If overeating and under-exercising were the only causes of this chronic disease, the rate of long-term success stories would not be an abysmal one in fifty cases. Some factors contribute to obesity, beginning with genetic inheritance. Doctors from Columbia University's College of Physicians and Surgeons studied 132 twins between the ages of three and seventeen. In each of the 66 couples, both young men had similar body mass indexes and body fat percentages, leading the researchers to conclude that a child's body composition is 80 percent programmed at conception.

Playing devil's advocate for a moment, it stands to reason that the twins would have the same body type. After all, they live in the same house and have probably adopted the family's eating habits. But other studies have found that even siblings raised in different households typically share nearly identical body mass indexes. A groundbreaking Danish study compared the BMIs of adopted adults with those of their biological parents and those of the couples who adopted them. For the most part, the body composition of adopted men and women reflects that of their biological parents and not that of their adoptive parents.

Heredity also largely determines a person's metabolism: the process by which the body converts nutrients from our food into energy (calories). Basal metabolic rate (BMR) is the rate at which we burn energy while at rest. Sixty to seventy-five percent of our total energy is spent in this state to maintain vital functions such as respiration, circulation, body temperature, digestion, and glandular activity.

One person's metabolic "rhythm" can be up to 20 percent faster or slower than another person's. This equates to a difference of four hundred calories a day. So two teens can go biking and eat the same number of calories, but the one with the lower BMR will burn fewer calories. When more calories are eaten than expended, the excess is stored as body fat for future use. Obese adolescents often had lower resting metabolic rates as children, before they became overweight.

However, it is other organic factors that partly determine which children can eat what they want and can't seem to gain an ounce, and which children face a lifelong struggle to keep their weight under control.

Insulin resistance.

Normally, the hormone insulin binds to tissue cells and helps them absorb blood sugar (glucose), the body's fuel. In young people who are resistant, insulin does not work effectively. Instead of being burned for energy, sugar builds up in the body. Insulin resistance can eventually lead to non-insulin dependent diabetes, also known as type II diabetes.

Low leptin levels.

Some people are deficient in leptin, a hormone that appears to regulate weight in two ways. The substance, which is produced by fat cells, essentially sends signals to the brain when the body has received enough food. It also inhibits the production of an enzyme crucial for fat production. In studies of overweight men and women, blood tests consistently revealed extremely low concentrations of leptin. The substance is currently being tested in volunteers as a potential weight loss drug.

Chromosomal abnormalities and endocrine abnormalities.

Very few adolescents are obese due to a birth defect or a disorder of the endocrine (hormonal) system, such as hypothyroidism.