Fat parents have fat kids, fat people marry other fat people and groups of friends all tend to be the same size – every noticed this? Well, now it seems that it isn't simply a matter of sharing bad habits. Living in among obese people may actually increase your tolerance for obesity and lead to becoming fat yourself.
The prevalence of obesity in the USA has increased from 23% to 31% in recent years and 66% of adult Americans are overweight. The explanations for this increase tend to concentrate on changes in society that have lead to an increase in food consumption and a decrease in exercise. But, as Nicolas Christakis and James Fowler point out, while obesity has been stigmatised in the past, attitudes appear to be changing. The increasing numbers of obese people in society may have led to an acceptance of overweight as a norm. People within social networks are influenced by the appearance and behaviour of other people. As they say, “Having obese social contacts might change a person's tolerance for being obese or might influence his or her adoption of specific behaviours (e.g. smoking, eating and exercise).”
These researchers looked at a network of 12 067 people. They repeatedly measured them over a period of 32 years and examined several aspects of the spread of obesity, including the existance of clusters obese people within the network, the association between a person's weight gain and weight gain among his or her social contacts, how the nature of the social ties influenced weight gain and the influence of gender, smoking behaviour and geographic distance between people within the social network.
They found that obesity may spread in social networks in ways that can be measured and in distinct patterns that depended on the nature of the social ties. Social distance was a more important factor than geographical distance in these networks. Essentially, if your social network includes a lot of obese people, you have a greater chance of becoming obese yourself, if you are not already. But, as the authors point out, social networks can also be harnessed for positive behaviour, so, if you are overweight, mixing with people who are careful about what they eat and who exercise regularly may be all you need to change your ways.
Christakis NA and Fowler JH. NEJM 2007; 357: 370
For the full text of the article, see here http://content.nejm.org/cgi/content/full/357/4/370
