Could your best friend make you fat? Researchers who have studied "networks" of obesity think so: they found that if someone's friend becomes obese, that person's chances of becoming obese increase by more than half.

Siblings and spouses also have an influence, although a reduced one - people whose siblings became obese were themselves 40 per cent more likely to grow obese, while people whose spouses became obese were 37 per cent more likely to.

"This is the first (study) to show how obesity spreads through the social network from person to person to person," James Fowler of the University of California, San Diego, who worked on the study, told a telephone briefing.

Writing in the New England Journal of Medicine, the researchers said their findings show that obesity is contagious - not like a virus is contagious, but in a social sense.

It may help explain why obesity is worsening exponentially across the United States, with two-thirds of Americans overweight and almost a third technically obese.

"It has become very popular to speak of the obesity epidemic. We began to wonder if it is truly an epidemic," Dr Nicholas Christakis, a professor of medical sociology at Harvard Medical School in Boston, told the briefing.

Christakis and Fowler studied the records of 12,067 people taking part in the Framingham health study, which included most of the residents of the mostly white, middle-class town of Framingham, Massachusetts.

They had their health and habits regularly monitored beginning in 1948. In 1971, offspring and spouses were invited to join the study, even if they had moved away.

TRACKING DOWN FRIENDS

Christakis and his team noticed that everyone had given alternative contacts for follow-up - they had named friends and relatives who could be called in case they for some reason could not be reached for the regular appointments.

In many cases, the people named as alternative contacts were also participants in the study. So the researchers looked to see which friends and relatives were also in the study, and then looked to see who became obese and when.

If someone became obese, their friends were 57 per cent more likely to become obese over the 32 years, they found. If people named one another as contacts, they were 171 per cent, or more than double, as likely to become obese if the other did.

The effect held for three degrees of separation. If a person became obese, their friends were more likely to become obese, but also friends of friends. It also occurred in shorter periods within the 32-year span.

"We think that these findings reinforce the idea that obesity is not just an individual problem but a collective problem," Christakis said.

"People look around them and see people gaining weight and it might change their attitude about what constitutes an acceptable body size," he added. "You might say it's OK to be heavier."

The findings held even if people lived 500 miles apart or even further. But having an obese neighbor did not affect a person's likelihood of becoming obese - which suggests that common environments are not to blame.

The findings struck a chord with Dr Amy Wachholtz of the Duke University Diet and Fitness Center in North Carolina.

"I do see this quite a bit clinically - that an individual will come in and say, 'I have been best friends with this person since college and we just slowly gained weight together,'" Wachholtz, who was not involved in the research, said in a telephone interview.

She said it reinforces the idea of encouraging people to diet and exercise in groups or with a friend.