Journal Title (Medline/Pubmed accepted abbreviation): Sports Med
Page number: 773-792
doi (if applicable):
Background: The majority of clinical evidence supports carbohydrate (CHO) supplementation as an ergogenic aid. The mechanism for enhanced exercise performance is thought to be maintenance of CHO oxidation that sustains exercise intensity and delays fatigue. Because of the comprehensive amount of literature on CHO supplementation, a meta-analysis of CHO effects on exercise performance was conducted.
Hypothesis/Purpose: The purpose of this meta-analysis was to evaluate the effects of CHO supplementation on exercise performance across the published literature.
Subjects: Seventy-three randomized, cross-over studies in which CHO supplements were consumed alone or with protein before or during exercise were included in the analysis.
Experimental design: Meta-analysis
Treatments Protocol: Google Scholar was used to search for studies in adults that measured performance effects in time-to-exhaustion or in time trials with or without a preload and used CHO supplements consumed alone or with protein before or during exercise (years, 1979-2009). Publication bias was reduced by excluding studies that had a random effect vs standard error > 1.25%. Studies were also excluded if the rest intervals were > 5 minutes between preload and exercise or if the rest-to-work ratio was > 0.25, if inappropriate controls were used, or if there was an unrealistically high error and performance effect. All performance effects in time-to-exhaustion or in preloaded time trials were converted to power output in non-preloaded time trials. The percentage effect of supplements on time-trial power was calculated. The meta-analysis was performed using the mixed linear modeling procedure. Dichotomous variables were type of exercise (cycling or running), performance test (time-to-exhaustion or time trial), blinding, salt in drink, and levels of CHO and/or protein (low, moderate, and high). Continuous predictors included temperature, maximal oxygen uptake (VO2max), ingestion timing, fasting period, fraction of males, ingestion intervals, amount of supplement ingested, and exercise duration.
In this study, CHO ingestion either improved or impaired exercise performance depending on the type and amount of CHOs provided. The best single source of CHO ingested at a high rate was glucose polymers, possibly because of reduced osmolality and/or gastrointestinal distress. Although not considered in this analysis, a good regimen might be CHOs before and during exercise in many boluses with the first bolus up to 4 hours before the exercise starts. The results also suggest that fructose should be consumed only at low rates. Protein addition at moderate amounts increased exercise performance in this analysis. The trivial effect from salt addition was unexpected and could possibly be from the poor reporting in many studies leading to a crude estimate in the analysis. Extrapolation of these results to high-intensity short-duration and ultra-endurance exercise is inappropriate. Also, individual responses were not accounted for in this analysis; individual athletes may sometimes experience either no benefit or even potential adverse effects from CHO supplementation.