Here's bad news for the overweight and the obese: Apparently, we're not going to lose weight by doing moderate exercises and dieting, research has shown.

Americans have been getting fatter for years and with 66 percent being overweight or obese, this is certainly not good news.

Scientists have concluded our body composition is predetermined, even before birth and is dictated by DNA and monitored by the brain.

"People don't know that a 20-minute walk burns about 100 calories," said Dr. Madelyn Fernstrom, director of the weight-management center at the University of Pittsburgh Medical Centre. "People always overestimate the calories consumed in exercise and underestimate the calories in the food they are eating."

Scientists now believe that each individual has a genetically determined weight range. Those who force their weight below nature's preassigned levels become hungrier and eat more. Several studies have also shown that their metabolisms slow in a variety of ways as the body tries to conserve energy and regain weight.

On the other hand, people who try to exceed their weight range face the opposite situation: eating becomes unappealing and their metabolisms shift into high gear.

Studies have also shown that the environment most responsible for rising obesity rates is the womb. A child's body composition is influenced by the mother's diet and lifestyle during the course of pregnancy.

Source: Obesity [NY Times Health]