Discussion
The main result of this study was that H-A/LB NW women exhibited lower MLOR and Lipoxmax compared with L-A/LB NW women. Previously our laboratory had reported that H-A/LB NW women exhibited lower lipid oxidation rates and less metabolic flexibility during prolonged moderate-intensity exercise compared with L-A/LB women [4]. Thus, the present findings are complementary to our initial results and underline the fact that adipose tissue distribution in NW women alters MLOR during exercise.
As individuals who can oxidize more lipids during exercise are those who are better at losing weight [10], it may be hypothesized from our study that NW women who tend to store fat in the abdominal region have a limited ability to reach high fat oxidation rates during exercise, and this may increase their risk of future excess FM accumulation, insulin resistance and dyslipidaemia.
Our results are relevant in the field of energy metabolism as no study has previously investigated the impact of FM ratio on Lipoxmax and MLOR. A previous study reported differences in lipid oxidation rates after low-intensity training between upper-body obesity and lower-body obesity in women [11]. Lipoxmax and MLOR disparities between the two groups may partly explain these discrepancies. Furthermore, our present results suggest that FM distribution is a major determinant of gender differences in lipid oxidation during exercise [12].
Excess abdominal FM stimulates chronic lipolytic activity with lower glucose oxidation under resting conditions, thereby leading to insulin resistance [13] and [14]. Conversely, a greater ability to oxidize lipids during exercise is protective against metabolic disturbances. In our present study, the H-A/LB women displayed reduced flexibility of maximum fat utilization, which predisposes to potential metabolic disorders [15]. When our two groups were analyzed together, correlations between total or abdominal FM and Lipoxmax or MLOR further highlighted the adverse impact of abdominal FM on maximum lipid oxidation in NW subjects. Lower Lipoxmax and MLOR are characteristic of an unhealthy metabolic phenotype. Thus, NW women with a preferentially abdominal fat distribution may be at greater risk of the metabolic syndrome compared with NW women with a preferentially peripheral FM distribution [16].
Several hypotheses may explain our results. The decreases in glycerol, FFA, growth hormone and atrial natriuretic peptide (ANP) plasma levels, and increases in glycaemia and insulinaemia, observed in our previous study [4] may partly explain both the reduced lipid oxidation rate during prolonged submaximum exercise and lower Lipoxmax and MLOR in H-A/LB women. This is corroborated by the positive correlations observed between lipid oxidation rate during submaximum exercise and FFA, glycerol, noradrenaline and ANP concentrations [4].
Raynaud et al. [17] reported that moderately overweight women with predominantly lower-body FM exhibited greater insulin sensitivity than those with predominantly trunk fat. Lipid accumulation is known to increase both insulin resistance and lipotoxicity. Thus, a greater ability to maximize the rate of lipid oxidation with lower CHO oxidation during exercise may be associated with greater insulin sensitivity.
From a methodological point of view, Lipoxmax presents some limitations, particularly when used for prescribing exercise-training programmes based on intraindividual variability. Also, training at Lipoxmax maximizes lipid oxidation during exercise but, conversely, limits glucose oxidation. It is well known that dyslipidaemia is involved in insulin resistance and, thus, maximizing lipid oxidation during exercise could act against these metabolic features.
To achieve a rate of fat oxidation similar to those measured during the exercise test, subjects should train in the fasted state, which may be a barrier in sedentary subjects. For this reason, the Lipoxmax may be considered an interesting diagnostic tool that may be used for planning exercise-training programmes under well-standardized conditions.
In conclusion, the present study indicates that FM distribution alters Lipoxmax and MLOR in NW women, with lower values observed in H-A/LB subjects. However, whether this reduced ability to maximize lipid oxidation during exercise in NW women with a predominantly abdominal FM distribution increases the risks for further obesity and the development of the metabolic syndrome requires further investigation.