Whether you're obese or slim is 'socially contagious,' study finds

Provided by: Canadian Press
Written by: SHERYL UBELACKER
Jul. 25, 2007

TORONTO (CP) - Look around at your friends, family and spouse or partner. Are you and they slender and fit? Or could you and others in your social circle stand to lose a few pounds? Or are most of you, dare we say it, even obese?

A landmark study has found that whether those closest to you are overweight or slender can significantly influence your own body shape and that of others in your social group - and that could have huge implications for public health measures to battle the growing obesity epidemic.

"Really what's unique or special about our study is to my knowledge this is the first study that's been able to look at an entire social network at once and tease out not just how there's person-to-person transmission, but rather how there's person-to-person-to-person transmission," said co-author Dr. Nicholas Christakis of Harvard University.

The propensity to be overweight or obese - or alternatively, to be slim - is "a kind of social contagion," Christakis said from Boston. "Our study takes seriously the fact that because people are interconnected, their health is interconnected."

The study, published Thursday in the New England Journal of Medicine, analyzed 32 years of data for 12,067 adults, who had repeated medical assessments as part of the Framingham Heart Study. The researchers were able to map socially interconnected clusters of subjects living in Framingham, Mass., by using tracking sheets that recorded not only their family members but also their friends.

Christakis and co-author Dr. James Fowler of the University of California, San Diego, found that when an individual became obese, the chances that a friend would become obese increased by 57 per cent.

Furthermore, the person's siblings had a 40 per cent increased risk of obesity and their spouse a 37 per cent increased risk.

Gender also played a key role: Among friends of the same sex, if one became obese, the other had a 71 per cent greater chance of piling on the pounds. The effect held true for siblings as well, although to a lesser extent (44 per cent among brothers; 67 per cent for sisters). Friends and siblings of opposite genders showed no increased risk.

"There's a kind of spreading process within the network," said Christakis. "So it's not just that people are influenced by those to whom they're immediately connected. They're also influenced by the actions of the people to whom those people are in turn connected."

"People's weight status is related to the weight status of people who are up to three degrees of separation from them."

Obesity experts not involved in the study lauded the research for its insights and implications.

"I think their observations are incredibly important," said Dr. Diane Finegood, scientific director of nutrition, metabolism and diabetes for the Canadian Institutes of Health Research.

Still, Canadians should be cautious in how they interpret the findings, said Finegood, a professor of kinesiology at Simon Fraser University.

"I certainly think it's dangerous to draw the conclusion that, 'Oh, if I'm surrounded by obese people, I should get rid of these friends,"' she said Wednesday from Burnaby, B.C.

"But if you are surrounded by a lot of overweight or obese people, it might give you another pause for thought about how their behaviours - not their obesity, but their behaviours - might influence your behaviours."

So why is someone's body shape so influenced by others in their social nexus?

Christakis speculates that certain behaviours may spread among people tied to each other by blood or friendship.

"So my friends or my social contacts say, 'Let's go have burgers and beer,' and we hang out and eat together and I adopt unhealthy eating behaviours," he said, citing an example. "Or running. Let's say my buddies say, 'Lets go running together,' and we lose weight together.

"Another possibility is that it's not the spread of behaviours, but rather the spread of norms."

Dr. Ian Janssen, an obesity researcher at Queen's University, agreed that when a person's friends and family members are on the heavier side, there is a tendency to have more tolerance for a bigger body type.

"In the past we've had this great stigma associated with obesity and overweight," Janssen said from Kingston, Ont. "But when you're around that all the time, it becomes the norm and it becomes more acceptable to you perhaps."

"And maybe it would contribute to weight gain or you're not as concerned if you are starting to put on the pounds, because . . . a lot of the people around you are looking like that as well."

With an estimated 60 per cent of Canadian adults being overweight (including 25 per cent who are obese), he said, "Obviously the stigma associated with obesity is going away."

However, the study also showed that having slender, fit friends and family had a symmetrical effect, said Christakis. "So your best friend becoming non-obese would decrease your risk of being obese by 57 per cent as well."

That relationship suggests novel ways to think about battling society's collective bulge from a public health standpoint, obesity experts say, and it underscores the notion that there is strength - for change - in numbers.

"The good news is we definitely can change," said Janssen. "You have an impact on the health of those you are related to, and to me that's very good news. . . . And from a public health perspective or clinical health perspective that's good news because if we treat one individual, that can also help treat the individuals around them."

Finegood said it also shows that one person can transmit their healthy habits to others, seemingly by osmosis.

"It tells me as an individual that my being active doesn't just affect me. It affects the people around me. So I may be doing more good by being physically active than just what I'm doing for myself." 

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