Researchers at Harvard University are predicting that the worst of the obesity epidemic is yet to come. If current trends continue, they say, the obesity rate in the U.S. won't level off until it reaches at least 42%, circa 2050.

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By Amanda Gardner

THURSDAY, November 4 ( — Just over one-third of American adults are obese. Though alarmingly high, this rate has remained relatively steady over the past decade, leading some public health experts to suggest that the obesity epidemic has peaked.

Now, researchers at Harvard University are predicting that the worst is yet to come. If current trends continue, they say, the obesity rate in the U.S. won't level off until it reaches at least 42%, circa 2050.

"The recent slowdown in the increase in obesity prevalence is a natural part of the obesity epidemic reaching a saturation," says the lead researcher, Alison Hill, a doctoral candidate in Harvard's department of biophysics. But, she adds, obesity rates "will still continue to increase, although at a slower rate, if no interventions are introduced."

This bleak forecast, which appears in a study published this week in the journal PLoS Computational Biology, is based on the social networks research of one of Hill's co-authors, Nicholas Christakis, MD, a professor of medical sociology at Harvard Medical School, in Boston.

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In a 2007 study published in the New England Journal of Medicine, Dr. Christakis and a colleague suggested that obesity can spread through social networks, much like the flu. In the new study, the researchers took this theory a step further and used the "infectious disease" model of obesity to predict future trends.

Obesity rates have a tendency to snowball, they found, because a person's likelihood of becoming obese increases with each additional obese family member, friend, or acquaintance he or she has. What's more, obese people appear to have a stronger influence on their friends and family now than they did in 1971, when the earliest data used in the study was collected.

"Over the past 40 years, there has been a steady increase in the rate of infection, and it is now the highest it has been," Hill says. "What's changing over time is how much each obese friend influences you."

Unfortunately that influence doesn't work the other way around. Only weight gain—not weight loss—is "contagious," according to the study.

Hill and her colleagues predicted the national obesity rate by applying their social networks model to 40 years of obesity data from the Framingham Heart Study, which has followed several generations of people in a single Massachusetts town. (Obesity rates in that study have roughly mirrored national trends: 14% of the study participants were obese in 1971, but by 2000 that number had reached 30%.)

Scott Kahan, MD, the co-director of the George Washington University Weight Management Center, in Washington, D.C., says that the new study is "worthwhile" but should be taken with a grain of salt.

The data the researchers used to make their projections were collected before the obesity epidemic became a major public health concern, Dr. Kahan notes, and anti-obesity interventions have since been ramped up.

"We're in a different place," he says. "We never addressed obesity until five years ago. … We're addressing policies and schools and social norms [now]."