Is Obesity About Willpower Or Wiring?

For Scottie Hill, losing weight has always been a case of one step forward, two steps back.

She's 28, a social worker in New York City, and has spent most of her adult life either starting a diet or breaking one, CBS Evening News anchor Katie Couric reports. Hill says it's "extremely frustrating, and extremely tiring" to diet, only to have the weight come back.

Hill has weighed as much as 283 pounds, and she says it infuriates her when people assume she is undisciplined.

"I work like crazy. And I have not spent my life filling my mouth with food and lying around the house all day," she says.

In fact, there is a growing school of thought that what makes Hill heavy is determined by her brain chemistry and genetic makeup.

"It's like your eye color, or your height or your blood pressure," says Dr. Rudy Leibel, a molecular geneticist at Columbia University Medical Center. "These are all instances of very strong genetic predispositions."

Leibel has been studying the science of weight for 30 years. He says people like Hill have actually been blessed with the heartiest of genes that have helped the human race survive. But in today's society, where food is everywhere, those genes are more of a curse.

"There are a set of genes that we have acquired as a result of evolution over the past several millions of years that are designed to save energy; to make us efficient; to favor the ingestion of food," Leibel says.

Doctors have isolated at least 30 to 40 genes that affect body weight. Some of those genes produce hormones like grehlin, which tells your body that it's hungry. Another hormone is leptin, which is secreted by fat cells. It signals the hypothalamus that you have enough stored fat to survive and don't need to eat. But in many overweight people, the brain may not be getting the message the leptin is sending ... the message of: "I'm full."

You can even see this in mice with a mutation in the leptin gene. "This animal is much fatter, virtually inactive. And if we were to measure its food intake, it would be eating more food," Leibel says, describing the mice in his lab. But if the mice were given very low doses of leptin, the obesity would go away, he explains.

Leibel believes within the next 10 years, there will be medications that will help reduce our collective girths, much like statins have reduced cholesterol.

So does willpower have nothing to do with obesity?

"I'm saying that when you look at something as complicated as body weight, there is a biology. There are basic hormonal and neurologic phenomena that actually dictate this kind of very complex behavior," Leibel says.

As for Hill, "I think for me, I finally decided in order to lose weight and to maintain an ideal weight it's something that I have to be conscious and aware of every single day," she says.

Hill has lost 57 pounds and hopes to lose another 50. While she is relieved to learn that her weight may be, in large part, determined by her genes, she is determined to fight Mother Nature and prove that her behavior can be more powerful than her biology.