At the Center for Weight Loss Surgery (CWLS) at Newton-Wellesley Hospital, we understand that there are many reasons for obesity. Often, morbid obesity is not simply a result of overeating. Research has shown that in many cases a significant, underlying cause of morbid obesity is genetic.

Factors that can contribute to obesity include:

• Genetics
• Environment
• Metabolism
• Eating Disorders and Medical Conditions
• Contributing Factors
• The Pima Paradox

Genetics 
Many scientific studies have shown that your genes can play an important role in your tendency to gain excess weight.

For example, the body weight of adopted children is often not the same as the body weight of their adoptive parents - the people who feed them and teach them how to eat. Adopted children have an 80 percent chance, however, of having a similar weight as their genetic parents whom they have never met.

Identical twins, who have the same genes, have a much higher chance of being a similar weight than fraternal twins who have different genes.

In certain groups of people, such as the Pima Indian tribe in Arizona, there is a very high rate 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 our 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.

Environment
Environmental and genetic factors are closely connected. If you have a genetic predisposition towards 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.

Metabolism
We used to think of weight gain or loss as only a function of calories ingested and then burned. If you take in more calories than you burn, you will gain weight. If you burn more calories than you ingest, you will lose weight. But now we know this idea 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 your 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.

Eating Disorders and Medical Conditions
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.

Contributing Factors
The underlying causes of severe obesity are not known. There are many factors that contribute to the development of obesity including genetic, hereditary, environmental, metabolic and eating disorders. There are also certain medical conditions that may result in obesity like intake of steroids and hypothyroidism.

The Pima Paradox
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 facing times without food, 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 doing physical activity for only two hours a week.

The Pima seem to 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.