Why is thin in and fat out, anyway?

Who deemed it so? How did it get that way?

And moreover, while thin is definitely in for mainstream America, what about the rest of the world?

The story goes back, the anthropologists tell us, a couple of million years, to the advent of the life form that eventually became man.

For most of history and prehistory, certainly for the past 40,000 years or so -- and even now in many parts of the world -- there were recurrent periods of famine and starvation. For the vast majority of our existence, humans were "hunters and gatherers," foraging for their food, moving from temporary camp to temporary camp, following the seasons and the saber-tooths (or whatever animals they killed for food and fur).

Both hunting and gathering were catch-as-catch-can operations, with hunted meat (and therefore the ingestion of saturated fats) rather more catch-can't than the vegetable carbohydrates and fibrous plants that were gathered.

So not many of these prehistoric forbears of ours had either the quality of life or quantity or type of food to get fat. Still, the ones who did put down some fat were the ones who tended to survive. Especially in women, especially when pregnant or nursing, a little extra fat was a big survival boost. To the anthropologists, the new studies that show a distinct genetic influence in obesity are no great surprise.

Dr. Peter J. Brown, anthropology professor at Emory University in Atlanta, discussed some of the evolutionary and subsequent cultural factors surrounding fatness at a recent symposium on obesity. He traced fatness from the survival mechanism it was over the course of about 99 percent of human history to the health hazard it often is today, from its cultural position then -- prized as a symbol of wealth -- to today's negative image where the mere hint of fleshiness is horrific enough to some teen-aged girls that they would sooner starve themselves to death than harbor the perception of a single fat cell.

It was the recurrent food shortages for so many millenia that started making some folks fat. "Since shortages were ubiquitous for humans under natural conditions, selection favored individuals who could effectively store calories in times of surplus," Brown said.

Natural selection is part of the theory of evolution. It says that a genetic trait that confers some survival advantage to an organism will be passed to subsequent generations. In the case of fatness, for example, there appear to be genes that increase the propensity to store fat. Fat individuals survived periods of famine more easily than their thin contemporaries and bore healthi- er children. In that case, the fat folks were the fittest. They survived and passed on their genes to be fat.

This selection worked especially in women, Brown said. "Females with greater energy reserves in fat would have a selective advantage over their lean counterparts in withstanding the stress of food shortage, not only for themselves, but for their fetuses or nursing children."

"In this evolutionary context," Brown said, "the usual range of human metabolic variation must have produced many individuals with a predisposition to become obese.

"Yet they would, in all likelihood, never have the opportunity to do so. Furthermore, there could be little or no natural selection against such a tendency. Selection could not provide for the eventuality of continuous surplus because it had simply never existed." It isn't only a predisposition to fat that got into our genes, but also a predisposed craving for the things that make us fat. "I think," said Brown, "our appetite for sweet and fatty things is deeply engrained in us. Hunting and gathering populations including several still existing in the world make a big deal out of fat, even though it is relatively uncommon in their diet, and they go way out of their ways to get sugar in the form of honey, for example. These are not just a big deal, they are a part of social camaraderie."

Brown added: "Any human population on record, when they had the opportunity to eat a lot of fat and sugar did so, and did so avidly." Fatness, or at least plumpness, then, became a standard of beauty and high economic status in most of the world's cultures.

And as undeveloped countries began developing, as relatively primitive populations began to become "westernized," the populations began to become obese. Now, for the first time in history, the appetites or metabolism which provided the additional fat storage were confronted with enough continued prosperity to get out of hand, and there was no inbred cutoff for fat storage because the prolonged prosperity was such a novelty of history. Obesity, he noted, has been called "the first of the western diseases to appear" as poor countries became modernized.

Still, except in the United States and western Europe, fat -- even the very word -- has positive cultural connotations.

"Fattening huts," in which prepubertal brides-to-be spend as long as two years eating extra food to plump up, still exist in certain African cultures. Among the Havasupai Indians of the American Southwest, if a girl at puberty is thin, a fat woman places her foot on the girl's back so she will (magically) become attractively plump.

"Among the Amhara of the Horn of Africa," Brown said, "thin hips are called 'dog hips' in a typical insult.

"Societies that favor plumpness as a standard of beauty are found in all of the major world culture areas, with the exception of Asia."

And within the United States itself, Brown said, "there are significant differences in ideal body preferences" among ethnic groups. Mexican Americans coined a new phrase -- godura mala, meaning bad fatness, because the word for fatness, godura, continues to have positive connotations.

Other specialists have also noted that the eating disorders anorexia and bulimia are extremely rare among black adolescents, although it is increasing among white middle class teen-agers.

Brown cites an obesity prevention campaign in a Zulu community outside of Durban, South Africa, as an example of the vast cultural differences in how obesity is regarded. "One of the health education posters depicted an obese woman and an overloaded truck with a flat tire, with a caption, 'both carry too much weight.' Another poster showed a slender woman easily sweeping under a table next to an obese woman who is using the table for support; it has the caption 'Who do you prefer to look like?' The intended message of these posters was misinterpreted by the community because of a cultural connection between obesity and social status.

"The woman in the first poster was perceived to be rich and happy, since she was not only fat, but had a truck overflowing with her possessions," Brown said.

"The second poster was perceived as a scene of an affluent mistress directing her underfed servant."

The American abhorrence of obesity has produced not only the eating disorders now widespread on high school and college campuses and among young professional women, but also genuine discrimination against overweight citizens, studies have demonstrated.

According to Dr. Albert Stunkard, psychiatrist and obesity expert from the University of Pennsylvania, everything else being equal -- test scores, grades and activities -- fat college applicants are significantly less likely to be accepted by prestigious colleges than thinner classmates. And a study of corporate executives, Stunkard said, suggested that each pound of overweight cost an executive $1,000 a year in salary.

Said Brown: "American ideals of 'thinness' occur in the setting of a generally fat population where it is easy to become fat, and other societies' preference for plumpness occurs in the setting of a lean population where it is easy to remain lean. In context, both standards require the investment of individual effort and economic resources in order to be attained. Furthermore, each in its context involves a display of wealth.

"In poor societies the rich impress the poor by becoming fat, which the poor cannot do. In rich societies even the poor can become fat, and avidly do; therefore, the rich must impress by staying thin, as if to say, 'We have so little doubt about where our next meal is coming from that we don't need a single gram of fat store.' "

Nor, said Brown, is fatness necessarily outmoded by permanent abundance. "Although we have focused on the role of food shortages in human history," he said, "they are unfortunately not limited to the past.

"The drought and famine in the Horn of Africa and the Sahel has justifiably received world attention. Even in the U.S., arguably the richest nation in human history, an estimated 20 million people are hungry. This continuing worldwide epidemic of hunger presents a powerful and tragic counterbalance to our contemplation of the new epidemic of obesity."