How Much Do Genes Determine Your Body Type?

New research has also uncovered a gene that may affect how much you eat. Neurexin 3, one of the genes recently implicated in regulating waist circumference, is also involved in brain function and has been linked to addictive behaviors such as alcoholism. Scientists believe this gene, which is carried by about 20 percent of the human population, may trigger a compulsion to overeat—which could explain why obesity tends to run in families the same way certain body shapes do. "Considering how many factors are involved in obesity, it's interesting that research is increasingly pointing to the brain's involvement in its development," says Kari E. North, Ph.D., an associate professor of epidemiology at the University of North Carolina at Chapel Hill. "Because this gene has been associated with addiction, we need to think about the psychology of weight gain too."

Regrettably, these shape-determining genes can be stubborn. Even disciplined dieters often hit a wall after losing the first few pounds or regain weight they've lost. Researchers believe this is because each person has a baseline weight, a genetically influenced set point where the body naturally wants to be. If you end up more than 10 percent below your set point, your body will fight back. "The more weight you lose, the harder your body works to compensate," says David E. Cummings, M.D., a professor of medicine at the University of Washington. "You become hungrier, and your metabolism becomes more efficient. Increasingly, you begin to crave food—and such a drive is very difficult to resist."

The Lifestyle Link

These new scientific findings are certainly compelling, but don't count nurture out just yet. "Environment and personal choice can have an impact on body shape," says North.

The national obesity rate is one clue to the big role that environment can play. According to the National Center for Health Statistics, 65 percent of Americans are overweight or obese, a 16 percent increase in fewer than 10 years. Genes have been around as long as human beings have, but the current obesity epidemic is brand-new.

One simple explanation, says Cummings, is the supply of calorie-rich food in our culture. "A couple hundred years ago, not many people had ready access to a lot of food, so only those with an extremely high susceptibility to weight gain became overweight." Today, for a few bucks, even someone with skinny genes can buy enough food to supersize herself. "We are living in an environment for which our genes just weren't designed," Cummings says.

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But perhaps one of the biggest wild cards in determining body development is fitness. Women in their twenties and thirties who exercised as kids have less typically "feminine" body types than what was common amongst that same age-group 25 years ago. They have wider middles and narrower hips, and more muscular legs and defined arms—the result of years spent playing sports.

Credit Title IX, legislature that was passed in 1972 giving girls the same athletic opportunities as boys. "Women in their thirties and early forties today are the first generation to benefit from Title IX, and many of them have bodies that look different from those of their mothers, who exercised sparingly, if at all," notes exercise physiologist Cassandra Forsythe, Ph.D., R.D. "When you build a lot of muscle as a teenager, your testosterone levels can get slightly higher, and this could contribute to a slightly wider, more boyish middle. You don't see a lot of 23-inch waists these days." Exercise also limits body fat in the hip and butt area—where women typically store flab—which explains the slimmer hips.

Diane, a 31-year-old graphic designer from Macungie, Pennsylvania, is a perfect example. "I know I have a tendency to get my mother's stocky body type," she says. "But because I grew up swimming and started competing in triathlons when I was in my twenties, and my mom never exercised, my body doesn't really resemble hers. I'm much leaner and fitter."

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