Day to day variability in fat oxidation and the effect after only 1 day of change in diet composition

Indirect calorimetry is a common and noninvasive method to estimate rate of fat oxidation (FatOx) during exercise, and test-retest reliability should be considered when interpreting results. Diet also has an impact on FatOx. The aim of the present study was to investigate day to day variations in FatOx during moderate exercise given the same diet and 2 different isoenergetic diets. Nine healthy, moderately-trained females participated in the study. They performed 1 maximal oxygen uptake test and 4 FatOx tests. Habitual diets were recorded and repeated to assess day to day variability in FatOx. FatOx was also measured after 1 day of fat-rich (26.8% carbohydrates (CHO), 23.2% protein, 47.1% fat) and 1 day of CHO-rich diet (62.6% CHO, 20.1% protein, 12.4% fat). The reliability test revealed no differences in FatOx, respiratory exchange ratio (RER), oxygen uptake, carbon dioxide production, heart rate, blood lactate concentration, or blood glucose between the 2 habitual diet days. FatOx decreased after the CHO-rich diet compared with the habitual day 2 (from 0.42 ± 0.15 to 0.29 ± 0.13 g·min(-1), p < 0.05). No difference was found in FatOx between fat-rich diet and the 2 habitual diet days. FatOx was 31% lower (from 0.42 ± 0.14 to 0.29 ± 0.13 g·min(-1), p < 0.01) after the CHO-rich diet compared with the fat-rich diet. Using RER data to measure FatOx is a reliable method as long as the diet is strictly controlled. However, even a 1-day change in macronutrient composition will likely affect the FatOx results.