If you have fat friends, you run a higher risk of becoming fat yourself.

This week's New England Journal of Medicine reports on a study that used social network analysis to show that:

Network phenomena appear to be relevant to the biologic and behavioral trait of obesity, and obesity appears to spread through social ties.

A person's chances of becoming obese increased by 57% (95% confidence interval [CI], 6 to 123) if he or she had a friend who became obese in a given interval.

Our study suggests that obesity may spread in social networks in a quantifiable and discernable pattern that depends on the nature of social ties. Moreover, social distance appears to be more important than geographic distance within these networks. Although connected persons might share an exposure to common environmental factors, the experience of simultaneous events, or other common features (e.g., genes) that cause them to gain or lose weight simultaneously, our observations suggest an important role for a process involving the induction and person-to-person spread of obesity.