The purpose of this study was to examine how fat mass affects the maximal fat oxidation rates of women. Fourteen active, healthy women (age, 21–31 years) with body composition ranging from 18.6 to 30.0% fat were divided into 2 groups (15–24.9% = lower-fat group; 25–35% = higher-fat group). On day 1, subjects performed a graded exercise test on the treadmill to determine maximal oxygen consumption (V[Combining Dot Above]O2max). On day 2, subjects were measured for % fat and performed a maximal fat oxidation test. Fat and carbohydrate oxidation rates were determined using gas exchange analysis. Fat oxidation in absolute (in gram per minute) and relative to fat-free body mass (in milligram per kilogram of fat-free mass per minute) was determined using stoichiometric equations and appropriate energy equivalents. There were no significant differences (p > 0.05) in maximal fat oxidation rates between the women in lower-fat (0.39 ± 0.10 g·min−1, 8.52 ± 2.69 mg·kg−1 FFM·min−1) and higher-fat (0.49 ± 0.13 g·min−1, 10.81 ± 2.80 mg·kg−1FFM·min−1) groups. Maximal fat oxidation occurred at an exercise intensity of 55.7 ± 11.1% and 59.1 ± 5.4% V[Combining Dot Above]O2max for the lower-fat and higher-fat groups, respectively, with no significant difference between groups (p > 0.05). The maximal fat oxidation rate (g·min−1 and mg·kg−1 FFM·min−1) was not significantly correlated with any of the descriptive variables (fat mass, fat-free mass, percent body fat, or V[Combining Dot Above]O2max). In conclusion, personal trainers and health practitioners can use the exercise intensities that elicited the highest rate of fat oxidation to prescribe exercise programs to women, despite their body composition, that prevent weight gain and/or promote body fat and body weight loss.
Alex’s Notes: Have you ever heard of the “fat-burning” zone when doing cardio? Well that’s not what the focus of our discussion, but it does serve as a good ice breaker for fuel use during exercise. You see, during exercise you are always burning both carbohydrates and fats, and the maximal rate of fat oxidation is the highest observed use of fat as an energy source that you can obtain during aerobic activities. From a biochemical standpoint, it would make sense that having more fat mass (FM) allows for increased fasting free fatty acid levels in the blood and increased uptake by the muscles, promoting higher rates of fat oxidation. If this were true, then we would expect overweight people to burn more fat during exercise than their leaner counterparts.
So the study at hand recruited 14 healthy young women aged 21-31 years who were all physically active as defined by participating in at least 3 aerobic exercise training sessions per week for at least two months prior to the study. These women were divided into two groups: the “higher-fat” group included women over 25% body fat and had an average of 28%, and the “lower-fat” group had below 25% body fat with an average of 22%. It should also be noted that the lean-body-mass and aerobic capacity of the two groups were identical. Both groups completed two graded exercise tests following a 4-hour fast to determine maximum substrate oxidation (the tests were separated by 48 hours).
And there was no significant difference in maximal fat oxidation rates between the groups, and both had maximal fat oxidation occur between 55-60% VO2Max. Moreover, there was no correlation between maximal fat oxidation or any anthropometric variables, although fat-free mas did correlate significantly with the relative rate of maximal carbohydrate oxidation.
The above findings are definitely counter-intuitive. Increased substrate supply increases oxidation; this is a biochemical reality. In fact, a different study by Schutz et al. investigated obese women maintaining stable weight and obese women losing weight and demonstrated that resting fat oxidation was positively correlated with fat mass, which led to the conclusion that fat mass significantly affected fat oxidation. Moreover, when comparing the obese women to themselves after weight-loss, it was found that fat oxidation fell 42%, demonstrating that there is indeed a relationship between fat mass and fat oxidation. So what gives?
Well, if you didn’t skim over the second paragraph you should recall that even our current study’s “higher-fat” women were barely overweight. It could simply be that these women were not fat enough to significantly affect free fatty acid substrate availability. Alternatively, given that both groups had similar lean-body-mass, it could also be that the metabolically active muscle tissue had similar uptake and usage of fatty acids in both groups.
Overall, the participants being all women, nonobese, and physically fit do serve as limitations on the study at hand. Then again it is nice to see this population getting more research attention. So the takeaway from this study is basically that body fat does not affect the maximal fat oxidations rate during exercise, nor does it affect the intensity required to elicit the greatest fat oxidation rate, at least in young, healthy, and fit women.