Obesity 'socially contagious'
Size could depend on other people
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Thursday, 26, Jul 2007 01:49
Whether or not someone is obese could be down to the size of those who make up their social network, scientists have found.
Although there are obvious factors which contribute to obesity, such as weight and level of exercise, researchers from Harvard medical school and the University of California (UC), San Diego, believe obesity spreads through social ties.
They studied 12,067 individuals and identified 38,611 social and family ties.
As they analysed the data, they found that when an individual becomes obese, the chances that a friend of theirs will become obese increases by 57 per cent.
This increases to a 71 per cent increased risk if the friends are of the same sex.
The obese person's siblings and partner have a 40 per cent and 37 per cent increased risk respectively. Same-sex relationships also had an impact on the risk; if a man's brother becomes obese, his chances of becoming obese increase by 44 per cent. Among sisters, the risk is 67 per cent.
Writing in the New England Journal of Medicine, the researchers argue that although there could be a biological and possibly genetic link to obesity – potentially explaining the siblings' increased risk observed in this study – the patterns seen among friends suggest there are other factors at work as well.
"Most likely, the interpersonal, social network effects we observe arise not because friends and siblings adopt each other's lifestyles," said Harvard's Dr Nicholas Christakis.
"It's more subtle that that. What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads."
UC San Diego researcher Dr James Fowler added that it is important to understand a "significant part" of an individual's health is affected by their social network.
"In fact, we really need to revisit our whole notion of cost-effectiveness. The fact that certain healthcare approaches won't just affect the individual but will also cascade through their social ties means that healthcare interventions are far more cost-effective than previously thought."