Eric Ravussin, a researcher with the National Institute of Diabetes and
Digestive 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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