On July 24, 2025, the BSI Medical Society Journal Club, hosted by Bob Kaplan with contributions from Pete Shaw, discussed the paper High-Fat Diet Improves Metabolic Flexibility During Progressive Exercise to Exhaustion (VO2max Testing) and During 5 km Running Time Trials. They reviewed how metabolic flexibility—the ability to switch between fat and carbohydrate as fuel—relates to performance and health. While traditional views place carbohydrates as the dominant fuel for high-intensity exercise, newer evidence shows that fat oxidation can be sustained at surprisingly high rates under low-carbohydrate, high-fat (LCHF) dietary conditions. The discussion drew connections to earlier research, particularly the “crossover” concept, and examined how measurement tools like respiratory quotient (RQ) and substrate oxidation calculations shape interpretations of fuel use.

The meeting emphasized that athletes on an LCHF diet demonstrated higher fat oxidation during both progressive exercise and 5K trials, though overall performance (VO2max and average pace) was not consistently superior to a high-carbohydrate, low-fat diet. This suggests that metabolic flexibility itself—rather than reliance on a single fuel—may be key for endurance and health. They also noted the practical implications for training and fueling strategies, including “train-low, compete-high” approaches that aim to optimize fat oxidation capacity while still leveraging carbohydrates for peak performance. Ultimately, they concluded that individualized nutrition strategies, grounded in metabolic flexibility, may offer the best path forward for both athletic output and long-term health.

A 10-minute summary of the webinar is available here free, for anyone, while the full video is available for Medical Society Members and MetFix affiliates in their dashboard.

https://brokenscience.org/high-fat-diets-and-metabolic-flexibility-summary