On any given day, more than half of women in the U.S. are on a diet. In hopes of slimming their figures, millions take on Atkins, South Beach, Lean for Life or Hollywood 48. Some never eat after 5 p.m.; others only eat Subway sandwiches. While the diet industry has a less than noble reputation, it's clear that American women, far more than men, remain obsessed with dieting. But what can evolutionary biology tell us about gender difference and eating habits?

In a new book called "Why Women Need Fat," Steven J.C. Gaulin, an evolutionary biologist, and William D. Lassek, a retired doctor of public health at the University of Pittsburgh, explain the science behind women’s unique relationship to their diet. In the book, Lassek and Gaulin make a surprising argument for a more positive outlook on fat and illustrate the differences between the ways women and men gain weight. Think of it as the evolutionary biology diet.

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Salon spoke over the phone with Gaulin, who explained why one common ingredient in much of our food is making us fatter, why women are very different from men when it comes to weight and health, and how it really pays to think like an evolutionary biologist.

You and William Lassek co-authored the book. It’s surprising that two men would co-author a book on women’s health. How did each of you come to focus your research on this topic?

When I was teaching at the University of Pittsburgh, they had a policy that if you were over 55 and you weren’t trying to accumulate credits for a degree, you could take any course you wanted. Will, who was retired, showed up in my introductory level course, Sex and Evolution. From the first day he started asking questions that were so far over the heads of the students. So I told him to come to my office hours instead of confusing all of my students with a Ph.D.-level dialogue. Something we began to discuss was this finding that men have a preference for women with a small waist and larger hips. No one had really explained why men should have such a strong preference for this shape, and it’s not immediately interpretable in terms of comparisons with our close relatives. For example female chimpanzees don’t have that shape and male chimpanzees don’t seem to care anything about female shape when they mate. So it was a bit of a puzzle. That was the question that got us started and eventually led us to work related to women’s body type and weight.

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In the first chapter of the book, you talk about the “polyunsaturated explosion,” during the 1950s that led Americans to eat much differently than they had in the past. What changed and why did it happen?

I don’t know if I normally subscribe to the principle that history is driven by the actions of a few influential people, but in this particular case there were two people who did exert a very big influence on our national diet. One was coming from an economic perspective and the other was coming from (what he believed) was a nutritional perspective. After Dwight Eisenhower had a major heart attack, when the American public became much more focused on heart health and nutrition, a popular nutritionist by the name of Ansel Keys made a lot of impact. He was committed to the notion that saturated fat was the culprit in the heart disease epidemic in the U.S. He advised Americans to replace saturated fat with polyunsaturated fats, in particular corn and soybean oils. Meanwhile Earl Butz, Nixon’s secretary of agriculture, had been tasked to get food prices lower. He decided to heavily subsidize and commoditize corn and soybeans in order to make them really cheap. So corn and soybeans became the basis of our entire food production system. And it continues today. The amount of these oils in the American diet increases significantly every year.

And you point out in the book that corn and soybean oil are high in a compound called omega-6, which is detrimental to health, especially for women. What is omega-6 and why does it make people fatter?

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Omega-6 is a category of fat. It is technically a fatty acid. Omega-6s are one category of polyunsaturated fats found in seeds and grains. Now, it’s not bad to eat grains, it’s not bad to eat corn, and it’s not bad to eat soybeans. What is bad is that food processors extract and concentrate these oils from plants. In an ear of corn there isn’t that much corn oil, but when you subject it to industrial processing and extract everything but the oil, now you’ve got a lot of omega-6. It’s this heavy industrial processing of seed crops that makes our diet so unnatural. Omega-6s make us fat in a variety of ways. They promote fat storage. Omega-6 is also the precursor for certain signaling molecules called endocannabinoids. Will likes to call them the body’s home-grown version of marijuana. Endocannabinoids give you the munchies just like cannabis does. So the omega-6s are telling the body, “Store the fat you have.” And they are also telling the body, “Eat more, I’m hungry!”

But later in the book, you also give some reasons why gaining weight is quite natural in women. You provide an evolutionary answer to the question: Why do women gain weight after having children? It’s not the typical reasons that many women tend to assume — being too busy to exercise, eating poorly because of stress, etc.

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Interestingly, human brain size plays a big role in why women need fat and why they tend to gain weight after having children. Humans have ridiculously big brains, which makes it more difficult to give birth to our infants. While chimps, orangutans and gorillas can literally sleep through a birth, human births, especially first births, are typically more than a day of very difficult labor. Women tend to weigh less before they have had their first baby because with a first infant, evolutionarily, it pays not to grow a baby that is too large. They can get stuck in the birth canal. It’s not so much of a problem for us in 21st-century North America because most women have fairly ready access to cesarean section. But for 99.99 percent of human evolution, it was a really big problem. The result of natural selection is that women tend to be lighter before they have a child because they need their first infant to be smaller in order to survive childbirth. Each infant that a woman has remodels the pelvis so that each subsequent infant can grow somewhat bigger. There is a positive correlation between birth order and birth weight. So the way to grow a bigger infant is for the mother to have more fat on her body.

American culture tends to vilify fat and fat people. You mention a particular instance in 2004 when the director of the Centers for Disease Control and Prevention appeared on national television claiming that obesity was approaching the No. 1 preventable cause of death. You think this crusade was misguided. Why?

Many M.D.s have bought this fallacious line that the optimal weight for women in terms of their health is what M.D.s call normal weight, a BMI between 18.5 and 25. And they have thought this to be true because women with higher BMIs exhibit a series of physiological measures that are indeed risk factors for disease in men. But they are not systematically risk factors for disease in women. If you actually look at the data from the National Health and Nutrition Examination Survey and data from studies done in other countries, the optimal weight for women who have had a kid is what doctors currently call “overweight.” I’m not saying that obesity is optimal, but all the findings show that overweight women survive better than normal weight women. We walk a fine line in the book because we argue that being overweight is not nearly as bad as your doctor has been telling you, but on the other hand, Americans are heavier than they need to be. There are diseases that still correlate with heavier weights, like diabetes. But if we ate a more natural diet, by that I simply mean the diet that we evolved to eat, we would all weigh less.

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And we all seem to have a “set point” for weight. We have biological constraints that keep us from veering too far from our genetically determined “set weight.”

Yes, there are studies where they starve people and there are studies where they feed people huge amounts of food to see if they can fatten them up. In both kinds of cases, the body seems to have a lot of inertia in this regard. It does not want to lose weight and it does not want to gain weight regardless of where the person happens to fall on the BMI curve before the experiment. If you are starving the body, the metabolic rate slows down, the activity level goes down, a variety of mechanisms kick in to try to hold on to the weight that the body has. Likewise if we feed people twice as many calories as they normally eat, many are quite resistant to gaining weight. When we feed them three times as many calories, they finally gain weight, but the weight goes right away when they return to their normal calorie intake. The body knows where it wants to be. It’s interesting in that people differ greatly in what their set points are, but everyone seems to have a set point.

So what kinds of implications does this have for women who diet? Why do diets seem to fail women again and again?

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One thing that’s important for women to understand is that your set point can change. That’s what “yo-yo dieting” does. When humans were hunter-gatherers, they never could count on where their next meal was coming from. They didn’t have grocery stores or refrigerators. In cases of bad luck foraging for food, the only thing they had for backup was stored body fat. There is an optimal amount of fat to store, which depends on how frequent and how severe your food shortages are. That is the point; a diet tells your body that there is a food shortage. Your body doesn’t know that you’ve decided to lose weight. Instead, the body takes a diet and goes, “Oh damn, I live in a food insecure world. The next time I get some food I better up my set point so that I have more fat for next time!” It’s so natural and obvious isn’t it?

It’s kind of bitterly ironic when you think about the history and intensity of the relationship between women and dieting in this country.

And it’s quite obvious once you start thinking like an evolutionist. But since barely half of the people in this country believe in evolution, a lot aren’t in a good position to think like one. Evolutionary biology isn’t just crazy theories about fossils from humans that are long gone, this is stuff that is highly relevant to decisions we make everyday in our lives.

In the book, you emphasize that instead of dieting to lose weight, women can change the way they eat in order to return to what you call a “more natural weight.” How do we determine what our natural weight might be and how do we get closer to it?

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I think the best way to do that is just start eating the kind of diet that drastically reduces the amount of polyunsaturated omega-6s in the diet. The best way to do that is to stop eating processed food and to avoid commercially fried foods because they are always fried in these omega-6 fats. Potato and corn chips, for example, are a huge contributor of omega-6s in the diet. There is more than a gram of omega-6 in every single potato chip that a person eats. So that’s my solution. Many studies in the U.S. and other countries show that the single best predictor of how much a woman will weigh is how much omega-6 is in her diet.

Reading the book, I couldn’t help but consider how regional and socioeconomic factors might take influence over the different ways that women tend to eat. In the book, you advise women to eat wild (not farmed) fish, grass fed meat, as well as a diversity of organic fruits and vegetables. But is it possible for all women in the U.S. have access to this diet?

I’m a big advocate of family farms. I don’t think there’s any reason why we can’t have family farms in virtually any part of the country. Because the U.S. has commoditized corn and soybeans, there’s been a progressive consolidation of farms into big industrial agribusinesses. But family farms, that raise animals on the land, are a really good alternative. And when animals are grass-fed it changes the fatty acid profile of their meat -- how much omega-6 and how much omega-3 is in it, which makes it healthier meat to consume. I don’t think that grass-fed or free-range is an elitist kind of food, I think it’s the natural, normal kind of food that we could have anywhere if we patronized our local farmers.