Does a low-carbohydrate diet impede endurance sports performance? Yes

The complex determinants of endurance sports performance vary according to exercise demands, the specific event, and the individual athlete. A key issue is the availability and integration of the oxidative use of muscle fat and carbohydrate (CHO) stores to supply energy. Current competition nutrition guidelines promote "high CHO availability," using personalized strategies pre-, during, or between events to match glycogen and blood glucose supplies to the specific event demands. Alternatively, adaptation to a ketogenic low CHO high-fat (LCHF) diet makes fat the principal fuel, but impairs performance in some scenarios. Indeed, at oxidative thresholds, important in high-performance sport, metabolic pathways of fat oxidation provide a lower energy yield, leading to lower power/speed. Moreover, the consumption of CHO during exercise in keto-adapted athletes often enhances endurance capacity or performance. Therefore, the LCHF diet alone is suboptimal for endurance performance, as is as a one-size-fits-all approach to sports nutrition.