When it comes to endurance events, the differences in performance between men and women becomes smaller and smaller, the longer the event. In fact, the last few years we have seen run splits for the women at Ironman Hawaii, that are at least as fast as the men’s! It is often hypothesised that this is because of differences in metabolism between men and women. It has been suggested that women are actually better at metabolising fat and can therefore sustain exercise for longer. Women would thus be better suited for very prolonged endurance exercise. This is an interesting theory, but are women really better at burning fat than men?
In the literature we find a number of studies comparing metabolism in men and women, but these studies are notoriously difficult to interpret because there are many ways you can make such comparisons.
The design of a study will have a major impact on the conclusions. For example, should we compare men and women exercising at the same absolute intensity (pace) or at the same relative intensity (effort). Should we compare fat metabolism expressed in grams per minute or should this be scaled to muscle mass? Women are generally lighter than men, so should be scale to body weight? Women also have a different body composition, so should we express our data per kg fat free mass (FFM)? And since fat oxidation changes with training how can we make sure that the men and women we compare, are trained in exactly the same way? These crucial study design questions have a major effect on the outcome of the studies and it is therefore perhaps not surprising that the conclusions have not been unanimous.
Although some studies report no differences between men and women, the majority of studies seems to conclude that fat oxidation is higher in women. We performed a study in 300 men and women (approximately 50:50) and measured fat oxidation over a wide range of intensities (1). Because we were interested in the intrinsic differences in the muscle in the ability to oxidize fat, we compared fat oxidation over a wide range of exercise intensities (from very low to high) in men and women. We expressed fat oxidation per kg fat free mass.
The results are shown in the infographic. Women were slightly better at oxidizing fat than men. Their peak fat oxidation was just over 10% higher and this happened at a higher exercise intensity. In other words, women had a right and upwards shift of the fat oxidation curve compared with men. This is the same change we would expect in a person who is better trained. So based on these findings we would have to conclude that women are better at burning fat than men, and might be more suitable for very prolonged endurance exercise where fat is the main fuel.
However, although this may be true, there are other considerations as well. In absolute terms men are actually better at burning fat because they have more muscle and higher energy expenditures.
The theory is that women would burn more fat and spare more of the small, but essential, carbohydrate stores. This would prevent "hitting the wall" in the last part of a race! Maybe this is true, but direct evidence is lacking.
Finally, it is important to note that the differences in the studies are actually quite small and maybe the practical relevance is not as significant as often suggested. For example, the difference in the figure here is 0.66 milligrams of fat per kilogram fat free mass per minute. For a 55kg (121 lbs) female athlete with 12% body fat this means that she would burn 42 mg more fat per minute than her male counterpart.
Per hour that would mean 2.5 grams! Or 22 kcal: the amount of energy we can find in 10 peanuts, one cup of sports drink, a quarter of a gel, a 5th of a banana or half an Oreo cookie? Would that make a difference? We will not know the answer until more research is done in female athletes!
Until then, it will remain fascinating to see how women and men battle for positions in the world’s longest endurance races!
Venables MC, Achten J, Jeukendrup AE. Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. J Appl Physiol (1985). 2005 Jan;98(1):160-7.