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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.