KEY POINTS:
If you want to stay thin, don't choose fat friends.
Researchers have found that obesity is socially contagious - it spreads from person to person within the same social group.
A study of 12,000 people whose height and weight were measured repeatedly over 32 years has revealed that when one person gained weight those around them tended to gain weight, too.
Unexpectedly, the greatest effect was seen not among members of the same family (who shared the same genes) or household but among friends.
A person's chances of becoming obese rose 57 per cent if they had a friend who was obese, but only 40 per cent if they had an obese sibling and 37 per cent if their spouse was obese.
People of the same sex had more influence on each other than people of the opposite sex.
The study, conducted in the US and published in the New England Journal of Medicine, showed that over the period the whole network of individuals grew heavier, reflecting the general increase in weight in the population. But it also revealed distinct clusters of heavy and thin people with long standing social ties.
Nicholas Christakis, professor of healthcare policy at Harvard Medical School and co-author of the study, said: "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 ... and this sensibility spreads."
James Fowler, an expert in social networks at the University of California and a co-author, said: "This is about people's ideas about their bodies and their health. Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much."
"There has been intensive effort to find genes responsible for obesity and physical processes responsible but what our paper suggests is that you really should spend time looking at the social side as well."
The upside was that helping one person control their diet and increase their exercise could result in many losing weight. Thinness was contagious, too.
- INDEPENDENT
Packing it on
* During a given time period, the likelihood that a person would become obese rose by 57 per cent if they had a friend who became obese.
* If a sibling or a spouse became obese, a person's risk of becoming obese increased 40 and 37 per cent, respectively.
