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Eric Ravussin, a researcher with the National Institute of Diabetes andDigestive and Kidney Diseases (NIDDK), has compared Pimas in Arizona with their distant relatives in Maycoba, Mexico, who still live on subsistence farming and ranching. Although the groups share most of the same genes, Pimas in Maycoba are on average 57 pounds (26 kilograms) lighter and about one inch (2.5 centimeters) shorter. Few have diabetes. Maycobans also eat about half as much fat as their counterparts to the north, and they spend more than 40 hours a week engaged in physical work. The fact that Mexican Pimas remain lean provides strong evidence that the high rate of obesity among American Pimas is the result not of a genetic defect alone but of a genetic susceptibility--exceptionally thrifty genes--turned loose in an environment that offers easy access to high-energy food while requiring little hard labor.

Because all human populations seem to share this genetic susceptibility to varying degrees, "we are going to see a continuing increase in obesity over the next 25 years" as standards of living continue to rise, predicts F. Xavier Pi-Sunyer, director of the obesity research center at St. Luke's-Roosevelt Hospital in New York City. He warns that "some less developed countries are particularly at risk. It is projected that by 2025, more than 20 percent of the population of Mexico will have diabetes."

Studies of Pimas, islanders and migrants "all seem to indicate that among different populations, the prevalence of obesity is largely determined by environmental conditions," Ravussin concludes. A few doctors have proposed changing those conditions by levying a "fat tax" on high-calorie foods or raising insurance rates for those who fail to show up at a gym regularly.

But economic and legal punishments are unlikely to garner much popular support, and no one knows whether they would effectively combat obesity. So most researchers are turning back to factors they think they can control: the genetic and biological variables that make one person gain weight while others in the same circumstances stay lean.

Finding Genes That Fit

Doctors have long known that the tendency to gain weight runs in families--how strongly is still under debate. Numerous analyses of identical twins reared apart have shown that genetic factors alone control a large part of one's body mass index, an estimate of body fat commonly used to define obesity [see the sidebar "A Shifting Scale"]. A few have found weight to be as dependent on genes as height: about 80 percent. But the majority have concluded that genetic influences are only about half that potent.

Investigators at the National Institutes of Health who examined more than 400 twins over a period of 43 years concluded that "cumulative genetic effects explain most of the tracking in obesity over time," including potbellies sprouting in middle age. Interestingly, the researchers also determined that "shared environmental effects were not significant" in influencing the twins' weight gain. That result is bolstered by five studies that compared the body mass indexes of adopted children with their biological and adoptive parents. All found that the family environment--the food in the refrigerator, the frequency of meals, the type of activities the family shares--plays little or no role in determining which children will grow fat. Apparently, only dramatic environmental differences, such as those between the mountains of Mexico and the plains of Arizona, have much effect on the mass of a people.

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