Genetic Influences - Why People Become Overweight: Obesity

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Genetic influences

To date, more than 400 different genes have been implicated in the development of overweight or obesity, although only a handful appear to be major players. Genes contribute to obesity in many ways, by affecting appetite, satiety (the sense of fullness), metabolism, food cravings, body-fat distribution, and the tendency to use eating as a way to cope with stress.

A 2006 report in Science that studied more than 900 people showed that those who have two copies of a specific gene variant (called Insig-2) were 22% more likely to have a BMI higher than 30. Researchers believe the gene variant affects the regulation of another gene involved in fat production. In follow-up studies of more than 9,000 people (including people with Western European ancestry, African Americans, and children), they found that about 10% carried two copies of the gene variant.

In another 2006 study, published in the Proceedings of the National Academy of Sciences, researchers studied the activity levels of three different genes in fat samples from people who were normal weight, overweight, or obese. They took fat samples from around the participants' internal organs and under their skin and found different levels of activity (known as gene expression) in the different samples. In overweight people, increased expression of two of the genes correlated with a tendency to be "apple-shaped." These and related studies have helped researchers better understand how and why obesity occurs. They may also spur the development of new weight-loss treatments.

The strength of the genetic influence on weight disorders varies quite a bit from person to person. Research suggests that for some people, genes account for just 25% of the predisposition to be overweight, while for others the genetic influence is as high as 70% to 80%. Having a rough idea of how large a role genes play in your weight may be helpful in terms of treating your weight problems (see "How much of your weight depends on your genes?").

People with only a moderate genetic predisposition to be overweight have a good chance of losing weight on their own by eating fewer calories and getting more vigorous exercise more often. These people are more likely to be able to maintain this lower weight.

In contrast, people with a strong genetic predisposition to obesity may not be able to lose weight with the usual forms of diet and exercise therapy. Even if they lose weight, they are less likely to maintain the weight loss. For people with a very strong genetic predisposition, sheer willpower is ineffective in counteracting their tendency to be overweight. Typically, these people can maintain weight loss only under a doctor's guidance. They are also the most likely to require weight-loss drugs or surgery.

The prevalence of obesity among adults in the United States has been rising since the 1970s (see Figure 3). Genes alone cannot possibly explain such a rapid rise. Although the genetic predisposition to be overweight varies widely from person to person, the rise in body mass index appears to be nearly universal, cutting across all demographic groups. These findings underscore the importance of changes in our environment that contribute to the epidemic of overweight and obesity.

Figure 3: Trends in adult weight Percent of adults ages 20–74* who were at a healthy weight, overweight, or obese† *Data are age-adjusted to the 2000 U.S. standard population. †Healthy weight, body mass index (BMI) = 18.5–24; overweight, BMI = 25–29; obese, BMI ≥30. Sources: National Health and Nutrition Examination Survey (National Center for Health Statistics); Cancer Trends Progress Report — 2005 Update (National Cancer Institute, 2005).

Last updated:	June 20, 2007