Pre-Exercise Carbohydrate Intake and Reactive Low Glucose Concentrations in Athletes: Mechanisms, Individual Susceptibility and Practical Implications – A Narrative Review

Abstract

Carbohydrate intake before endurance exercise can cause a transient decline in blood glucose at exercise onset. Its clinical and functional significance is less clear. This narrative review examines the underlying mechanisms, individual responses, and implications for nutrition and glucose monitoring. English-language literature identified through PubMed and Google Scholar searches in September 2026 was synthesised thematically. Experimental studies were considered alongside reviews, position statements, and methodological studies. Pre-exercise carbohydrate intake increases glucose and insulin concentrations. Muscle contraction adds a stimulus for glucose uptake, so disposal may temporarily exceed appearance. The resulting decline varies between individuals and nutritional conditions. Pre-exercise insulin concentrations and exercise modality do not consistently identify those who will develop low glucose. Evidence that susceptibility is reproducible under identical conditions remains limited. Transient low glucose is not consistently accompanied by symptoms, increased perceived exertion, or impaired performance. However, incomplete outcome assessment limits conclusions about its consequences. Continuous glucose monitoring adds uncertainty because interstitial readings may differ from concurrent blood glucose. Interpretation should therefore integrate the glucose trajectory, episode duration, symptoms, performance, nutritional context, and measurement method. An early decline alone does not justify routinely avoiding pre-exercise carbohydrate. Strategies should meet exercise demands and accommodate individual tolerance. Repeated standardised studies are needed to establish which responses recur and affect performance.