A few months ago, I addressed the role gender plays in how we respond to intermittent fasting . That post sparked a great discussion, and I’ve since received a fair number of emails from readers eager to learn other ways in which gender plays a role in our health and nutrition. One email in particular set me off on a round of research. So, a hat tip to you, Winifred, for giving me something to think, learn, and write about. I hope everyone finds it to be helpful.
As you may know, women and men store and metabolize fat differently from each other, and a 2008 paper (PDF ) reviewed the evolutionary reasons for these differences. Here’s a summary of their findings and few other noteworthy factoids:
Women carry more fat than men. They are better at storing fat than men. Moreover, when women store fat, they do so in different places than men. They’ll preferentially store fat in in the hips, butt, and legs, whereas when men gain weight, it usually goes to the upper body (hence why you see massive beer bellies atop stick legs). Furthermore, when both men and women store upper body fat, men are more likely to develop visceral fat – the abdominal fat associated with metabolic syndrome – while women are more likely to develop subcutaneous fat .
On women, body fat seems to be healthier and less problematic. The characteristically female lower body “gluteofemoral fat” is actually a strong sign of metabolic health , whereas abdominal fat is not. In men, high body fat levels correlate strongly with insulin resistance, while this relationship is much weaker in women (probably because of their lower tendency to store visceral fat).
Women burn fat differently than men. Upper body fat goes first, while lower body fat tends to stay put. Except during pregnancy and lactation, when the lower body begins to give up lower fat stores far more readily. Interestingly (and not by coincidence), women tend to preferentially store the long chain omega-3 fatty acid DHA  – the one that’s so important to the baby’s development during and pregnancy – in their thighs .
Women are better at burning fat in response to exercise. During endurance exercise, they exhibit lower respiratory exchange ratios than men, which indicates more fat burning and less carb burning .
Women are better at converting ALA into DHA , and they also tend to have more DHA and AA circulating throughout their serum than men, who have more saturated and monounsaturated fat.
These differences in fat metabolism aren’t seen in isolated muscle cells of men and women , which isn’t really surprising. We’re made with the same basic building blocks; we just run on different software. The differences are systemic and hormonal.
Why does this sexual dimorphism in fat metabolism exist?
Well, the name of the game in evolution is reproduction, and reproduction is far more nutritionally expensive for women than it is for men. I don’t think I have to spell out why – for a man, the reproductive process is a brief moment in time, a half tablespoon’s worth of effort; for a woman, the reproductive process lasts the better part of a year and represents a significant drain on nutrient stores. As such, women are “designed” to hold onto said nutrients because, as far as evolutionary fitness is concerned, her primary purpose is to feed, nurture, and cultivate an entire other human being inside her body for nine months. Think about that for a second: women have to create and support another life inside their bodies. They have to provide the food, the water, and the shelter. If something goes terribly wrong in the “outside world,” that nutrient flow to the fetus could be interrupted, thus putting her evolutionary purpose at risk.
Now, imagine if the body didn’t know best. Imagine if the expectant mother had to know precisely what the fetus needed at any given moment – what precise nutrients were needed, which foods to eat and when to eat them in order to provide said nutrients, what to drink and how much of it, what not to eat nor drink – and then make a conscious decision to provide those things right on schedule? It wouldn’t work. We wouldn’t be here. Luckily, the body “knows.” The body will draw on what’s stored and what’s provided to make a health baby. And if it’s not all there, it’ll even convert other stuff into the stuff that it actually needs. Sure, a good diet will absolutely improve fetal health, but we’ve all known parents with less-than-ideal diets who have healthy kids. The body knows.
Which is why women store and burn fat differently than men. In order to be able to provide those nutrients to the growing child, female bodies store certain types of fat in certain places on the body. Female bodies “hoard” certain types of fat and are loathe to relinquish them “just cause you had a simple caloric deficit.” Whereas a man could go low carb Primal and lose weight pretty easily because all he “has” to be able to do is provide a bit of sperm, a woman’s body has more important things in mind, like having enough body fat on hand to produce enough leptin  for optimal fertility , or enough DHA stored in lower body fat to build a robust baby brain.
How does this affect my recommendations or your diet, if at all?
Women – don’t be concerned about a little (or more than a little) subcutaneous body fat, especially on your lower body. If you’ve been trying in vain to lose that stubborn jiggle on your thigh, consider that maybe, just maybe it’s there for a reason. Even if you’re not interested in having a child, it’s likely that the presence of lower body fat indicates good health. You don’t have to get pregnant, but the ability to do so is probably a marker of good health, and the research outlined above suggests that classically feminine patterns of fat deposition are healthier than classically male patterns. And even if you don’t like your glutofemoral fat, rest assured that the males in your life (even ones as far away as Papua New Guinea – PDF ) likely do!
Men – most of the fitness and health literature is geared toward you, so I’ll just suggest that you take this information on gender differences in fat metabolism into consideration.
I’ve always stressed the relativity of a person’s ideal body composition . I’ve discussed my own failings at trying to eat big and lift big to get big. I managed to put on five pounds of muscle by eating and working out way more than was comfortable or natural, but it made me lethargic, and as soon as I skipped a meal or workout , muscle would just peel off. That’s my comfortable body composition. Your ideal body composition might look very different, and, if you’re a woman, it might look very different from a man’s. That’s fine. That’s natural. That’s attractive. That’s how it’s supposed to be, and by trying to fight it – in either direction – your health may suffer.
What do you folks think about all this? Does this take a load off your mind, or does it open up new avenues of inquiry? Let me know in the comment section!