A new American study shows that obesity spreads among friends and family with close social ties, even when they don’t live near each other. The research is the first to explore the impact of social networks on the spread of obesity, and will help to develop new clinical and public health initiatives.
The research was funded by the US National Institute on Aging (NIA), part of the National Institutes of Health (NIH) and is published in the current online edition of the New England Journal of Medicine (NEJM).
One third of adult Americans, that is 66 million men and women, are obese, which significantly increases their risk of developing serious health problems such as type 2 diabetes, heart disease and stroke.
“We need to learn as much as we can about contributing factors. This study describes social network influences that might be an important part of that equation,” said Richard J. Hodes, Director of the NIA.
The researchers who conducted the study were Drs Nicholas Christakis of the Harvard Medical School, and James Fowler of the University of California in San Diego. They said that a sedentary lifestyle and eating more high calorie food are important reasons why obesity has risen steeply, but they suggest that the attitudes, behaviours and acceptance of obesity among family and friends in a person’s social network also play a strong role.
For their research, Christakis and Fowler used data from the Framingham Heart Study (FHS). The FHS is administered by the National Heart, Lung and Blood Institute, (NHLBI), also part of the NIH, and comprises a large scale, long term project that is now taking data from grandchildren of the original cohort enrolled in the 1940s.
Christakis and Fowler examined weight, height and other data from over 5,000 FHS participants at seven dates between 1971 and 2003. At the start of the study their ages ranged between 21 and 70, with an average of 38. They called these key participants “egos”.
They also looked at the data relating to the parents, spouses, siblings, children and close friends of the key participants, to find out whether obesity spreads from person to person in social networks. These were called “alters”.
The total number of people they explored, egos and alters together, formed a large social network of over 12,000 individuals.
Christakis said that:
“We were able to reconstruct a large network of individuals who had been repeatedly weighed over time as part of the Framingham Heart Study, and we could see that as one person gained weight, those around him or her gained weight.”
“We didn’t find that people who were overweight simply flocked together. Rather, we found what seemed to be a spread of obesity and that the likelihood of a person becoming obese depended on the nature of the relationship,” explained Christakis.
The findings showed that:
- The chance of a key participant becoming obese went up by 57 per cent if he or she had a close friend who became obese.
- If he or she had a same sex friend who became obese, this figure went up to 71 per cent.
- But no such increase in a person’s chance of becoming obese was found in opposite sex friendships.
- Perception of friendship was also important: when both the key participant and his or her friend said their friendship was close, the key participant’s chance of becoming obese was 171 per cent if the friend became obese.
- They were not likely to become obese however if only the friend claimed the friendship was close.
- Among pairs of brothers and sisters, one becoming obese increased the chance of the other one becoming obese by 40 per cent.
- This chance was higher among same sex siblings as opposed to opposite sex siblings.
- Among married couples, the chance of a husband or wife becoming obese if the spouse became obese was increased by 37 per cent.
- The spread of obesity in a social network was not dependent on geographic distance.
- Social distance, the degree of separation between two people in the network, had greater influence than geographic distance.
- A geographic neighbour becoming obese did not increase a key participant’s chance of becoming obese.
- Smoking behaviour did not affect the spread of obesity from person to person.
Christakis said:
“We identified distinct clusters of obese people within social networks, and the clusters spread about three people deep.”
“People who were only one degree removed from each other socially, such as siblings or close friends, influenced one another twice as much as people who were two degrees removed from each other,” he added.
The study concluded that:
“Network phenomena appear to be relevant to the biologic and behavioral trait of obesity, and obesity appears to spread through social ties. These findings have implications for clinical and public health interventions.”
“The rising rate of obesity threatens to reverse the decline in disability in the older population, with major implications for the health care system,” said Dr Richard Suzman, Director of the NIA’s Behavioral and Social Research Program.
“This seminal study breaks important new ground in showing how social networks may amplify other factors and help account for the dramatic increase in obesity across the population,” he said.
“The Spread of Obesity in a Large Social Network over 32 Years.”
Nicholas A Christakis and James H Fowler.
N Engl J Med 2007 357: 370-379.
Volume 357:370-379, July 26, 2007, Number 4
Written by: Catharine Paddock