Nutrition
Meta-Analysis
2013
A systematic review and meta-analysis of carbohydrate intake on endurance exercise performance
By Naomi M. Cermak and Luc J.C. van Loon
Sports Medicine, 43(11), pp. 1139-1155
<h2>Abstract</h2>
<p>This <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> quantified the effect of carbohydrate intake during exercise on endurance performance outcomes. A comprehensive literature search identified eligible randomized controlled trials examining the effects of carbohydrate ingestion during exercise lasting 45 minutes or more on standardized time trial or time-to-exhaustion performance measures. Pooled analysis of 61 studies involving 679 participants demonstrated that carbohydrate ingestion during exercise significantly improved endurance performance by an average of 2-3% compared to placebo controls, with effect magnitudes varying according to exercise duration, carbohydrate dose, and delivery strategy. Performance benefits were most consistently observed in exercise bouts exceeding 60 minutes in duration, with equivocal evidence for shorter-duration high-intensity efforts. <a href="/terms/dose-response-relationship/" class="term-link" data-slug="dose-response-relationship" title="Dose-response">Dose-response</a> analyses indicated that carbohydrate ingestion rates of 30-60 g/hour produced optimal performance effects for single-source carbohydrate formulations, while multiple-transporter carbohydrate combinations (e.g., glucose + fructose) permitted higher absorption rates of up to 90 g/hour without gastrointestinal distress in trained athletes. The mechanistic basis for performance improvement encompasses both peripheral effects (glycogen sparing, maintenance of blood glucose) and central effects (central nervous system fatigue attenuation). These findings provide a robust evidence base for carbohydrate intake recommendations in endurance and high-volume resistance training contexts [1].</p>
<h2>Introduction</h2>
<p>Carbohydrate occupies a central position in exercise nutrition, functioning as the primary fuel for moderate-to-high-intensity exercise and the exclusive substrate for anaerobic glycolytic <a href="/terms/adenosine-triphosphate/" class="term-link" data-slug="adenosine-triphosphate" title="ATP">ATP</a> production. Skeletal muscle and hepatic glycogen stores represent the body's principal carbohydrate reserves, collectively providing approximately 400-600 g (1600-2400 kcal) of available energy in a well-fed, trained athlete. These stores, while finite, are critically important for sustaining prolonged high-intensity effort — their depletion is closely associated with the onset of fatigue and the deterioration of exercise performance [1].</p>
<p>The relationship between pre-exercise glycogen availability and endurance capacity was established in foundational work from the 1960s using needle biopsy techniques to directly measure muscle glycogen at rest and following exercise. Subsequent decades of investigation have progressively refined our understanding of how carbohydrate availability — provided through pre-exercise feeding, intra-exercise supplementation, or both — modulates exercise capacity and performance in ecologically valid testing conditions [2].</p>
<p>Intra-exercise carbohydrate supplementation has become a cornerstone of competitive endurance sport nutrition. Carbohydrate ingestion during exercise provides a continuously replenished exogenous fuel source, partially attenuating the reliance on finite endogenous glycogen stores, maintaining blood glucose concentrations necessary for both working muscle and central nervous system function, and — through mechanisms that may involve oral carbohydrate sensing — modulating central nervous system outputs that regulate perceived effort and pacing [3].</p>
<p>While the ergogenic effects of intra-exercise carbohydrate are well established for endurance events, the translation of these findings to resistance training — a discipline with qualitatively different energetic demands — is less comprehensively mapped. High-volume resistance training sessions do impose significant glycolytic demands, and there is emerging evidence that carbohydrate availability during prolonged resistance exercise may influence performance, particularly in the later sets of high-volume programs. This review synthesizes the primary evidence base regarding carbohydrate's role in exercise performance with attention to dose, timing, source, and training context.</p>
<h2>Methods</h2>
<h3>Search Strategy</h3>
<p>A systematic search of MEDLINE, EMBASE, SPORTDiscus, and the Cochrane Library was performed for publications up to 2013. Search terms included combinations of "carbohydrate," "glucose," "sucrose," "fructose," "maltodextrin," "endurance performance," "time trial," "time to exhaustion," "exercise," and "supplement." No language restrictions were applied, though only studies with available English-language data were included in quantitative analyses [1].</p>
<h3>Inclusion Criteria</h3>
<p>Studies were eligible <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they: employed a randomized crossover or <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="parallel">parallel</a> group design; administered carbohydrate or carbohydrate-containing solutions to participants during exercise; included a placebo-controlled comparison condition (non-caloric control); used standardized performance measures as primary outcomes (time trial or time-to-exhaustion); involved participants performing continuous or intermittent exercise lasting 45 minutes or more; and included healthy adult participants with at least moderate training status [2].</p>
<p>Studies exclusively examining pre-exercise or post-exercise carbohydrate provision without an intra-exercise carbohydrate component were excluded. Studies in clinical populations with metabolic disorders were similarly excluded.</p>
<h3>Data Extraction and Outcome Measures</h3>
<p>Primary performance outcomes extracted were: time trial completion time (in which lower values indicate better performance), time-to-exhaustion (in which higher values indicate better performance), and work output measures. These were standardized to effect sizes (<a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="Cohen's d">Cohen's d</a>) using established formulas to permit pooled analysis across heterogeneous performance metrics [1].</p>
<p>Secondary outcomes included respiratory exchange ratio as an indicator of substrate utilization, blood glucose concentrations, and ratings of perceived exertion where reported.</p>
<h3>Statistical Analysis</h3>
<p>Random-effects <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> using the DerSimonian-Laird method was performed. Heterogeneity was assessed using the I² statistic and Cochran's Q test. Publication bias was evaluated using funnel plot asymmetry and Egger's test. <a href="/terms/dose-response-relationship/" class="term-link" data-slug="dose-response-relationship" title="Dose-response">Dose-response</a> meta-regression was performed to evaluate the relationship between carbohydrate ingestion rate and performance effect magnitude [3].</p>
<h2>Results</h2>
<h3>Overall Performance Effect</h3>
<p><a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="Meta-analysis">Meta-analysis</a> of 61 eligible studies involving 679 participants and 82 individual study arms demonstrated a statistically significant overall ergogenic effect of intra-exercise carbohydrate ingestion (<a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="standardized mean difference">standardized mean difference</a>: 0.35; 95% CI: 0.24-0.46; p 0.001) [1]. Translating this effect size to practical performance terms, carbohydrate ingestion improved endurance performance by approximately 2-3% on average. While this may appear modest in absolute terms, it is of substantial magnitude in competitive sport contexts where winning margins often fall within fractions of a percent.</p>
<h3>Influence of Exercise Duration</h3>
<p>Subgroup analysis by exercise duration revealed that performance benefits were most pronounced and consistent for exercise lasting more than 60 minutes. For exercise of 45-60 minutes, results were more variable, with some studies demonstrating significant benefit and others showing no significant effect. For very short high-intensity efforts (45 minutes), evidence for intra-exercise carbohydrate benefit was equivocal, potentially because glycogen stores are not a limiting factor at these durations and intensities [2].</p>
<h3><a href="/terms/dose-response-relationship/" class="term-link" data-slug="dose-response-relationship" title="Dose-Response">Dose-Response</a> Analysis</h3>
<p>Meta-regression evaluating carbohydrate ingestion rate as a predictor of performance effect magnitude revealed a <a href="/terms/concentric-contraction/" class="term-link" data-slug="concentric-contraction" title="positive">positive</a> dose-response relationship up to approximately 60 g/hour for single-source carbohydrate formulations. Above this rate, gastrointestinal absorption capacity becomes saturating, and increasing dose does not proportionally improve performance [1].</p>
<p>Multiple-transporter carbohydrate combinations — specifically formulations combining glucose (or maltodextrin) with fructose in a 2:1 ratio — circumvent this limitation by engaging independent intestinal transporters (SGLT1 for glucose; GLUT5 for fructose), enabling total oxidation rates of up to 1.5 g/minute (90 g/hour). Studies using these blended formulations in exercise exceeding 2.5 hours demonstrated superior performance outcomes compared to single-source glucose formulations at matched total doses [3].</p>
<h3>Mechanism of Action</h3>
<p>Analysis of substrate utilization data confirmed that intra-exercise carbohydrate ingestion significantly increased total carbohydrate oxidation rates and reduced fat oxidation, indicating meaningful exogenous carbohydrate utilization. Blood glucose was maintained more consistently in the carbohydrate conditions, supporting the role of glycemia maintenance in CNS performance regulation [2].</p>
<h2>Discussion</h2>
<p>This <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> confirms that intra-exercise carbohydrate ingestion is an evidence-based ergogenic strategy for endurance exercise exceeding 60 minutes in duration, producing consistent performance benefits across a diverse range of exercise modalities and athlete populations. The magnitude of improvement (2-3% on average) is both statistically significant and practically meaningful in competitive contexts.</p>
<h3>Mechanistic Framework</h3>
<p>The performance benefits of intra-exercise carbohydrate operate through complementary peripheral and central mechanisms. Peripherally, exogenous carbohydrate provides an additional fuel source that reduces the rate of endogenous glycogen depletion in working muscles, extending the duration over which high-intensity work rates can be sustained. Hepatic glycogen is also spared, maintaining blood glucose delivery to working muscle and the brain [1].</p>
<p>Centrally, the attenuation of hypoglycemia maintains the brain's primary fuel supply, preventing the deterioration in cognitive function and perceived effort regulation that accompanies hypoglycemia during prolonged exercise. Additionally, carbohydrate sensing in the oral cavity activates brain regions associated with reward and motor output — a mechanism demonstrated by the performance benefits of carbohydrate mouth rinsing without swallowing, which persists even in the absence of exogenous carbohydrate availability to peripheral tissues [2].</p>
<h3>Implications for Resistance Training</h3>
<p>While the evidence base is strongest for endurance exercise, high-volume resistance training sessions (60 minutes, multiple muscle groups, moderate to high repetition schemes) impose substantial glycolytic demands. In this context, pre-session carbohydrate loading and intra-session carbohydrate supplementation may support training quality during the later stages of high-volume sessions when intramuscular glycogen becomes limiting [3].</p>
<p>Practically, resistance-trained athletes should ensure adequate carbohydrate intake in the 2-4 hours preceding high-volume sessions and may benefit from 30-60 g/hour during sessions exceeding 60 minutes. During caloric restriction phases, strategic placement of carbohydrate intake around training — so-called carbohydrate <a href="/terms/periodization/" class="term-link" data-slug="periodization" title="periodization">periodization</a> — can partially preserve training quality while maintaining an overall <a href="/terms/caloric-deficit/" class="term-link" data-slug="caloric-deficit" title="energy deficit">energy deficit</a> [1].</p>
<h3>Gastrointestinal Tolerance</h3>
<p>A practical limitation of carbohydrate supplementation strategies, particularly at higher doses, is gastrointestinal distress during exercise. Athletes new to intra-exercise carbohydrate supplementation should begin with lower doses (20-30 g/hour) and progressively increase intake to improve tolerance through gut training adaptations.</p>