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

To determine whether eating a breakfast cereal with a moderate glycemic index could alter substrate utilization and improve exercise duration.

Six active women (age, 24 +/- 2 yr; weight, 62.2 +/- 2.6 kg; VO(2peak), 46.6 +/- 3.8 mL x kg(-1) x min(-1)) ate 75 g of available carbohydrate in the form of regular whole grain rolled oats (RO) mixed with 300 mL of water or water alone (CON). The trials were performed in random order and the meal or water was ingested 45 min before performing cycling exercise to exhaustion (60% of VO(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 immediately after exercise for glycogen determination. Glucose kinetics (Ra) were determined using a [6,6-(2)H] glucose tracer.

Compared with CON, plasma 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 also higher (P < 0.05) for the first 120 min of exercise for the RO trial. At exhaustion, glucose, insulin, FFA, glycerol, EPI, NE, RER, and muscle glycogen 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 x kg(-1) x min(-1), respectively). Exercise duration was 5% longer during RO, but the mean times were not significantly different (253.6 +/- 6 and 242.0 +/- 15 min, respectively).

Increased hepatic glucose output before fatigue provides some evidence of glucose sparing after the breakfast cereal trial. However, exercise duration was not significantly altered, possibly because of the sustained suppression of lipid metabolism and increased carbohydrate utilization throughout much of the exercise period.