High dietary fat intake increases fat oxidation and reduces skeletal muscle mitochondrial respiration in trained humans

What is it about?

High-fat, low carbohydrate diets have increased in popularity due to whole body and skeletal muscle adaptations which lead to increased utilization of fat during submaximal exercise. However, a consequence of this increased fat utilization is a reduction in the ability to utilize carbohydrates as a fuel source; a robust adaptation which persists even following 24 hours of high carbohydrate intake. As athletes typically require the use of both carbohydrate and fat-based fuels during training and competition, the current study aimed to determine whether high fat diet-induced adaptations are driven by the increased fat or low carbohydrate availability. Trained cyclists were fed either a high-fat or a high-protein diet for five days with carbohydrate clamped at less than 20% of total daily energy intake. This was followed by 24 hours of high-carbohydrate intake. We report that the high rates of fat utilization are primarily driven by increased dietary fat and that a high-fat diet reduces skeletal muscle mitochondria's ability to convert stored nutrients into energy. A 24 hour period of high carbohydrate intake was able to rescue mitochondrial fuel utilization, but was unable to fully rescue the ability of the whole body to utilize carbohydrate as a fuel source. Together this study has broad significance for human health and athletic performance by providing new insights into the mechanisms driving metabolic adaptations in response to high-fat, low carbohydrate diets.

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