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updated 2/8/2010 8:29:45 AM ET 2010-02-08T13:29:45

Peeking into your future isn't as simple as taking a look at your mom.

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Studies suggest that while your genes may determine up to 80 percent of your weight and body shape, environment and personal choice still play a significant role. So even if you're a dead ringer for your mother in old family photos, it doesn't mean you'll enter middle age with the same body. See, she grew up in a world where women never sweat — and never passed up a slice of pie — while you grew up with soccer and diet-meal delivery services, and experts say this distinction can make all the difference. Women's Health dissected the variety of factors that count ... and looked at what control you can exert over them.

Body of Evidence
In the 1990s, studies done on identical twins indicated that genes pretty much determined adult shape and size. But new research is uncovering a more nuanced view. Some aspects of shape and size, it turns out, are more closely tied to genes than others. The ease with which you develop muscle mass, for example, is a highly inherited trait. A study that appeared in the International Journal of Obesity found that while you need physical activity in order to build muscle, people who have "muscular" genes require far less exercise than others to look fit.

The other major finding: Apple-shaped bodies are more genetically linked than pear-shaped or skinny ones. Some speculate this is because you also inherit genes from your father, and men typically store extra pounds in their guts. So if your mother carries weight in her stomach too, it could increase your chances of being an apple. From a medical standpoint, this is worrisome because central abdominal fat is associated with several serious conditions, including type 2 diabetes and coronary artery disease.

"You inherit half of your genes from your mother and half from your father, so you're a blend. You can be unlucky and get the worst possible combination from both parents, or be lucky and get the best," says Harvard medical professor C. Ronald Kahn, M.D.

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.

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.

You can't override a genetic predisposition, but building muscle can reshape your body to a degree and delay the point at which your figure starts to widen, according to Forsythe. However, muscle mass begins to diminish as you reach menopause, so eventually your body will probably wind up nudging its way back to its genetic set point. The glass-half-full perspective: That hard work will buy you more years of owning a hot bod, and if you stay active into your fifties, you'll put on less weight than someone who has been using her gym membership card as a bookmark.

Fear Factor
Still, to some women, their mother's shape is a black cloud that perpetually hangs over their head.

Clinical psychologist Sherrie Delinsky, Ph.D., hears this sentiment in her private practice in Wellesley, Massachusetts. "When talking with patients who have body-image issues or disordered eating, it often comes out that women have very specific feelings and anxieties about their mothers' bodies. Women who have never been overweight can become paranoid about gaining, because they're concerned about looking like their heavy moms," she says. 

On the other hand, you'd think the daughters of moms with Christie Brinkley-like bodies would have it made. Truth is, girls who aren't as thin as their moms often feel inadequate. "There's a lot of competition between mothers and daughters in general, but it often manifests itself in terms of weight and size, because so much cultural importance is placed on appearance," Delinsky says.

Winning the Genetic War
Despite the slew of new research indicating that certain body shapes are largely preordained, it's by no means a fat sentence. At the end of the day, you're in the driver's seat of your own life and the navigator of your own body. "No matter what your genes or your environment might be, you can't gain weight unless you're taking in more calories than you're expending," Kahn says. In other words, while you may not be able to change being apple-shaped, it's certainly well within your power to be the healthiest, fittest apple possible.

© 2012 Rodale Inc. All rights reserved.

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