A person's circle of friends influence his/her body weight, reveals
study. Students were more likely to gain weight if they had friends who
were heavier than they were. Conversely, students were more likely to get trimmer -- or gain weight at a slower pace -- if their friends were leaner than they were.
Results of the study by David Shoham, PhD, and colleagues are
published in the journal PLoS ONE
. Shoham is an assistant
professor in the Department of Preventive Medicine & Epidemiology of
Loyola University Chicago Stritch School of Medicine.
A student's social network also influences how active he or she
is in sports. (By social networks, researchers mean face-to-face
friends, not Facebook friends.)
"These results can help us develop better interventions to prevent
obesity," Shoham said. "We should not be treating adolescents in
The study was designed to determine the reason why obesity and
related behaviors cluster in social networks. Is it because friends
influence one another's behavior? (This explanation is called "social
influence.") Or is it simply because lean adolescents tend to have lean
friends and heavier adolescents tend to have heavier friends? (This
explanation is called "homophily, or more informally, "Birds of a
feather flock together.") Researchers used a sophisticated statistical
technique to determine how much of the link between obesity and social
networks is due to social influence and how much is due to homophily.
This statistical technique is called "stochastic actor-based model," or
The researchers examined data from two large high schools that
participated in the National Longitudinal Study of Adolescent Health
(Add Health). One school, referred to as "Jefferson High," is in a rural
area and has mostly white students. The second school, "Sunshine High,"
is an urban school with a substantial racial and ethnic diversity.
Students were surveyed during the 1994-95 school year and surveyed again
the following school year. Researchers examined data from 624 students
at Jefferson High and 1,151 students at Sunshine High. Previously,
researchers not affiliated with the current study asked students about
their weight, friendships, sports activities and screen time. The body
size measure they used was body mass index (BMI), which is calculated
from a student's height and weight. A BMI over 25 is considered
overweight and a BMI over 30 is considered obese.
Researchers found that part of the reason why obesity clusters in
social networks was due to the way students selected friends. But even
after controlling for this friend-selecting process, there still was a
significant link between obesity and a student's circle of friends. For
example, if a borderline overweight student at Jefferson High School had
lean friends (average BMI 20), there was a 40 percent chance the
student's BMI would drop in the future and a 27 percent chance it would
increase. But if a borderline overweight student had obese friends
(average BMI 30), there was a 15 percent chance the student's BMI would
decrease and a 56 percent chance it would increase.
The findings, researchers concluded, show that social influence
"tends to operate more in detrimental directions, especially for BMI; a
focus on weight loss is therefore less likely to be effective than a
primary prevention strategy against weight gain. Effective interventions
will be necessary to overcome these barriers, requiring that social
networks be considered rather than ignored."
Shoham noted the study has several limitations. All of the
measures were based on self-reported data, which has known biases.
Social network studies are observational rather than experimental, which
limits researchers' ability to call the associations causal. The model
also makes assumptions about how friendships form, are maintained, and
dissolve over time, and these assumptions could not be directly tested.
Also, the data were collected more than a decade ago -- before Facebook
and at a time when childhood obesity rates were much lower.
Nevertheless, Shoham believes these results add to the vigorous
debate over the relative importance of selection and peer influence in
network studies of health. "Our results support the operation of both
homophily and influence," he said. "Of course, no one study should ever
be taken as conclusive and our future work will attempt to address many
of these limitations."