Obesity - A Friends-and-Family Problem

Summarized by Robert W. Griffith, MD
September 11, 2007

Summary

The likelihood of a person becoming obese is heavily influenced by existing obesity in their friends, siblings, and spouse. In the case of friends and siblings, same-sex relationships are stronger than opposite sex. Geographic distance, neighbors, and smoking status are without influence.

Introduction

Obesity - or at the very least, overweight - is increasing in the USA rapidly. Many factors have been blamed, ranging from fast foods to excessive television; genetics, so far, is not one of them. Obesity often seems to occur more commonly in clusters (e.g. in less affluent neighborhoods), suggesting localized spread of one or more of the causative factors.

It's been suggested that weight gain in one person might influence weight gain in another, based on the appearance and behavior of a nearby person. In order to explore further how this might occur within a social network, Harvard scientists used 32-year data from the Framingham Heart Study that contained information on relatives and close friends of participants that could be used to track the member. They published their findings in the New England Medical Journal, and we summarize them here.

What was done

The Framingham Heart Study began in 1948 with 5000-odd enrollees; from 1971 onward most of the children of the original set, together with their spouses, were enrolled in the 'offspring cohort', and 5 years ago 4000 grandchildren of the original group were enrolled as the 'third generation cohort'. All participants undergo a physical exam and complete a questionnaire at regular intervals. The body mass index (BMI) is available for all participants; for the purposes of this study, a BMI of 30 or above constituted obesity.

The present study used the offspring cohort of 5124 'subjects' as their central group for analysis. The network was formed of the subjects plus any Framingham participants who were linked-to in a way that might alter the behavior of the subject; this resulted in a network of over 12,000 persons. Close friends were found because they had been listed by the subjects as contact persons in case of emergency. The geographic distance between linked network members were obtained from the addresses held by Framingham, which were frequently updated. There was an average of 7.5 links, or ties, per offspring-cohort subject; however, there were only about 0.7 'friendship' ties per subject.

Software was used to graph the network at intervals of 5 or 10 years. Clusters of obese people were identified, which extended to three degrees of separation (e.g. a friend's friend's friend, or a spouse's sister's friend). To quantify the role of obesity in formed clusters, the distribution of obese individuals was compared with that of randomly-distributed obesity in a simulated network with the same configuration.

What was found

At all time intervals, the risk of obesity occurring among network members who were linked to an already obese subject was about 45% higher in the observed network than in the simulated, random network; it was about 20% higher for two degrees of separation, and 10% higher for three degrees of separation.

A subject's chance of becoming obese increased by 57% if they had a friend who became obese in a given interval. The likelihood was increased 40% if a sibling became obese, and 37% if a spouse did so.

If sampling was restricted to same-sex friendships (87% of the total), the probability of obesity increased by 71% if the friend became obese, while for friends of the opposite sex there was no significant association. (The increased risk was 100% for male/male friends, but for female/female friends it was 38% - not significantly different from zero.) The gender effect was similar for siblings. For sisters, the risk increased by 67%, for brothers by 44%, while for siblings of opposite sex there was no increase.

If the subject described a network member as his or her friend the increased risk was 57%, but there was no increase if the friendship was perceived by the member but not by the subject. Between mutual friends, however, the subject's risk for obesity following the friend's obesity rose by 71%.

The obesity of a friend living far away was just as strongly associated as if the friend lived nearby. And an immediate neighbor's obesity had no influence. Smoking/non-smoking status or quitting smoking had no influence on the findings.

Conclusions

This study shows that the likelihood of a person becoming obese is heavily influenced by obesity in their friends, siblings, and spouse. The 'reach' of obesity influence amounts to three degrees of separation in someone's social network; beyond that, obesity occurrence is unaffected. Friends in the study did not, for the most part, become obese at the same time, effectively denying the influence of environmental factors.

Mutual friends have the highest likelihood of exerting this effect. Females appear to be more readily influenced - at least in the case of friends and siblings. Geographic distance, immediate neighbors, and smoking status are without influence.

One can speculate that the findings represent a role for "spending time together" so that the behavior and appearance of the other person influence one's own behavior. Thus explains why same-sex network members have a greater influence than opposite-sex members.

This important study will provide considerable food for thought, both for the obese and non-obese. Friends obviously have an eminent influence on a person's risk of obesity. So how about the risk of pathological thinness, or other health-influencing conditions? Perhaps mother was right when she said: "Choose your friends carefully". Often easier said than done.

Source

• The spread of obesity in a large social network over 32 years. NA. Christakis, JH. Fowler, New Engl J Med, 2007, vol. 357, pp. 370--379

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