Are your friends making you fat?  

Public-health officials have been working hard to account for the dramatic rise in U.S. obesity rates. Many obvious factors, such as poor diet and a sedentary lifestyle, certainly contribute to the swelling statistics. But these and other explanations tend to focus exclusively on how individuals’ choices and behaviors affect their weight.  

Now, researchers from Harvard Medical School and the University of California, San Diego, have found that obesity is hardly a private matter. Reporting in the July 26 edition of the New England Journal of Medicine, the researchers assert that obesity spreads through social ties. When an individual gains weight, it dramatically increases the chances that friends, siblings, and spouses will likewise gain weight. The closer two people are within a social network, the stronger the effect. Interestingly, geographical distance between people in a social network appears to have no effect.  

“What we see here is that one person’s obesity can influence numerous others to whom he or she is connected, both directly and indirectly,” says Nicholas Christakis, MD, PhD, a professor in Harvard Medical School’s Department of Health Care Policy. “In other words, 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.”  

Over the last 25 years, the incidence of obesity among U.S. adults has more than doubled, shooting from 15% to 32%. In addition, roughly 66% of U.S. adults are considered overweight.  

The study found that when an individual becomes obese, the chances that a friend will become obese increase by 57%. His or her siblings have a 40% increased risk of obesity and his/her spouse a 37% increased risk. But the individual’s neighbor, if not a part of his/her social network, has no effect.  

Gender played an important role in how these statistics broke down. In same-sex friendships, individuals experienced a 71% increased risk if a friend became obese. This pattern was also observed in siblings. If a man’s brother became obese, his chances of becoming obese increased by 44%. Among sisters, the risk was 67%. Friends and siblings of opposite genders showed no increased risk. While the researchers note that correlations among siblings provide evidence for a biological, and possibly even a genetic, component to obesity, patterns seen among friends indicate there’s more than biology at work.  

Social connections seem to be key.  

“The fact that neighbors don’t affect each other and that geographic separation doesn’t influence the risk among siblings or friends tells us that environmental factors are not essential here,” Dr. Christakis says. “Most likely, the interpersonal, social-network effects we observe arise not because friends and siblings adopt each other’s lifestyles. It’s more subtle that that. 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 OK to be bigger since those around them are bigger, and this sensibility spreads.”  

“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,” adds UC San Diego researcher James Fowler, PhD. “Social effects, I think, are much stronger than people before realized. There’s been an intensive effort to find genes that are responsible for obesity and physical processes that are responsible for obesity, and what our paper suggests is that you really should spend time looking at the social side of life, as well.”  

Obesity, the authors conclude, needs to be seen not simply as a clinical issue, but as a public-health problem.  

“We need to understand that a significant part of an individual’s health is embedded in their network,” Dr. Fowler says. “In fact, we really need to revisit our whole notion of cost-effectiveness. The fact that certain healthcare approaches won’t just affect the individual, but will also cascade through their social ties, means that healthcare interventions are far more cost-effective than previously thought.  

“When we help one person lose weight, we’re not just helping one person; we’re helping many. And that needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier.”  

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