April 18, 2003 -- What determines body weight? Is it just some people's destiny to be thin, while others are fat?

In Boston, members of the National Association to Advance Fat Acceptance once held a demonstration they called the Every Body Good Body Festival. They called it that proudly because, they said, there's nothing wrong with being fat, and there's not much they can do about it.

They say some people are thin, while they are fat, because being fat is their genetic destiny. Some obese belly dancers said they went to the demonstration to explain to the public that they cannot lose weight.

"Everyone knows the facts, that we're metabolically different, and the serious research hasn't really been done to figure out why," says one dancer.

Their size is not their fault, they say, so people should just accept them, the way they are.

Addicted to Food

Celebrities tell us that overeating is an addiction. Oprah Winfrey has said, "It's like a crack addict going to crack."

Today Show weatherman Al Roker has the willpower to get up at 4:30 in the morning and freeze in the cold, but to lose weight he had to get his stomach stapled so it's the size of an egg, because, he says, he's addicted to food. "People will look at this and say, 'Oh, what a crock, but it's true," Roker says.

Jeffrey Schaler, author of Addiction Is a Choice, disagrees: "These people are playing or pretending to be helpless. If they want real help, they need to confront the fact that they're lying when they say they can't do something that they can do."

Schaler says we're stronger than we think, and that overeating, smoking and other so-called addictions are all things we can choose to control. Our genes are not in charge, he says, we are.

Schaler says people may process food differently because of their genes. But, he says, "The activity of eating is not controlled by a gene."

Winfrey has demonstrated that she recently lost 33 pounds.

"It's a myth that we have no control over our body weight," says JoAnn Manson, an obesity expert and chief of preventive medicine at Boston's Brigham & Women's Hospital. She says genes do partly determine body shape, but not mostly. "Maybe a third is genetic and the remainder is lifestyle-based," Manson says.

The Tale of the Twins

Well, a recent study done at California's Lawrence Berkeley National Lab found that people with exactly the same genes — identical twins — could have big differences in their weight. The researchers studied 35 pairs of identical twins to try to learn if certain traits, like obesity, are mostly hereditary. Their conclusion — fat is not.

Researcher Paul Williams says he was surprised by the study's findings. Even though identical twins "have the same genes, they do have the freedom of choice. … They can decide how much they want to exercise, how lean they want to be," Williams says.

Take John Sovocool and his twin brother, Wayne. Despite having the exact same genes, John is 50 pounds lighter than his brother.

Paul Williams says he was surprised by his study's findings. "Even though these two have the same genes, they do have the freedom of choice. They can decide how much they want to exercise, how lean they want to be," Williams says.

This goes against what I've been hearing, which is that your body type, basically, determines how, how big you are.

John Sovocool says he doesn't buy that argument. "People are overweight," he says, because number one, they eat too much, and because they don't exercise enough. Period."

John runs nearly every day. Wayne lifts weights, but he doesn't exercise nearly as much as John. And, Wayne eats more than John.

We took the twins to a buffet so we could spy on their eating habits. Sure enough, John made a beeline for the fruit salad. Wayne went for the chow fun and the octopus, dumplings, deep-fried seafood, barbecue pork, prime rib, and baked, cheese-stuffed oysters.

For dessert, Wayne had coconut pie, melon custard and three pieces of cheesecake.

Wayne isn't uptight about his food choices. When pointed out that he had five desserts, he says, "Why deprive myself of what I enjoy?"

OK, that's a philosophy, but guess what? It makes you fatter.

John weighed in at 159 pounds, while Wayne weighed 205 pounds.

The California study found that height and cholesterol levels correlated almost exactly among identical twins, but the twins' body mass index, based on their weights, were very different.

Eating More and Moving Less

It shouldn't have surprised us that genes turn out not to be the main reason so many people are overweight. After all, 40 years ago, only 13 percent of Americans were obese. Now it's more than 30 percent. Our genes didn't change. What has changed is that we eat more, and move less.

Obesity specialists point out that we used to move more. A Mayo Clinic Study of energy expenditure found that simply going to drive-thrus, using remote controls for the TV or the garage door — even buying pre-sliced vegetables instead of chopping your own — makes a difference. Over a year, 20 such activities make a difference in weight of 30 pounds.

And gaining even less than 30 pounds is dangerous, says Manson.

"Even a moderate weight gain, 15, 20, 25 pounds of weight gained during adulthood, will increase the risk of Type II diabetes, high blood pressure, and even heart disease," she says.

Wayne says he wasn't entirely aware of these health risks. "Maybe John will serve as an inspiration. Maybe I'll do some more running," he says.

We'll expand this discussion about fat and other "addictions" in an hour-long special airing on Monday, April 21 at 8 p.m. ET.