The effects of low- and high-glycemic index meals on time trial performance

Int J Sports Physiol Perform. 2009 Sep;4(3):331-44. doi: 10.1123/ijspp.4.3.331.

Abstract

Purpose: 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.

Methods: 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.

Results: 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).

Conclusions: 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.

Publication types

  • Comparative Study

MeSH terms

  • Adult
  • Bicycling / physiology*
  • Blood Glucose / metabolism*
  • Body Mass Index
  • Dietary Carbohydrates / administration & dosage
  • Dietary Carbohydrates / pharmacokinetics*
  • Dose-Response Relationship, Drug
  • Energy Metabolism / physiology*
  • Heart Rate / physiology
  • Humans
  • Male
  • Oxygen Consumption
  • Physical Endurance / physiology*
  • Time Factors

Substances

  • Blood Glucose
  • Dietary Carbohydrates