The Effects of Low– and High–Glycemic Index Meals on Time Trial Performance

Abstract

The aim of this work was to determine whether the consumption of pre-exercise high- or low-glycemic index (GI) meals has a beneficial effect on time trial performance. Eight male cyclists were provided with either a high-GI or low-GI meal, providing 1 g x kg(-1) body mass of carbohydrate, 45 min before performing a 40-km time trial on a Velotron cyclePro. Time trial performance was significantly improved in the low-GI trial (92.5 +/- 5.2 min) compared with the high-GI trial (95.6 +/- 6.0 min) (P = .009). Blood glucose concentrations at the point of exhaustion were significantly higher in the low-GI trial (5.2 +/- 0.6 mmol x L(-1)) compared with the high-GI trial (4.7 +/- 0.7 mmol x L(-1)) (P = .001). There was no significant difference in estimated carbohydrate oxidation data between the low-GI (2.51 +/- 1.74 g x min(-1)) and high-GI (2.18 +/- 1.53 g x min(-1)) meals (P = .195). No significant difference in estimated fat oxidation was observed between the low-GI (0.15 +/- 0.15 g x min(-1)) and high-GI (0.29 +/- 0.18 g x min(-1)) diets (P = .83). The improvement in time trial performance for the low-GI trial may be associated with an increased availability of glucose to the working muscles, contributing additional carbohydrate for oxidation and possibly sparing limited muscle and liver glycogen stores.