[lionfish90]

I thought this paper might be of interest. It looked at the effect on measured RER (fat vs. carb oxidation contribution to work) of a change from the current habitual diet. In other words, they had a group of moderately trained women already on a moderately high fat diet do 1 day of a high fat or a high carb diet and then looked at the effect on fat oxidation during moderate exercise. The bottom line was that one day of carb-rich food intake can significantly change the fat utilization of the muscles (by 30%!), so don’t change your diet right before having your lab metabolic testing done. That is already the message from articles here on the site, but I thought this might be interesting anyway, to put some numbers to that.

Citation and link:
Applied Physiology, Nutrition, and Metabolism, 2016, 41(4): 397-404
http://www.nrcresearchpress.com/doi/abs/10.1139/apnm-2015-0334#.Ww3-lUgvzt8

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

Abstract:
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.

From the paper’s Discussion section:
…
We found no difference in FatOx between high-fat diet days and habitual diet days. The most obvious reason is that the participants’ habitual diets already consisted of a high relative proportion of fat (?42%TEI [total energy intake] compared with ?47%TEI in high-fat diet). Another reason may be the short duration of only 1 day of diet manipulation, since adaptations to a high-fat diet takes longer than the almost immediate response to a high-CHO diet (Jeukendrup 2003). Similar to our study, Guimaraes Couto et al. (2014) found no changes in RER during exercise between high-fat diet and habitual diet. Interestingly, this occurred although there was a higher difference in fat content between fat-rich diet days and habitual diets than in the present study, and despite that their habitual diets contained a higher proportion of CHO than in the present study.
This study has demonstrated the importance of dietary macronutrient composition on FatOx during exercise, and that shifts in substrate utilization during aerobic exercise may occur after only 1 day of change in diet composition.
Conclusion: There were no significant day to day variations in FatOx between 2 habitual diet days with equal macronutrient composition. A low-fat–high-CHO diet led to a 31% reduction in FatOx during moderate exercise compared with high-fat–low-CHO diet and habitual diet.

Best,
Rene’

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