Is Obesity Contagious?

July 25, 2007 -- New research on obesity shows that obesity may be contagious -- but don’t get the wrong idea about that.

The findings, published in The New England Journal of Medicine, have nothing to do with bacteria or viruses.

Instead, the new data show that obesity is "socially contagious." That means that people tend to follow suit when their friends and family become obese or lose weight to ditch obesity.

"We find that a person's chances of becoming obese increase by 57% if they have a friend who becomes obese, 40% if they have a sibling who becomes obese, and 37% if a spouse becomes obese," say researchers Nicholas Christakis, MD, PhD, and James Fowler, PhD.

"Mutual friends more than triple the risk to each other," note Christakis and Fowler. "If one of the two [mutual friends] becomes obese, the chance for the other to follow suit goes up 171%."

Christakis is a professor of sociology at Harvard University and professor of medical sociology at Harvard Medical School in Boston. Fowler is an associate professor of political science at the University of California, San Diego.

Obesity, Family, and Friends

Christakis and Fowler analyzed 32 years of obesity data on more than 12,000 people who participated in the Framingham Heart Study.

Every two to four years, participants got weighed during checkups. The researchers used participants' height and weight to calculate their BMI (body mass index). BMI of at least 30 is considered obese.

At their checkups, participants named family and friends that the Framingham Heart Study researchers could contact in case they lost touch with any participants.

Christakis and Fowler computerized that contact information and mapped each participant's social network -- the array of the friends and family they named over the years.

Christakis and Fowler then traced obesity trends through participants' social networks. "Because people are connected, their health may be interconnected," Christakis explains.

The study included many spouses, siblings, and friends. Mutual friends named each other as friends on their contact sheets.

Obesity Spreads Socially

A person's chances of becoming obese were influenced by his or her family and friends, even if they were hundreds of miles away.

"We were stunned" by that finding, Fowler says. He notes that immediate neighbors didn't affect a person's chances of becoming obese, which suggests that the findings weren't strongly tied to social class.

The researchers considered many other factors, including gender, natural weight gain with aging, and the tendency of people to associate with people similar to them.

The results held. In other words, the findings weren't about thin people favoring thin people as their friends.

How Does Obesity Spread?

The data don't show how obesity spread through the social networks. But social norms appear to play a role, Christakis notes.