This morning I sat on a panel for medical students; the subject was obesity. Nationally, as anyone who hasn't been hiding under a rock knows, the picture is not pretty-in fact it's pretty ugly. By the standard definition, obesity means a Body Mass Index (BMI; weight in kilos over height in meters squared) above 30, and in about 15 years starting in 1990 we went from 22 percent to 33 percent obese.

Now, I don't care what you call it or where you want to draw the line. Pick any BMI number and you'll see more Americans above it every year. This includes children and teens, who are showing up in pediatricians' offices in growing numbers with Type II diabetes, which used to be called "adult-onset." In some practices there are more of them than there are of Type I, once called childhood diabetes.

Don't want to call it an epidemic of obesity? Fine. Endemic, pervasive, skyrocketing, steeply increasing, whatever. Anyone who's telling these kids that it's okay and they should feed good about their bodies is doing them and the country a huge disservice. They are headed for diabetes, heart disease, colon cancer, arthritis, and many other conditions in record number. Help them with their body image issues, but don't tell them it's fine to be fat.

But why this trend? Isn't it maladaptive? In evolutionary terms, the answer is simple. Not just for millions but for hundreds of millions of years our ancestors were selected to store fat during times of abundance. That way, you could survive lean times. Trouble is, now there's no end to abundance.

Among the Bushmen of Botswana, who I lived with for two years, and other hunter-gatherers, there was no starvation but there was also no obesity. Foods were harder to get, eat, and digest, and they maintained a high level of aerobic and muscular fitness just because of activity in the food quest. Compared to our diet, theirs was much higher in fiber and much lower in refined carbs, saturated fat, and salt. Children were always outdoors and active.

Our genes and bodies evolved in that context, and they just can't handle what we do to them now.

Culture matters, of course. My colleague Peter Brown and I many years ago looked at the ideal of beauty in cultures throughout the anthropological record. Plumpness is the ideal in over 80 percent. Look at paintings by Rubens, Titian, and Tintoretto, and you'll see the women who were considered the greatest beauties of their time. Other eras have prized less plump women, but few thin enough to make Playboy, much less Vogue.

Slim was the ideal in a few cultures, very fat in none. Pleasantly plump but not obese was the goal in most, and the extra fat those beauties stored held just about the extra energy they would have needed to get through a pregnancy and a couple of years of lactation. In other words, they were beautiful because they could make and nourish a baby.

In some cultures in the past, frank obesity was evidently valued. Carvings like the "Venuses" of the late Stone Age looked like women with very high BMIs, possibly also pregnant. Some cultures in Africa had fattening huts for adolescent girls to prepare them for marriage. In many cultures you displayed your wealth and status by showing you could put on surplus fat. But it stopped with pleasantly plump, probably because serious signs of ill health became evident when you went further.

But we live in the Supersize Culture, which is producing a legion of boys and girls, men and women, who are far fatter than the plumpness that used to be (and in much of the world still is) the norm. Yet ironically ours is a culture with a cosmetic ideal that's uniquely thin. It's not a health ideal; mortality is lowest at a BMI of 22 or 23, which you would never see in a typical fashion ad. And at its worst the ideal leads to anorexia or bulimia.

Those are bad, sometimes deadly disorders, but the implications for the population are far more dire at the other end of the BMI spectrum. And like most bad things, the trend is worst for the poor. Our society consigns the poor to put on weight by making it hard and expensive to get good food and easy and cheap to get junk.

The trend affects Blacks (especially women) more than Whites, in part because of culture but mainly because of access. You can't find fresh fruit and vegetables in the inner city, and you don't want your kids playing outside on streets rife with drugs and gunfire. You're struggling to keep body and soul together, so when your kid sits for hours watching TV and eating snacks, you've got bigger worries than BMI.

But it's up to us as a society to recognize that there is more going on here than lack of will power. The human species was set up to store fat whenever possible and to resist weight loss with many fail-safe appetite mechanisms. And for very good evolutionary reasons, it's even harder to keep weight off than to lose it.

So we'd better rethink our laws, school food policies, farm subsidies, educational programs, and information campaigns before we have to change our name to Homo lipidens.

About the Author

Mel Konner MD, PhD
Melvin Konner's books include The Tangled Wing and The Evolution of Childhood. He has written for The New York Times, Newsweek, Nature, and Science.

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