Obesity apparently affected by social networks

Increase in obesity seen among clusters of family and friends.

Issue: August 2007

New research is suggesting that obesity may be socially contagious.

Nicholas A. Christakis, MD, PhD, MPH, professor, department of health care policy, Harvard Medical School, and James H. Fowler, PhD, associate professor of political science, political science department, University of California, San Diego, examined data from the Framingham Heart Study and found that obesity seems to spread through interconnected social networks.

“It’s not that obese or nonobese people simply find other similar people to hang out with,” Christakis said. “Rather, there is a direct, causal relationship.”

“Social effects are much stronger than people before realized,” Fowler said. “Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much.”

The researchers analyzed data from a period of 32 years for 12,067 adults who underwent repeated medical assessments during the Framingham Heart Study. They utilized tracking sheets that recorded information about participants’ family members and friends.

The results of the study were published in The New England Journal of Medicine.

Mutual friendship had strongest effect

Christakis and Fowler defined an “ego” as the person whose behavior was being analyzed; an “alter” was defined as a person connected to the ego who may have influenced the ego’s behavior.

They observed clusters of obese participants as defined by BMI >30 that extended to three degrees of separation.

The chance of becoming obese increased by 57% when an ego defined an obese alter as a friend. Between mutual friends, the ego’s risk for becoming obese increased by 171%. In contrast, there was no statistical association when friendship was perceived by the alter but not by the ego, according to the researchers.

Additional connections

Friends of the same sex had increased risk, whereas there was no statistical association for friends of the opposite sex. Among siblings, the risk for obesity increased by 40% if a sibling became obese. Between spouses, the chances increased by 37% if one spouse was obese.

Factors including geographic location and smoking cessation did not account for the spread of obesity. Neighbors did not influence the spread of obesity unless the ego and neighbors were friends.

“It’s important to remember that we’ve not only shown that obesity is contagious, but that thinness is contagious,” Fowler said. – by Christen Haigh

For more information:
Christakis NA, Fowler JH. The spread of obesity in a large social network over 32 years. N Engl J Med. 2007;357:370-379.