Researchers said data on more than 12,000 people suggested the risk was increased by 57% if a friend was obese, by 40% if a sibling was and 37% if a spouse was. They argued this showed social factors, such as the body sizes of other people, were important in developing obesity.
Researchers at Harvard Medical School and the University of California in San Diego looked at data collected over 32 years as part of a heart study. Participants gave personal information, including their body mass index, and the names of friends who could be contacted. The authors were able to map social connections including both friends and family members.
The effects were generally larger between people of the same sex. And their analysis suggested that the links could not be solely attributed to similarities in lifestyle and environment, for example, the impact of friends existed even where friends lived in different regions. The statistics may be meaningful, but in real life this is not very helpful to people who are overweight .
Author Professor Nicholas Christakis said: “It’s not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship. 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.”