A recent US study has added credence to the theory that eating trends leading to obesity, and at the other end of the spectrum, thinness, may often be socially contagious.

Study author and professor at Harvard University, Dr Nicholas Christakis, said, ‘This reinforces the idea that because people are interconnected, their health is interconnected. It takes seriously the embedded-ness of people within social networks and gives new meaning to the concept of public health’.

The study, published in the New England Journal of Medicine, involved evaluating over 12,000 socially interconnected people in New England, who had been part of a major three-decade cardiovascular study. Because that study had involved repeated measurements, including BMI over its duration, the researchers were able to study and apply the data to the obesity study.

It was found that both thin people, and overweight and obese people tended to be socially grouped together, and that the groups extended to three degrees of separation - that’s to say, if an individual was overweight, so was their friend’s friend’s friend. Researchers also found that a person was 57 per cent more likely to become obese if they had a friend who became obese over a certain time period.

The degree of closeness between individuals hugely affected the odds on becoming obese as well; if one sibling became obese, the other was 40 per cent more likely to follow suit, and in marriages one partner’s weight gain resulted in a 37 per cent likelihood of their partner joining them. The team also found that, outside of marriage and de facto relationships, people were more likely to be influenced by members of their sex, rather than by the opposite sex.

Chairman of the department of psychiatry and behavioural sciences at the University of Miami, Dr Julio Licinio, said, ‘Trying to address the problem on an individual level has been so hard, and it may be because we’re not addressing the network, which could be family, neighbourhood, community, school. This is a fascinating way to look at the problem, and it may be a very good reason why treatments have been so difficult, because we’re only addressing one member of the network’.

The research authors suggested that people’s inclination to be influenced by those around them could result in ‘copycat’ behaviour, even if was not deliberate. ‘It brings up another component of our environment that influences obesity. This would need to be proven, but it suggests that, to be effective in treating obesity, we have to not just treat the person who’s obese but also the social network’ commented Dr Samuel Klein, director of the Centre for Human Nutrition at Washington University School of Medicine.