THE EFFECTS OF DIFFERENT AMOUNTS OF PRE-EXERCISE CARBOHYDRATE FEEDING ON METABOLISM AND CYCLING PERFORMANCE

Abstract

Studies on the effect of pre-exercise carbohydrate ingestion on metabolism and performance have produced conflicting results and this may be because of differences in the study designs. PURPOSE: The purpose of the present study was to examine the effects of different amounts of pre-exercise glucose feeding on glucose and insulin responses during cycling exercise and on time trial performance. METHODS: Nine trained male cyclists (VO2max: 64.1 ± 2.1 mlkg−1min−1) completed 4 exercise trials in random order. Forty-five min before the start of exercise subjects consumed 500 ml of a beverage containing either 0 g (PLAC), 25 g (LOW), 75 g (MED) or 200 g (HIGH) of glucose. The exercise trials consisted of 20 min of submaximal steady state exercise (SS) at 65% of maximal power output (72 ± 1%VO2max) immediately followed by a 691 ± 12 kJ time trial (TT). RESULTS: Plasma insulin concentrations at the onset of SS exercise were significantly higher (P < 0.05) in MED and HIGH compared with LOW and PLAC. Plasma glucose fell rapidly (P < 0.05) during SS exercise in all glucose trials, while plasma glucose remained stable in PLAC. No difference in plasma glucose concentration was observed between the glucose trials at any of the time points. Hypoglycemia (< 3.5 mmolL−1) was observed in 5 subjects during SS after pre-exercise glucose feeding, while none of the subjects developed hypoglycemia after PLAC. However, there was no difference in TT performance between the four trials. CONCLUSION: The ingestion of 0, 25, 75 or 200 g of glucose 45 min before a 20-min submaximal exercise bout did not affect subsequent TT performance. In addition, mild rebound hypoglycemia following pre-exercise glucose ingestion did not negatively affect TT performance.