Saturday, November 17, 2007

Obesity Among Friends, Spouses, Siblings and Neighbors

Obesity has become an epidemic. Right? We hear this all the time. Well, isn't the term "epidemic" used often for diseases that are spread from a person to a person, like infectious diseases? Could the phenomenon of the prevalence of obesity be actually behaving as an infectious process? Could the benefits of obesity control, likewise, spread in an epidemic (good) way? Should the treatment of obesity be considered not only a form of individual therapy, but also, and probably more importantly, a treatment of public health proportions and general community benefits?

A very important article appeared in the July 26, 2007 issue of the New England Journal of Medicine "The Spread of Obesity in a Large Social Network over 32 Years" by Drs. Nicholas A. Christakis and James H. Fowler from Harvard Medical School, Boston and University of California, San Diego, San Diego. The researchers analyzed the nature and extent of the person-to-person spread of obesity as a possible factor contributing to the obesity epidemic. To do so, they evaluated a social network of 12,067 people assessed repeatedly from 1971 to 2003 as part of the Framingham Heart Study.(1) They examined whether weight gain in one person was associated with weight gain in his or her friends, siblings, spouse, and neighbors.

What they found? A person's chances of becoming obese increased by 57% if he or she had a friend who became obese. The type of friendship appeared to be important. Between mutual friends, a person's risk of obesity increased by 171% if the other became obese. In contrast, the influence did not appear to be statistically significant when one person, but not the other, defined the relationship as a friendship. The sex also appeared to be important. When analysis singled out same-sex friendships, the probability of obesity in a person increased by 71% if the friend became obese. For friends of the opposite sex, however, the probablity of obesity did not increase significantly. Among friends of the same sex, a man had a 100% increase in the chance of becoming obese if his male friend became obese, whereas the female-to-female spread of obesity was not as significant.

How about siblings? If one sibling became obese, the other's chance of becoming obese increased by 40%. As for married couples, if one spouse became obese, the likelihood that the other spouse would become obese increased by 37%. By the way, those effects were not seen among neighbors.

If social networks are so influential in the spread of obesity, then this may actually explain another well-known observation. Individuals in weight loss programs or after weight loss (bariatric) surgery, who attend regular support group activities, that modify the person's social network, are more successful than those that do not.

This is a great study that will certainly be quoted over and over in the future.

(1) The Framingham Heart Study is an ambitious project that was initiated in 1948, when 5209 people were enrolled in the original cohort. The Framingham Offspring Study began in 1971, when most of the children of members of the original cohort and their spouses were enrolled in the offspring cohort. In 2002, the third-generation cohort, consisting of 4095 children of the offspring cohort, was initiated. All participants undergo physical examinations (including measurements of height and weight) and complete written questionnaires at regular intervals.


Christakis NA, Fowler JH. The spread of obesity in a large social network over 32 years. N Engl J Med. 2007 Jul 26;357(4):370-9. [PMID: 17652652]