Strategic carbohydrate feeding improves performance in ketogenic trained athletes

Background & Aims: Ketogenic diets (KDs) induce significant metabolic adaptations related to exercise performance and alter the glycaemic response to carbohydrates. Research examining the efficacy of KDs on endurance performance is mixed, with sub-elite athletes showing no impairment in endurance performance following a KD intervention. Despite the increased volume of KD research, no one has explored the metabolic and performance impacts of carbohydrate re-introduction before exercise in chronically ketogenic (≥12 months) individuals. This study aimed to investigate the effects of varying carbohydrate feeding timings and dosages on performance and physiological responses in 13 recreational athletes who followed a KD for 25 ± 12 months. 

Methods: Participants visited the lab six times. The first visit determined their V˙ O_2max. The second accustomed them to test procedures. In a single-blinded crossover design, visits 3–6 tested four conditions using a Latin square design. Each condition involved two days of consuming 2.27L of either 200g of CHO or a placebo, followed by a 750 ml bolus of either 60g of CHO or a placebo 30 min before exercise. 

Results: Conditions involving a pre-exercise carbohydrate bolus significantly improved 16.1 km time trial performance compared to placebo (P < 0.05). Carbohydrate consumed in the 48 h before exercise had no impact on performance compared to placebo (P > 0.05). 

Conclusion: Our results indicate that carbohydrate ingestion enhances performance only when consumed immediately prior to exercise. This suggests that the ergogenic effects are likely mediated through central nervous system mechanisms or via the prevention of hypoglycaemia, rather than through delayed metabolic pathways. Furthermore, these findings imply that muscle and liver glycogen stores may influence performance through distinct mechanisms, warranting further investigation into their independent roles.