Effect of pre-exercise carbohydrate diets with high vs low glycemic index on exercise performance: a meta-analysis

Context: Although pre-exercise consumption of a low-glycemic-index (LGI) carbohydrate meal is generally recommended, the findings regarding subsequent exercise performance are inconsistent. Objective: This meta-analytic study was conducted to determine whether a pre-exercise LGI carbohydrate meal leads to greater endurance performance than a pre-exercise high-glycemic-index (HGI) meal. Data sources: The following electronic databases were searched until April 2016: MEDLINE, SPORTDiscus, Web of Science, and Cochrane Central Register of Controlled Trials. The reference lists of selected articles were searched manually. Study selection: Randomized controlled or crossover trials comparing the effects of LGI and HGI preexercise carbohydrate meals on subsequent exercise performance of healthy participants were included. Data extraction: The Jadad scale was used to assess the methodological quality of the included studies. A fixed-effects model was used to evaluate overall and subgroup estimates. Results: In total, 15 eligible studies from 727 articles were included in this meta-analysis. All included studies were of low research quality. The synthesized effect size (d = 0.42, z = 3.40, P = 0.001) indicated that the endurance performance following an LGI meal was superior to that following an HGI meal. Subgroup analyses demonstrated that the treatment effect did not vary across outcome measures (exercise to exhaustion, time trial, and work output) or athletic status (trained or recreational participants). Conclusions: Weak evidence supports the claim that endurance performance following a pre-exercise LGI meal is superior to that following a pre-exercise HGI meal. Further high-quality research in this area is warranted.