Discussion of “Body Fat Has No Effect on the Maximal Fat Oxidation Rate in Young Normal and Overweight Women”

Abstract

To the Editor: Blaize et al. (4) recently published an interesting paper dealing with the effect of body fat mass on the maximal fat oxidation rate in normal and overweight women in line with some of their previous publications (3,10). Although the paper adds some interesting results to the current literature in this field, we believe that some issues warrant discussion. First, we noticed some inconsistencies in the participants' description. In their title, the authors specified “normal and overweight women” and the 14 women were separated in 2 number-matched sub-groups (n = 7 in each group) based on a body fat cut-off point of 25%, as described in the Methods section (2). However, it seems difficult to consider women as overweight based only on this criterion, and body mass index data—the reference standard used by the World Health Organization (12) and by the Center for Disease Control and Prevention (13) to define overweight—are missing to corroborate the results. Moreover, this is reinforced by the population description indicating that all 14 women were physically active and fit (based on the V̇o2max and fat free mass values). By calculating the mean body mass index (using the mean height and weight values presented in Table 1), we reached the conclusion that all participants were within the normal weight range (i.e., body mass index < 25 kg·m−2; 21.44 kg·m−2 for the low and 23.48 kg·m−2 for the high fat group) (12). This misclassification is also underscored by the standard error presented for the 2 groups. Thus, although the 2 groups can be justified based on the body fat cut-off point (25%), it is difficult to classify them as normal or overweight women and this has important implications when interpreting the results. Second, the main result of the paper by Blaize et al. is that the maximum lipid oxidation rate during exercise was not affected by the body fat mass. The authors concluded that the intensity that elicits the highest lipid oxidation rate could be prescribed in exercise programs, whatever the women's fat mass level. Based on the current knowledge on the effect of body fat mass on metabolism, the body fat mass localization should also have been assessed and considered for the analysis and interpretation of the results (7–9). Indeed, there are some studies indicating that abdominal and peripheral adipose tissues differentially affect substrate oxidation at rest and during exercise. For instance, we previously reported that, during submaximal exercise sessions, normal weight women with high abdominal to lower body fat mass ratio had lower lipid mobilization and maximum lipid oxidation rate values than women with low abdominal to lower body fat mass ratio (7,8). Negative correlations between abdominal fat mass and maximum lipid oxidation rate and the specific metabolic and hormonal responses to exercise confirmed that high abdominal fat storage affects lipid metabolism (7,8). Thus, not only the total body fat mass, but also the adipose tissue distribution should be considered when assessing the maximum lipid oxidation rate during exercise. Although air displacement plethysmography does not allow the assessment of fat mass localization, waist and hip circumference measurement and waist to hip ratio calculation are important indicators of adiposity that could have been reported and discussed in this paper. Finally, there is a minor methodological point. Although we agree that the stage duration (3 minutes) of the experimental protocol to determine the maximum lipid oxidation rate is correct for active people (1), overweight subjects (as described by the authors) need more time to achieve V̇o2 and V̇Co2 steady state and this could have led to inaccurate fat oxidation rate calculations (5,6,11). In conclusion, the inconsistency between the title and the participants' description (body mass index, fat-free mass, V̇o2max and physical activity level required to be enrolled in the protocol) does not allow drawing rigorous conclusions. Overall, the paper by Blaize et al. provides interesting results, but also highlights the need for rigorous methodologies to ensure valid conclusions. Furthermore, the inclusion of very recent data on the effect of adipose tissue distribution on substrate metabolism appears necessary.