Journal Title (Medline/Pubmed accepted abbreviation): J Nutr
Summary of background and research design
There are numerous studies supporting recommendations for consumption of carbohydrate (CHO) to optimize endurance exercise performance; however a systematic review and meta-analysis has not yet been done. This study assessed randomized, placebo-controlled research trials including CHO ingestion not greater than 8% (to maintain proper gastric emptying) at 30-80 g/hour during exercise of ≥1 hr duration. This amount/rate of ingestion spans typical recommendations.
Fifty studies were selected from over 40,000 that were initially screened from numerous databases, including Embase, Medline, PubMed, SportDiscus, and Web of Knowledge. Studies were screened for numerous criteria, including forms of carbohydrate and placebo delivery, age (≥16 yr) and health, and excluded for any non-CHO ingestion (e.g. protein, caffeine) or pre-exercise CHO ingestion.
Studies were included if performance was measured by either time trial (for distance/work or distance/work over a time interval), submaximal exercise immediately followed by a time trial, time to exhaustion at a predetermined exercise intensity, or submaximal exercise immediately followed by time to exhaustion evaluation. Findings were stratified according to type of performance assessment.
Summary of research findings
Studies evaluated typically included young adult males, with relatively few containing exclusively female or mixed gender populations. Cycle ergometry was the most common means of assessing performance. Participants fasted for 1-21 hours prior to assessment, with half of studies fasting for ≥8 hrs.
There was a significant effect size favoring CHO in all strata of performance evaluation. In time to exhaustion trials, the weighted mean performance benefit was 15.1%. In submaximal exercise following time to exhaustion, the weighted mean performance benefit was 54.2%. In time trials, the weighted mean performance benefit was 2.0%. In submaximal exercise followed by time trial, the weighted mean performance benefit was 7.5%.
Interpretation of findings/Key practice applications
The meta-analysis of numerous studies overall supports the recommendation that CHO ingestion at 30-80 g/hour in appropriately concentrated solution (<8%) during extended endurance exercise can improve performance. The findings were similar and consistent if participants were fasted ≥8 hrs or consumed a pre-exercise meal, so the ergogenic effect of CHO is apparent even when individuals optimize endogenous CHO availability prior to exercise (e.g. glycogen loading). A strong bias towards young male athletes limits the ability to generalize results to women, or to older or untrained individuals.
The performance improvements seen in time trials (+2.0%) and submaximal exercise followed by time trial (7.5%) more closely mimic competitive endurance events, and these improvements, although apparently small, may result in favorable competitive outcomes for competitive athletes. For example, 2.7% separated 1st from 5th place over a decade in the World TT Cycling Championships.
The benefits of CHO ingestion during endurance exercise are likely due to a combination of maintaining blood glucose levels and delaying use of liver glycogen to support dropping glucose levels. This meta-analysis provides a consensus that CHO ingestion during endurance exercise is likely to improve performance.