How switching to fat burning and then restoring carbs affects metabolism and performance during long cycling

Five days of a high-fat diet led to a significant reduction in respiratory exchange ratio during cycling.

• The respiratory exchange ratio (RER) was reduced from 0.90 +/- 0.01 on day 1 to 0.82 +/- 0.01 on day 6 for the high-fat diet, and partially restored to 0.87 +/- 0.01 on day 7 after consuming high carbohydrates.
• Estimated fat oxidation increased from 61 +/- 5 g on a high-carbohydrate diet to 94 +/- 6 g on a high-fat diet.
• Carbohydrate oxidation decreased from 342 +/- 14 g on a high-carbohydrate diet to 271 +/- 16 g on a high-fat diet.
• Muscle glycogen utilization showed a trend towards sparing, with values of 260 +/- 26 mmol/kg dry wt for the high-fat diet compared to 360 +/- 43 mmol/kg dry wt for the high-carbohydrate diet (P = 0.06).
• Time trial performance was similar between diets, with times of 30.73 +/- 1.12 min for the high-fat diet and 34.17 +/- 2.48 min for the high-carbohydrate diet (P = 0.21).

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