The finding is based on a study by researchers from Harvard Medical School and the University of California, San Diego, who found that obesity spreads through social ties.
They also found that when a person piles on the pounds, the chance that people around them - friends, siblings and spouses - will also gain weight dramatically increases.
The researchers also noted that the closer two people are in a social network, the more likely it is that they will put on weight - and that's something that doesn't change even if they happen to live miles apart.
"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. 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," said. Nicholas Christakis, MD, PhD, a professor in Harvard Medical School's Department of Health Care Policy.
As a part of the study Dr Christakis and U.C. San Diego researcher James Fowler, PhD, analysed data from the Framingham Heart Study (an ongoing cardiovascular study begun in 1948) to see if any social patterns affected the growing obesity rates.
They derived information from archived, handwritten administrative tracking sheets dating back to 1971. All family changes for each study participant, such as birth, marriage, death, and divorce, were recorded. In addition, participants had also listed contact information for their closest friends. Coincidentally, many of these friends were also study participants.
The researchers then aimed their focus on 12,067 individuals, who had a total of 38,611 social and family ties. Other factors that figured in were gender, smoking, socioeconomic status, and geographic distance.
The study found that when an individual becomes obese, the chance that a pal will become obese increases by 57 percent, siblings by 40 percent, and a spouse by 37 percent.
A factor that was found to play an important role obesity was gender, with the Christakis and Fowler finding that the risk of obesity increases by a whopping 71 percent in same-sex friendships. This pattern was also observed in siblings - among brothers the risk increases by 44 percent and among sisters it increases by 67 percent.
Friends and siblings of opposite genders showed no increased risk and neither did neighbours who were not in the social group.
"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," says Christakis.
"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 okay to be bigger since those around them are bigger, and this sensibility spreads," he added.
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. 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," says Fowler.
Funded by the National Institute on Aging, the study appears in the July 26 edition of the New England Journal of Medicine.