Obesity to affect at least 42 percent of adults, say Harvard researchers

By Will Parker on November 5, 2010 in News

Mathematical models of social contagion suggest that America’s obesity epidemic won’t plateau until at least 42 percent of adults are obese. The figures run counter to recent assertions by some experts that the obesity rate, which has been at 34 percent for the past five years, may have peaked. The new projections, by Harvard University researchers, are published in the journal PLoS Computational Biology.

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 gaining weight since 1971, they haven’t gotten any better at losing weight,” says lead researcher Alison L. Hill. “Specifically, the rate of weight gain due to social transmission has grown quite rapidly.”

Worryingly, the projections by Hill and her colleagues are a best-case scenario. 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.

The Harvard scenario 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 unhealthy foods or increasingly sedentary lifestyles;
The rate of “recovery” from obesity, defined as weight loss sufficient to push 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,” said co-researcher David G. Rand.

The researchers note 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.

“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 researchers conclude.