exercise time was prolonged (P < .05) after the MO D-GI meal compared with control, but the HI-GI trial was not different from control (MOD-GI, 165 +/- 11; HI-GI, 141 +/- 8; control, 134 +/- 13 minutes ). Thus, in contrast to the HI-GI meal or control, the MOD-GI breakfast cer eal ingested 45 minutes before exercise enhanced performance time, maintain ed euglycemia for a longer period during exercise, and resulted in greater total carbohydrate oxidation during the exercise bout. We conclude that a M OD-GI meal provides a significant performance and metabolic advantage when consumed 45 minutes before exercise. Copyright (C) 2001 by W.B. Saunders Co mpany.