Can friends ‘catch’ obesity?

Having an overweight friend, spouse or sibling increases a person’s risk of also being obese, a US study has found.

Researchers at Harvard Medical School and the University of California, San Diego, reported last week that a person’s chances of becoming obese increased by 57 per cent if he or she had a friend who became obese.

Among siblings, if one becomes obese, the chance that the other will also become obese is increased by 40 per cent.

Among spouses, obesity in one partner means the other is 37 per cent more likely to become obese, the study found.

The report’s two authors, Professor Nicholas Christakis and James Fowler, speculated that increased societal tolerance of obesity might be to blame.

‘Whereas obesity has been stigmatized in the past, attitudes might be changing,’ they said.

‘To the extent that obesity is a product of voluntary choices or behaviours, the fact that people are embedded in social network and are influenced by the evident appearance and behaviours of those around them suggests that weight gain in one person might influence weight gain in others.’

The report also found that friends of the same sex were far more likely to influence obesity in other friends than in opposite-sex friendships.

In same-sex friendships, the probability of obesity in one person increased by 71 per cent if a person connected to them became obese. For friends of the opposite sex, there was no significant association.

While the researchers found evidence of obesity spreading through social networks, they found no evidence that obesity spreads amongst non-friend neighbours.

‘These results suggest that social distance plays a stronger role than geographic distance in the spread of behaviours or norms associated with obesity,’ they reported.

The study, published in the New England Journal of Medicine on 26 July, evaluated a densely interconnected social network of 12,067 people assessed repeatedly from 1971 to 2003 in a heart study.

Arguing that obesity needs to be tackled as both a public health problem and a clinical problem, the report’s authors said the importance of social influence could be harnessed to slow the spread of obesity.

‘Network phenomenon might be exploited to spread positive health behaviours, in part because people’s perceptions of their own risk of illness may depend on the people around them.

‘People are connected so their health is connected. Consequently, medical and public health intervention might be more cost effective than initially supposed, since health improvements in one person might spread to others,’ they concluded.

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