When we think of how people gain weight, we often think of oversized soft drinks, McDonald’s restaurants on every corner, or a life that is increasingly positioned in front of a screen.
But that’s not where it ends. Our propensity to gain weight now seems, at least partly, to be determined by the genes inherited from parents, and some research even suggests that viruses and gut bacteria may also play a role.
Now, researchers around the world are investigating whether obesity is contagious, not transmitted through viruses or bacteria, but through our social networks.
There’s a famous adage that you are the average of the five people you spend the most time with. This was first used to describe the power our peers and close circle have in shaping our outlook and behaviours, but has increasingly been turned to as a possible explanation for the tripling of obesity rates across the world since 1975.
Globally, around two billion adults are overweight or obese. And if the people around us are heavier, we are more likely to be heavier.
Conversely, one person’s weight loss may also have a ripple effect on the people around them. Just this month, University of Connecticut researchers found that the partners of individuals enrolled in a weight-loss program seemed to mimic the participants’ weight loss.
“When one person changes their behaviour, the people around them change,” lead author, behavioural psychologist Professor Amy Gorin says. “Whether the patient works with their healthcare provider, joins a community-based, lifestyle approach like Weight Watchers, or tries to lose weight on their own, their new healthy behaviours can benefit others in their lives.”
The study, which was funded by Weight Watchers, tracked 130 cohabiting couples and found that despite not receiving any weight-loss intervention themselves, one in three untreated partners achieved a clinically significant 3% body weight reduction over the six-month follow up.
What’s more, couples appeared to follow the same weight-loss trajectories, meaning that if the participant lost weight more rapidly than expected, their partner was significantly more likely to have a steeper weight-loss trajectory.
Self-guided weight loss was just as effective as the Weight Watchers intervention, however.
According to the authors, changing our eating and exercise behaviours can affect those around us both positively and negatively.
“On the positive side, spouses might emulate their partner’s behaviours and join them in counting calories, weighing themselves more often, and eating lower-fat foods,” Professor Gorin says.
While it may seem intuitive that if your partner stops bringing home Tim Tams, you’ll be less likely to eat them, the effect appears to be much more far-reaching.
In a landmark study that brought the idea of obesity as a social contagion to the public, investigators found that it was friends, and even friends of friends, who could lead to you adding on the kilos.
In fact, the magnitude of the effect seemed to be even larger than the role of genes. The analysis revealed that if a person’s friend became obese, their chances of becoming obese increased by 57%, and if their sibling or spouse became obese then the chances rose by around 40%.
The study, published in the New England Journal of Medicine, applied advances in mapping theory and technology to the public health sphere.
Using the three decades of data from the Framingham Heart Study, the US researchers tracked the relationship between weight gain and friends and family among the 12,000 participants. Unsurprisingly, the entire network became heavier over the course of the study.
But after controlling for potentially confounding factors such as socioeconomic status, gender and predicted weight gain with age, the authors concluded that obesity seemed to be spread through social ties.
In fact, your risk of developing obesity would be 20% higher if a friend of a friend became obese, and 10% higher if it was a friend of a friend of a friend – or three degrees of separation.
Because participants were asked at the start of the study to nominate friends at the outset, and the investigators had access to medical data from three generations of families, they were able to map out the social connections and examine where obesity clustered over the generations.
When trying to understand whether weight gain is socially contagious, it is important to note that friends and family members often share the same environment. If this environment has little access to grocery stores, poor pavements for walking and few outdoor areas to do activities, then it would be no surprise that these friends and families tend to put on weight together.
Interestingly, the study revealed that physical proximity was not particularly significant. A neighbour gaining weight did not appear to have any impact at all on an individual’s waistline, whereas a good friend on the other side of the country did have a substantial effect.
“When we looked at the effect of distance, we found that your friend who’s 500 miles away has just as much of an impact on your obesity as [one] next door,” said study author Professor James Fowler, who was an associate professor of political science at University of California San Diego at the time.
Even more compellingly, they found that if both participants nominated the other as a friend, the risk of one becoming obese was threefold higher if the other became obese. But if the friendship wasn’t mutual, then it made no difference if someone who considered you a friend became obese.
Could the explanation be that birds of a feather flock together?
Humans have a tendency to be drawn towards people who are similar to them, whether it is in values, behaviours or appearance.
A study of 617 US adolescents found that teens who were overweight were twice as likely to have overweight friends, and it is quite plausible that people who are overweight are drawn to other people who are overweight because they enjoy the same activities, eating habits or the subconscious familiarity.
But Professor Fowler and his colleague controlled for participants’ starting weight to account for this tendency, known as homophily, as well as the possible influence of genetics and childhood experiences.
Given the spread of obesity through the social network regardless, the authors suggest that the explanation may lay in our friends changing our perception of what is normal.
“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,” explained study co-author Professor Nicholas Christakis, a sociologist and physician.
This was further illustrated by the finding that weight gain was greater among friends of the same sex, presumably because they are more likely to be models for our own behaviour.
“This is about people’s ideas about their bodies and their health,” Professor Fowler said. “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.”
Social influence had likely been underappreciated, Professor Fowler said.
“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.”
A 2013 study of 53 young Japanese women who had recently moved to New York City backs this idea up.
Investigators asked the women, who had moved countries within the last month, about their perception of their current weight and their ideal weight at the start of the study and again two months later.
After only two months of living in the United States, where body weight is higher than their home country, the women perceived their bodies as smaller and at the same time, their ideal body size increased.
The shrinking gap between current and ideal body size was also correlated with an increase in BMI, suggesting that being around bigger people makes you more comfortable with being bigger
On the flip side, a 2005 study of 119 young women found that being around a thinner than average person increased body dissatisfaction, even among those who had high self-esteem and were perceived by others to be attractive.
The potential for social networks to help us become a healthier weight was emphasised by Professor Fowler and Professor Christakis.
“When we help one person lose weight, we’re not just helping one person, we’re helping many,” Professor Fowler said. “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.”
“It’s important to remember,” Fowler said, “that we’ve not only shown that obesity is contagious, but that thinness is contagious”.
A NATURAL EXPERIMENT
While the last decade of research had made it clear that people with obesity tend to cluster within social networks, the jury was still out on whether it was a result of social contagion, or whether it could be better explained by the environment or some underlying tendency to be drawn to others like us.
Professor Christakis’ and Professor Fowler’s paper was not without criticism. For starters, replication is vital in science, and yet the uniqueness of the Framingham Heart Study, both in terms of its size, duration and information about friendship ties, meant that it would likely be too expensive to repeat.
Follow up analyses have suggested that the impact of friends and family on weight gain may have been less than the authors originally found. Other critiques argued that it was not possible to differentiate social contagion from homophily based on their observational data.
Then in January this year, Californian researchers published a unique paper in JAMA Pediatrics, which tracked the weight of military families as they moved across the country.
“Disentangling the extent to which the clustering of obesity within networks is due to social contagion vs the competing explanations of self-selection (i.e., homophily and residential selection) and shared environment is crucial because of their different implications for public health policy making,” said the study authors, Dr Ashlesha Datar, from the University of Southern California, and Dr Nancy Nicosia, from US think tank RAND Corporation.
The economists explained that if obesity was contagious, then policies should lean to targeting social networks, specifically targeting well-connected individuals to make the most of their wide-reaching effects.
Social contagion would also favour interventions that interfered with potentially harmful changing norms and attitudes, they wrote. On the other hand, if obesity is spread through a shared environment then money and resources should be aimed at the built environment or policies. Or if the relationship is explained by self-selection and homophily, there’s much less use for interventions to address either of these targets, they explained. Luckily, the way military families move across the country offered a natural experiment. The fact that the decision to move areas is in the hands of the military, not the families themselves, makes the case that any correlations between weight and the surrounding area are not due to self-selection and homophily.
Moreover, the investigators were able to factor in how much time the families spent living on the installation or within the community, as a proxy for how much they were exposed to the obesity rate in the community.
Among the 1500 military families who participated in the study, those who were sent to live in a county with a higher obesity rate were more likely to be overweight or obese than those sent to counties with lower obesity rates.
“Greater exposure to the county, proxied by longer time at installation and by off-installation residence, was associated with stronger observed correlation between county obesity rates and the body masses of military families,” the authors wrote.
In fact, for each percentage point higher the county’s obesity rate was, the odds of the parent being obese rose 5% and the odds of the child being overweight or obese rose by 4% to 6%.
But while this type of study appears to control for homophily, it is possible that there was something about the environment that caused both the native population and the new families to gain weight.
To account for that, Dr Datar and Dr Nicosia tracked both objective and subjective measures of the shared built environment.
They found that the association remained regardless of how close the population was to a park, grocery store or recreational facility, their incomes and whether the area was perceived as being safe, easy to move around in and be active in.
One explanation for social contagion is that individuals mirror the behaviours of those around them.
However, the authors noted that mirroring was more common when individuals admired those whose behaviours they were emulating. “It is therefore less likely to be a primary mechanism in our geography-based network,” they said.
However, for endocrinologist Emeritus Professor Joseph Proietto it is important to not overstate the role that others have in our own weight gain.
The obesity expert stresses the importance of genes and epigenetics as the predominate factor behind weight-loss.
While viruses or obese friends may contribute to a couple of extra kilograms, Professor Proietto says that “social interaction will not turn a genetically lean person into an obese person”.
Despite social influences garnering less attention from researchers than factors such as diet, exercise and genetics, Professor Kylie Ball from the Institute for Physical Activity and Nutrition at Deakin University said they were likely to be “potent”.
Her own research into this revealed that women who perceived others as having a healthy diet and lifestyle were more likely to engage in those behaviours themselves.
“We are social animals and we live in communities,” Professor Ball said.
“Conforming to social norms was a critical part of most societies.”
It’s likely that many people shared behaviours with people close to them, she said.
“If we sit down for meals together with our friends or family, often we’re sharing the same kinds of foods and the same kinds of portion sizes, and also potentially modelling behaviours.”
A growing body of research will be needed to understand the role that our social environment and connection to others plays on our own waistline.