Obesity rate will reach at least 42 percent, say models of social contagion

November 4, 2010

Researchers at Harvard University say America's obesity epidemic won't plateau until at least 42 percent of adults are obese, an estimate derived by applying mathematical modeling to 40 years of Framingham Heart Study data.

Their work, published this week in the journal , runs counter to recent assertions by some experts that the rate, which has been at 34 percent for the past five years, may have peaked. An additional 34 percent of American adults are overweight but not obese, according to the federal government's .

The Harvard scientists say that their modeling shows that the proliferation of obesity among American adults in recent decades owes in large part to its accelerating spread via social networks.

"Our analysis suggests that while people have gotten better at since 1971, they haven't gotten any better at losing weight," says lead author Alison L. Hill, a graduate student in Harvard's Program for Evolutionary Dynamics, Biophysics Program, and at the Harvard-MIT Division of Health Sciences and Technology. "Specifically, the rate of weight gain due to social transmission has grown quite rapidly."

The projections by Hill and colleagues are a best-case scenario, meaning that America's obesity rate could rise above 42 percent of adults. One silver lining is that their model suggests the U.S. population may not reach this level for another 40 years, making the future rate of increase much more gradual than over the past 40 years. Only 14 percent of Framingham Heart Study participants were obese in 1971.

Along with co-authors David G. Rand, Martin A. Nowak, and Nicholas A. Christakis, Hill broke down the spread of obesity into three components:

  • the rate at which obesity has spread through social networks, via transfer from person to person;
  • the rate of non-social transmission of obesity, such as through easier access to or increasingly sedentary lifestyles;
  • the rate of "recovery" from obesity, defined as weight loss sufficient to push body mass index (BMI) back below 30.
"We find that while non-social transmission of obesity remains the most important component in its spread, social transmission of obesity has grown much faster in the last four decades," says Rand, a research scientist in the Program for Evolutionary Dynamics and a fellow in Harvard's Department of Psychology and Berkman Center for Internet & Society.

Hill, Rand, and colleagues found that a non-obese American adult has a 2 percent chance of becoming obese in any given year -- a figure that has risen in recent decades -- and that this number rises by 0.4 percentage points with each obese social contact, meaning that five obese contacts doubles the risk of becoming obese.

By comparison, an obese adult has a 4 percent chance of losing enough weight to fall back to merely "overweight" in any given year. This figure has remained essentially constant since 1971.

"These results suggest that social norms are changing the propensity for becoming obese by non-social mechanisms, and also magnifying the effect that obese individuals have on their non-obese contacts," the scientists write in PLoS Computational Biology.

Explore further: Obesity doubled in British children

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3 / 5 (2) Nov 05, 2010
I have no clue how obesity can spread by social contacts. I don't assume the habits of other persons by just contacting them.
not rated yet Nov 05, 2010
Sure you do. Its called social expectation and fitting in with your peer group. Ever heard of a meme?

I think these estimates are too low and will not plateau until it hits closer to 65-75%. I also don't see it taking 40 years to reach this level either. Short of a massive change in society this problem will continue to get worse faster and faster. Have you noticed the current marketing trend towards grossly obese people in tv and fashion?
5 / 5 (1) Nov 05, 2010
I don't think we'll have a problem in 40 years. So much of our population is fat that drug companies will find it in their best interest to develop miraculous new diet drugs that block the absorption of calories or turn off the hunger signal in your brain. I say 20 years from now we'll have some amazing medical solutions for this. This is the only way, since these people will not change their lifestyle voluntarily until it's made simple by drugs. Maybe we should start supporting Bulimia, considering how much we eat it will probably bring us into normal BMI ranges!
1 / 5 (1) Nov 05, 2010
Sure you do. Its called social expectation and fitting in with your peer group.
I didn't realize that this phenomenon is a general one.
I've learnt decades ago that it's better for me to not just follow the mainstream. Which includes marketing trends.

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