July 26 (Bloomberg) -- Obesity, a modern health scourge afflicting 400 million people worldwide, is not just determined by what one eats. It also depends on who one associates with.

If someone close to you becomes overweight or obese, chances are you will too, according to a study published in the New England Journal of Medicine today. Friends of obese people were 57 percent more likely to gain weight, U.S. researchers found. The risk was 40 percent for siblings and 37 percent for spouses. Neighbors outside the person's social network weren't affected.

Researchers tracked more than 12,000 people over three decades and concluded that obesity spreads through social ties in more subtle ways than just adopting the habits of others. The study may explain why the rate of obesity has more than doubled in the U.S. over the past 25 years, and suggests the surge should be viewed as a social problem as well as a medical one.

``What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size,'' investigators led by Nicholas Christakis, a professor of medical sociology at Harvard Medical School, wrote in the July 26 edition of the New England Journal. ``People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads.''

About 1.6 billion people are overweight worldwide, with 400 million of them considered obese, according to the World Health Organization. About two-thirds of Americans are considered to be overweight, the researchers at Harvard and the University of California at San Diego said.

`Direct Relationship'

``What we see here is that one person's obesity can influence numerous others to whom he or she is connected both directly and indirectly,'' Christakis said. ``It's not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship.''

The New England Journal research looked at data since 1971 on 12,067 individuals included in the Framingham Heart Study, which began in 1948, to elicit social patterns. The study was funded by the U.S. National Institute on Aging.

``We need to understand that a significant part of an individual's health is embedded in their network,'' said James Fowler, an associate professor of political science at the University of California, in the report.

Obesity threatens to undo many of the advances of modern medicine that have helped reduce the disabilities that come with growing old. Being overweight can lead to heart disease and diabetes, a condition that affects more than 20 percent of Americans aged 60 or older.

Career Opportunities

Being overweight also can hamper a person's development and career opportunities.

A separate study by researchers at the University of Texas at Austin earlier this week showed that obese girls are half as likely to attend college as non-obese girls. The findings appear in the July issue of the Society of Education journal.

Researchers found that obese girls were more likely to consider committing suicide, use alcohol and marijuana, and have negative images of themselves.

While companies including France's Sanofi-Aventis SA are developing new medicines to treat obesity, regulators and public health officials frequently regard it as a lifestyle problem that should be treated with diet and exercise. Drugs based on previously untested biological pathways are meeting resistance from regulators and payers.

A U.S. Food and Drug Administration panel last month voted against recommending Paris-based Sanofi's anti-obesity drug Acomplia, the first in a new class of treatments that block hunger receptors in the brain, because of health risks including suicide.

To contact the reporter on this story: Angela Cullen in Frankfurt at acullen8@bloomberg.net

Last Updated: July 25, 2007 17:00 EDT