Why are We So Fat?

It's become a far too familiar headline: Today one out of three Americans is obese, twice as many as three decades ago.

It is enough for the Centers for Disease Control and Prevention to declare obesity an epidemic.

More disturbing are statistics relating to children: 15 percent of children and teens are overweight, a nearly three-fold jump from 1980.

Obesity is defined by your body mass index, or BMI, a fancy calculation in which your weight is divided by your height.

If it's 25, you're overweight. If it's 30, you're obese. Over 40, you're morbidly obese.