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The weaker sex? – Part I
I would like to thank Molly Meehan for suggesting this topic. I will likely follow this particular column with at least one more on the topic.
There are readily apparent differences in body composition, muscle mass and performance between the sexes. In only one Olympic sport do men and women compete on equal footing – equestrian. Title IX not only paved the way for equal participation in sport (at least more equal than it was), but also ushered in an age where physiologists were forced to consider more than just the standard 70 kg male (which is also a joke because these days because by today’s standards that is very skinny). Why are there these differences between men and women? Some of it comes down to genetics, the difference of having XX versus XY. However, a lot comes down to having differing amounts of sex hormones – estrogen/progesterone versus testosterone. When studying physiological differences between men and women, there are several different approaches, which basically come down to manipulating hormone concentrations in women (since by tradition, males were the “norm”) to see how manipulations of the hormones affect physiology.
Over the years as my research has evolved, I have taken a particular interest in the differences between males and females. Early in my research, we performed studies on how changes in female sex hormones change substrate (fat versus carbohydrate) use. Later, I was interested in why there are differences in muscle mass between the sexes. These days I am interested in why women live longer than men. Below, I will touch on the first of these topics – differences between the use of fat and carbohydrate between men and women.
Men and women have different body composition with women in general having a higher percentage of body fat. Of course this has evolutionary reasons in that a female has to carry a baby to term and has to simultaneously feed herself and a developing fetus. We were involved in a series of studies to determine whether the fat and carbohydrate used for energy were different in women versus men. Since we suspected that female sex hormones would be behind any changes, we then looked at whether substrate use changed in women because of the menstrual cycle (hormones low or high) or as a result of being on oral contraceptives (periods of almost no hormones and very high hormones). From these studies, it was concluded that women use more fat than men at an equal exercise intensity. However, changes in hormones (either endogenous from the menstrual cycle or exogenous from oral contraceptives) had very little effect on substrate use and could be overridden by other factors such as the intensity of the exercise itself. If one was to rank what factors had the most effect on substrate use during exercise in women, it would rank as exercise intensity > oral contraceptives > recent carbohydrate nutrition, and stage of the menstrual cycle. Therefore, exercise intensity was the most important while menstrual cycle phase had the least effect.
From these data, a couple of interesting questions arise. First and foremost, if women use more fat during exercise, why is it seemingly more difficult for women to lose fat mass? Well after I was involved with this research, a very bright graduate student answered this question. Remember that moderate to high intensity exercise is primarily sustained by carbohydrate, not fat. Therefore, even though women use relatively more fat than men during exercise, fat is still not the primary fuel used. So, why do fit people have less fat mass if it is not used to a huge extent during exercise? The answer is that fat use becomes very important in the period after exercise when the body is restoring glycogen (the storage form of glucose). Fat is also used in the rested states if you have not recently consumed food. What the subsequent research found was that men used a greater proportion of fat during the recovery period, and that this use of fat persisted to a greater degree than for women even after food was consumed. Very simply, men use more fat than women after exercise bouts in the time that fat oxidation is the predominant fuel.
There is one other important consideration when comparing the sexes on how much fat and carbohydrate is used. As mentioned, in general men have a greater muscle mass than women. The amount of lean mass (lean being the opposite of fat mass) largely determines how much energy is required to just keep the body running. If you have a man and a women that are each 150 lbs, and the man has 10% body fat and the woman has 20% body fat, that equates to a lean mass of 135 lbs for the man, and 120 lbs for the woman. Therefore, the amount of energy needed is greater in the man than in the woman resulting in a greater amount of substrates being used over time. On average, at a given weight, a male typically uses more energy and this is often derived from fat. Because lean mass is such an important component of energy use, weight training is an effective strategy for those wishing to lose weight (at least fat mass).
The main take home from this column is that there are slight differences in the fuel use patterns between men and women. On a relative scale they are pretty small, but over years can add to significant differences in overall fat used. It begs the question whether body composition drives changes in substrate use, or if differences in substrate use drive body composition changes. One way to answer that, of course, would be to look at energy use in pre-pubescent (prior to hormonal changes) males and females. However, there is a host of other problems with that approach, and it is interesting to note that some body composition changes are already apparent at that age. Last, there has been a recent introduction of sports drinks catering to female athletes. From a substrate point of view, I have some skepticism related to these drinks (for the reasons explained above), but admittedly I have done zero research into any differences in electrolyte balance between the sexes.
In the near future, I will discuss differences in muscle mass and fatigue between women and men and for that particular topic, the answers may surprise you.