Numerous scientific studies have established that your genes play an important role in your tendency to gain excess weight.

The body weight of adopted children shows no correlation with the body weight of their adoptive parents, who feed them and teach them how to eat. Their weight does have an 80 percent correlation with their genetic parents, whom they have never met.

Identical twins, with the same genes, show a much higher similarity of body weights than do fraternal twins, who have different genes.

Certain groups of people, such as the Pima Indian tribe in Arizona, have a very high incidence of severe obesity. They also have significantly higher rates of diabetes and heart disease than other ethnic groups.

We probably have a number of genes directly related to weight. Just as some genes determine eye color or height, others affect our appetite, our ability to feel full or satisfied, our metabolism, our fat-storing ability, and even our natural activity levels.

The Pima Indians are known in scientific circles as one of the heaviest groups of people in the world. In fact, National Institutes of Health researchers have been studying them for more than 35 years. Some adults weigh more than 500 pounds, and many obese teenagers are suffering from diabetes, the disease most frequently associated with obesity.

But here's a really interesting fact - a group of Pima Indians living in Sierra Madre, Mexico, does not have a problem with obesity and its related diseases. Why not?

The leading theory states that after many generations of living in the desert, often confronting famine, the most successful Pima were those with genes that helped them store as much fat as possible during times when food was available. Now those fat-storing genes work against them.

Though both populations consume a similar number of calories each day, the Mexican Pima still live much like their ancestors did. They put in 23 hours of physical labor each week and eat a traditional diet that's very low in fat. The Arizona Pima live like most other modern Americans, eating a diet consisting of around 40 percent fat and engaging in physical activity for only two hours a week.

The Pima apparently have a genetic predisposition to gain weight. And the environment in which they live - the environment in which most of us live - makes it nearly impossible for the Arizona Pima to maintain a normal, healthy body weight.

Environmental and genetic factors are obviously closely intertwined. If you have a genetic predisposition toward obesity, then the modern American lifestyle and environment may make controlling weight more difficult.

Fast food, long days sitting at a desk, and suburban neighborhoods that require cars all magnify hereditary factors such as metabolism and efficient fat storage.

For those suffering from morbid obesity, anything less than a total change in environment usually results in failure to reach and maintain a healthy body weight.

We used to think of weight gain or loss as only a function of calories ingested and then burned. Take in more calories than you burn, gain weight; burn more calories than you ingest, lose weight. But now we know the equation isn't that simple.

Obesity researchers now talk about a theory called the "set point," a sort of thermostat in the brain that makes people resistant to either weight gain or loss. If you try to override the set point by drastically cutting your calorie intake, your brain responds by lowering metabolism and slowing activity. You then gain back any weight you lost.

Weight loss surgery is not a cure for eating disorders. And there are medical conditions, such as hypothyroidism, that can also cause weight gain. That's why it's important that you work with your doctor to make sure you do not have a condition that should be treated with medication and counseling.