Carbohydrate (CHO) ingestion during exercise has long been associated with improved performance. Early Scandinavian research proposed that CHO ingestion mitigates exercise-induced hypoglycemia (EIH) through a central neural mechanism, preventing glycopenic brain damage. Subsequent studies linked muscle glycogen depletion to fatigue during prolonged exercise, suggesting an obligatory reliance on glycogen, while overlooking the simultaneous presence of profound EIH at exhaustion. However, emerging evidence challenges this paradigm highlighting EIH role in fatigue. We comprehensively review more than 100 years of evidence from more than 160 studies looking at CHO ingestion, exercise metabolism, and physical performance that demonstrates the following key findings: (1) EIH correlates strongly with exercise termination, while muscle glycogen depletion alone does not induce rigor or whole-body fatigue; (2) CHO ingestion reduces liver glycogenolysis, preserves blood glucose, and paradoxically accelerates muscle glycogen breakdown through conserved neuroendocrine mechanisms; (3) high-fat-adapted athletes demonstrate exceptional fat oxidation, equivalent exercise performance, despite lower glycogen and CHO oxidation, challenging the belief that glycogen and CHO oxidation are central to exercise performance or that CHO is an obligatory fuel; and (4) CHO ingestion during exercise significantly enhances performance, even in glycogen-depleted states, by eliminating EIH. These data demonstrate that the main benefit of CHO ingestion before or during exercise is to prevent EIH, highlighted in prolonged efforts (>2-3 hours) and individuals with insufficient hepatic gluconeogenesis. This has important implications for sports dietary recommendations (ie, habitual high- or low-CHO diets) and the amount of CHOs athletes should be encouraged to ingest during exercise to maximize performance.
Noakes et al. (Wed,) studied this question.