Obesity: A contagious disease?

It seems obvious that diseases such as colds and STDs are spread through social networks, but it now seems that obesity may also be. The Framingham Heart Study, a longitudinal study looking at health, specifically weight, among people and their friends, shows that obesity tends to cluster in social networks.

It does not seem surprising that obese people may tend to be friends with other obese people, but the Framingham study shows that if someone becomes obese in a given time interval, their friends have a greatly increased chance of also becoming obese. Possible reasons for this could be that friends may simultaneously adopt similar lifestyle choices such as diet, exercise or smoking, which could effect their weight. There could also be changing attitudes towards weight that could spread through social networks and make one more or less inclined to gain/lose weight because of social pressures. Similar effects were seen among siblings, where if one gained weight, another sibling is also likely to gain weight.

The effects both among siblings and friends are strongest among same sex relationships. They also depend primarily on social distance, ie. closer friends have stronger impacts, and the effects appear to be independent of geographic distance.

Another article on the spread of obesity through social networks, looks at how obesity, and other diseases, should be looked at as a hierarchy of networks (see figure below). On the bottom, there are all of the molecular networks such as gene regulation, protein interaction and metabolic networks which can be studied to look at the molecular basis of disease. Then the paper discusses a disease network where diseases such as obesity are also linked to other diseases such as heart disease and diabetes. Diseases could be connected because they frequently co-occur, one leads to another, they involve similar molecular components or other types of connections. Lastly, there is the social network. Diseases such as the flu or HIV could be passed directly through a social network or it could be a transfer of attitudes or lifestyle.
The relation between the networks adds an interesting angle to the subject. For example, genetics. One's genetics most obviously effect the molecular aspects of disease, but the social network has a big impact on genetics because within an obesity cluster, genetic predispositions for obesity are likely to be passed on to children from both parents. The combined factors of genetics, lifestyle and social pressures could make these children highly prone to obesity.

The interacting levels of networks may also lead to new strategies for treating obesity. The Framingham study emphasizes that their findings suggest that the social network provides an important resource in treating obesity as well and that perhaps social support groups could be a helpful strategy. It seems so human ecological to say it, but issues like obesity and other public health problems, need to be addressed on many levels, and the hierarchy of networks provides a model for how those levels interact.