Obesity May Be Contagious Among Friends and Family

BOSTON, July 25 -- Obesity tends to spread throughout a person's social and family ties, even as far as a friend's friend's friend, researchers found.

When individuals become obese, it dramatically increases the chance that their friends, siblings, and spouse will also gain weight, Nicholas A. Christakis, M.D., Ph.D., of Harvard, and James H. Fowler, Ph.D., of the University of California San Diego, reported in the July 26 issue of the New England Journal of Medicine.

Surprisingly, the researchers found, the greatest effect was not among those sharing the same genes or the same household, but among friends, even those living apart.

What appears to be happening, the investigators said, is that obese persons change what they see as appropriate body size, and they come to think it is acceptable to be bigger, inasmuch as those among them are bigger, and this sensibility spreads. Other mechanisms might include food consumption, but the data did not permit a detailed examination of this factor, they said.

To estimate the nature of the person-to-person spread of obesity as a possible factor contributing to the obesity epidemic, the researchers analyzed a densely interconnected social network of 12,067 people. These individuals were assessed repeatedly from 1971 to 2003 as part of the Framingham Heart Study.

The relationship data derived from archived tracking sheets used since 1971 to identify family members and friends who could be contacted within a few years.

Discernible clusters of obese persons (body mass index of 30 or higher) were present in the network at all time points, and the clusters extended to three degrees of separation (a friend's friend's friend). These clusters did not appear to be solely attributable to the selective formation of social ties among obese persons, the researchers said.

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

Among pairs of adult siblings, if one sibling became obese, the chance that the other would become obese increased by 40% (CI, 21 to 60).

If one spouse became obese, the likelihood that the other spouse would become obese increased by 37% (CI, 7.0 to 73). Husbands and wives appeared to affect each other similarly.

Gender played an important role in these relationships. When the sample was restricted to same-sex friendships (87% of the total), the probability of obesity increased by 71% (CI, 13 to 145) if the friend became obese. For friends of the opposite sex, the association was not significant.

The pattern was also seen among siblings. Among sisters, the risk was 67%, while among brothers the risk increased by 44%. However, siblings of opposite gender showed no increased risk.

Persons of the same sex had a relatively greater influence on one another than those of the opposite sex. It seems that people are influenced mainly by those they resemble, Dr. Christakis said.

These effects were not seen among neighbors in the immediate geographic location.

Finally, the researches reported, the spread of smoking cessation, with its frequent weight gain, did not account for the spread of obesity in the network.

The obesity of a friend living far away correlated as strongly with an individual's obesity as did having an obese friend living close by. Thus the fact that neighbors don't affect each other and that geographic separation doesn't influence the risk among siblings or friends suggests that environmental factors are not an explanation, Dr. Christakis said.

It is not just lifestyle, he said, or that obese or non-obese people simply associate with similar people, but rather that there is a direct, causal relationship.

So the relevance of social influence also suggests that network phenomena might be exploited to spread positive health behaviors. Thinness could become contagious, they suggested.

In an accompanying editorial, Albert-Lszl Barabsi, Ph.D., of the University of Notre Dame in South Bend, Ind., wrote that many new-found disease-associated genetic mutations account for only a tiny fraction of disease occurrences. There is a tendency to believe that the rest are hidden in more genes.

He said the answer is not always as simple as that. Networks that pertain to social influence may have just as strong an impact on the development of obesity as the otherwise strong genetic effects.

The role of links and connections does not stop here. "In the past few years," he wrote, "we learned that network effects increasingly affect all aspects of biologic and medical research, from disease mechanisms to drug discovery. It is only a matter of time until these advances will start to affect medical practice as well, marking the emergence of a new field that may be aptly called network medicine."