From Metabolism to Medals: Contemporary Perspectives and Revisiting Carbohydrate Guidelines for Fueling Endurance Athletes during Exercise

The effects of carbohydrate (CHO) intake on substrate metabolism, exercise capacity, and exercise performance have been studied for >100 y. From a metabolic perspective, the ergogenic effect of CHO intake is likely mediated by liver (and potentially muscle) glycogen sparing, maintenance of plasma glucose concentrations, and whole-body CHO oxidation rates, such that the required exercise intensity can be sustained for a longer duration thereby delaying fatigue. Accordingly, the 2016 sport nutrition guidelines from the American College of Sports Medicine recommend CHO intakes ≤90 g/h (from multiple-transportable CHOs, e.g., glucose/fructose mixtures), as targeted to exercise that is >2.5-3 h in duration. Although field observations report a trend for endurance athletes to consume (and experiment with) higher rates of CHO ingestion during training and racing (i.e., 120-200 g/h), the efficacy of such doses is not yet substantiated by current scientific research. Rather, contemporary research suggests that the upper limit of CHO intake could increase from 90 to 120 g/h (at least for trained participants), considering that both exogenous and whole-body rates of CHO oxidation can be increased with these higher ingestion rates. Such absolute doses may also modulate important physiological determinants of performance (e.g., durability and economy) across cycling, marathon running, and ultraendurance exercise. As such, the present paper provides a contemporary review of CHO metabolism during exercise, factors affecting exogenous CHO oxidation rates (i.e., CHO blend, ratio, format, environmental considerations, etc.) and sport-specific research (alongside personal author insights from practice), before presenting an updated and more nuanced model to guide CHO personalization strategies for endurance athletes. Directions for future research are also discussed, emphasizing the need for collaborative research to study both male and female athletes during ecologically valid exercise protocols that better address the real-world fueling challenges faced by elite athletes.