Effects of a moderate glycemic meal on exercise duration and substrate utilization

Purpose: To determine whether eating a breakfast cereal with a moderate gly cemic index could alter substrate utilization and improve exercise duration . Methods: Six active women (age, 24 +/- 2 yr; weight, 62.2 +/- 2.6 kg; (V) over dotO(2peak), 46.6 +/- 3.8 mL.kg(-1).min(-1)) ate 75 g of available ca rbohydrate in the form of regular whole grain rolled oats (RO) mixed with 3 00 mL of water or water alone (CON). The trials were performed in random or der and the meal or water was ingested 45 min before performing cycling exe rcise to exhaustion (60% Of (V) over dotO(2peak)). Blood samples were drawn for glucose, glucose kinetics, free, fatty acids (FFA), glycerol, insulin, epinephrine (EPI), and norepinephrine (NE) determination. A muscle biopsy was obtained from the vastus lateralis muscle before the trial and immediat ely after exercise for glycogen determination. Glucose kinetics (Ra) were d etermined using a [6,6-H-2] glucose tracer. Results: Compared with CON, pla sma FFA and glycerol levels were suppressed (P < 0.05) during the first 120 min of exercise for the RO trial. Respiratory exchange ratios (RER) were a lso higher (P < 0.05) for the first 120 min of exercise for the RO trial. A t exhaustion, glucose, insulin, FFA, glycerol, EPI, NE, RER, and muscle gly cogen were not different between trials. Glucose Ra was greater (P < 0.05) during the RO trial compared with CON (2.36 +/- 0.22 and 1.92 +/- 0.27 mg.k g.(-1)min(-1), respectively). Exercise duration was 5% longer during RO, bu t the mean times were not significantly different (253.6 +/- 6 and 242.0 +/ - 15 min, respectively). Conclusions: Increased hepatic glucose output befo re fatigue provides some evidence of glucose sparing after the breakfast ce real trial. However, exercise duration was not significantly altered, possi bly because of the sustained suppression of lipid metabolism and increased carbohydrate utilization throughout much of the exercise period.