Biologic and Medical Causes of Obesity

Specific Genetic Factors

Genetics may directly contribute to severe obesity in people with family histories of the problem. Genetic factors such as slow metabolisms may also make people more prone to being overweight. At least seven genetic mutations have been associated with specific and uncommon cases of severe obesity. Some are outlined below.

• HOB1 (human obesity 1) is a gene that is linked to a high BMI in women.
• Leptin gene variants have been linked to leptin deficiencies and obesity.
• Melanocortin-4 receptor is a gene that helps turn off the urge to eat. It may not work properly in those with a family history of obesity.
• Researchers have also identified a mutation in a gene for a protein called proopiomelanocortin, which results in a syndrome of obesity, red hair, and deficiencies in stress hormones.
• About 5% of severely obese people have mutations that over respond to agouti-related protein.

Genetics also determine the number of fat cells a person has. Some people are simply born with more. It should be noted that even when genetic factors are present, a person can still control their diet.

The Thrifty Gene. Some experts think the existence of a so-called "thrifty" gene regulates hormonal fluctuations to accommodate seasonal changes. Theoretically, it works in the following manner:

• In certain populations, hormones are released during seasons when food supplies have traditionally been low. This leads to insulin resistance and increased fat storage.
• The process is reversed in seasons when food is readily available.
• Because modern industrialization has made high carbohydrate and fatty foods available all year long, the gene no longer serves a useful function. Fat, originally stored for famine situations, is not used.

Such a theory could explain the high incidence of type 2 diabetes and obesity found in Pima tribes and other Native American tribes with nomadic histories and Western dietary habits. In the past, the traditional low-fat high-fiber foods (corn, lima beans, white and yellow teparies, mesquite, and acorns) of the Pima people may have protected them from obesity and type 2 diabetes. But today, they now have a high incidence of such disease.