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Madison “Mad” Nena nibbles on a tangerine picked from his garden on Kosrae, a tiny volcanic island in the Pacific Ocean some 4,670 kilometers southwest of Hawaii. The 53-year-old Nena is a rarity here. He’s thin in a place where fatty, sugary foods imported from the United States have caused an alarming number of people to inflate like dirigibles; obesity-related diseases such as diabetes and heart disease have struck the island’s 7,600 residents hard. Why Nena has stayed thin, and others have not, has drawn American researchers from Rockefeller University in New York City to this 109-square-kilometer patch of jungles, white beaches, mangrove swamps, and quiet villages for more than a decade, in a quest to tease out the genetic and molecular mechanisms of why humans are compelled to eat. And sometimes to eat and eat, far beyond what is healthy.

The Rockefeller team suspects that the proclivity of a person’s body to approach a certain weight is determined far more by genes than was previously thought – specifically, genes that control the impulse to eat. Growing evidence indicates that an individual’s weight is 40 to 70 percent decided by genes, which makes it about as heritable as height. (Height, however, is determined during infancy and childhood, whereas weight can continue to fluctuate throughout life.) Some people appear to be hardwired to be particularly ravenous. When access to food is unlimited, say hunger-gene experts, these people can will themselves to eat less, but their efforts will almost inevitably be overridden by the far more powerful force of genetics.

If true, this is not good news for many Kosraeans, or for anyone else unlucky enough to have the genes that compel him to gobble a second baked potato. “We have to realize that obesity is a disease, like cancer, that people have less control over than most of us think,” says Jeffrey Friedman, an obesity researcher and leader of the Rockefeller team.

Not everyone buys this, of course. Those in the diet industry and many nutritionists make their living on the contention that people can readily choose to reduce calories and to stay thin. “There are too many cases where people have willed themselves to lose substantial amounts of weight and keep it off,” says nutrition expert Marion Nestle of New York University.

Teasing out which perspective is right – or whether, as seems likely, obesity is a complex interaction of both genetics and lifestyle – will help determine our attitudes not only toward fat people but toward the effectiveness of dieting. Globally, more than one billion people are overweight, and that number is growing fast. Between 1991 and 2000, the average weight of Americans swelled by 10 pounds. The 1999-2000 U.S. National Health and Nutrition Examination Survey found that 64 percent of American adults are overweight or obese. Britain, Germany, and other Western countries are not far behind. Neither are developing countries that are rapidly modernizing, where diseases associated with obesity are increasing.

On Kosrae, more than 80 percent of adults are overweight or obese, and diabetes afflicts one in eight adults. Until the United States took control of Kosrae and the rest of Micronesia after World War II and began shipping in canned and processed foods, the people were predominately lean, eating fish, bananas, coconuts, and taro. For centuries before the arrival of the first European ship in 1824, the elites ate well. But most islanders lived a near-subsistence life, suffering through frequent droughts and stormy seasons that decimated crops. And they stayed thin.

The exact molecular mechanisms behind the rapid onset of obesity among island residents in this new dietary environment are still uncertain – and are the mystery that the Rockefeller researchers are intent on solving. Are many of the islanders genetically predisposed to large appetites, which, once food was plentiful, they were suddenly able to satisfy? Or as NYU’s Nestle and Kosrae health officials maintain, is it simply a case of a population’s sudden shift to an unhealthy lifestyle, which might be corrected by cutting down on frosted flakes and Spam? Kosrae’s genetically isolated population and its abrupt changes in eating habits make it almost the perfect place to examine such issues.

“The local food is high in fiber and balanced in minerals,” says Vita Skilling, who runs the public-health outreach program on the island. “But now it’s so easy to get refined flour and refined sugar. It’s also how we [now] prepare food. We take fresh bananas and we fry them with sugar.” Skilling says that cars and a newly paved road ringing most of the island mean that few people walk as much as they used to. “We have plenty of food all the time,” she says. “We don’t exercise, because it wasn’t the thing to exercise–that was work.”

On Kosrae, crates full of goods and canned and processed foods arrive regularly in a container ship from overseas. The supplies are paid for mostly by the salaries and assistance that a large U.S. grant to Micronesia – part of a compact agreed to as part of Micronesian independence in 1986–provides each year.

I see this Western bounty in Thurston’s, a general store in the town of Lelu. Cooled by a massive fan, the cavernous store displays rows of canned and packaged foods familiar to Americans: pork and beans, canned peas, soft drinks, and Spam. Tails of turkey and chicken – the fatty portion at the rump–are also popular. The store sells a smaller stock of homegrown produce in a stall in front: bananas, taro, limes, and tangerines.

Skilling takes me on a tour of the island’s 40-bed hospital, built in 1978 on a hill below a jagged volcano. Dozens of people wait in the dark, humid halls to have their blood pressure taken and their blood tested for glucose and other indicators of diabetes and hypertension. Others wait in line at the small air-conditioned pharmacy for insulin and other medications. “Most of the admissions for adults, other than day-to-day minor trauma, are really to do with complications from diabetes and hypertension,” says Skilling. She says the hospital might perform several amputations a month and treats patients with heart disease, eye problems, kidney failure, and other diseases associated with diabetes and obesity. “I was told that 70 percent of the admissions to the surgical unit are due to complications from diabetes,” she says.

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