updated 11/4/2010 5:43:01 PM ET 2010-11-04T21:43:01

Obesity in America won't plateau until 42 percent of adults are obese, according to a new study.

The figure takes into account the effects social networks have on people's likelihood to become obese, researchers at Harvard University said.

The finding goes against a prediction made earlier this year by the Centers for Disease Control and Prevention that obesity in the United States — which for the past five years, has stood at 34 percent of adults — has already hit its plateau.

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According to the new study, it might be another 40 years before this plateau is reached, said study researcher Alison L. Hill, a graduate student at Harvard's Program for Evolutionary Dynamics.

"Our model suggests that, based on rates of [people] becoming obese that we've measured, it seems that even though it's slowed down, it will continue to increase," Hill told MyHealthNewsDaily.

The study showed obese people have the potential to create a ripple effect among their non-obese peers, Hill said. That's because the more non-obese people an obese person comes into contact with, the greater the chance those people will become obese.

Hill and her colleagues based their findings on a model of obesity's spread through social networks, which was similar to models used to trace the spread of infectious diseases like the flu.

"Data suggests the dynamics are the same for obesity, even if the mechanisms are different," she said.

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The researchers found the obesity rate is affected by three factors: person-to-person contact through social networks, personal factors like diet and exercise, and the rate at which obese people lose weight.

Although obesity can't actually spread like an infectious disease, contact with other obese people does influence the likelihood of becoming obese yourself, Hill said.

After analyzing 40 years of data from 7,500 people in the Framingham Heart Study, researchers found the average person has a 2 percent chance of becoming obese in any given year, due to personal factors such as an unhealthy diet or lack of exercise. That chance increases by 0.5 percent for every obese family member, friend or coworker a person has regular contact with, Hill said.

An obese person has a 4 percent chance of losing enough weight to no longer be considered obese in any given year, the study said.

"Maybe it's mimicking behavior or adopting similar lifestyles, or changing what you consider to be a normal and acceptable weight for yourself based on the weight of people around you," Hill said. "It's most likely to be some social influence."

Hill has used the same mathematical model to look at how positive and negative emotions can spread among people. She said she plans to use the model to look at how social contact can affect other behaviors.

The study was published today (Nov. 4) in the journal PLoS Computational Biology.


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