|New NIH-Supported Study Characterizes Social
Networks of Family, Friends Influencing Obesity
People wondering about excessive weight gain might look to their
relationships with family and friends for one clue, suggests new
research reported July 26, 2007, in The New England Journal
of Medicine. The study showed that obesity spreads within
social networks and that the closer the social connection — even
if people live in different households many miles apart — the
greater the influence on developing obesity. The study, funded
by the National Institute on Aging (NIA), a component of the National
Institutes of Health (NIH), is the first to provide a detailed
picture of the social networks involved in obesity and could prove
useful in developing both clinical and public health interventions
The analysis was conducted by Nicholas Christakis, M.D., Ph.D.,
of Harvard Medical School, and James Fowler, Ph.D., of the University
of California, San Diego, using data from the Framingham Heart
Study. The Framingham Heart Study is supported by the National
Heart, Lung and Blood Institute, another NIH component.
“Nearly one in three American adults — 66 million men and
women — are obese, which puts them at risk for a number of
serious health problems, including type 2 diabetes, heart disease
and stroke. With the sharply rising rates of obesity in this country,
we need to learn as much as we can about contributing factors.
This study describes social network influences that might be an
important part of that equation,” says NIA Director Richard J.
A sedentary lifestyle and increased consumption of high-calorie
foods are critical factors in the steep rise in the prevalence
of obesity, the researchers note. But they suggest that a hierarchy
of influence exists among family and friends on developing obesity,
in which the attitudes, behaviors, and acceptance of obesity also
might play an important role.
To explore whether obesity spreads from person to person within
social networks, the research team gleaned weight, height and other
data from the records of 5,124 Framingham Heart Study participants
at up to seven time points between 1971 and 2003. In addition,
they analyzed similar information from the Framingham records of
these key participants’ parents, spouses, siblings, children and
close friends. Together, these individuals formed a large, intertwined
social web totaling 12,067 people. The average age of key participants
at the inception of the study was 38 years, with a range of 21
to 70 years.
“We were able to reconstruct a large network of individuals who
had been repeatedly weighed over time as part of the Framingham
Heart Study, and we could see that as one person gained weight,
those around him or her gained weight,” says Christakis. “We didn’t
find that people who were overweight simply flocked together. Rather,
we found what seemed to be a spread of obesity and that the likelihood
of a person becoming obese depended on the nature of the relationship.”
“The rising rate of obesity threatens to reverse the decline in
disability in the older population, with major implications for
the health care system,” says Richard Suzman, Ph.D., director of
the NIA’s Behavioral and Social Research Program. “This seminal
study breaks important new ground in showing how social networks
may amplify other factors and help account for the dramatic increase
in obesity across the population.”
- A key participant’s chances of becoming obese increased by
57 percent if he or she had a close friend who became obese.
- In same-sex friendships, a close friend becoming obese increased
a key participant’s chance of becoming obese by 71 percent. However,
no such association was found in opposite-sex friendships.
- The perception of friendship also was an important factor.
When two people identified each other as close friends, the key
participant’s risk of becoming obese increased by 171 percent
if his or her friend became obese. In contrast, a key participant
was not likely to become obese if someone claimed a close friendship
with him or her but the key participant did not report the friendship.
- Among pairs of siblings, one’s becoming obese increased the
other’s chance of becoming obese by 40 percent. This finding
was more marked among same-sex siblings than opposite-sex siblings.
- In married couples, one spouse’s becoming obese increased the
likelihood of the other spouse becoming obese by 37 percent.
Husbands and wives appeared to affect each other equally.
- Obesity spread across social ties, despite geographic distance
from one person to another. Further, social distance — the
degree of social separation between two people in the network — appeared
to make more of a difference than geographic distance in the
spread of behaviors and norms associated with obesity.
- An immediate neighbor’s becoming obese did not affect a person’s
risk of becoming obese.
- Smoking behavior was not associated with the spread of obesity
from person to person.
“We identified distinct clusters of obese people within social
networks, and the clusters spread about three people deep,” Christakis
says. “People who were only one degree removed from each other
socially, such as siblings or close friends, influenced one another
twice as much as people who were two degrees removed from each
NIA leads the federal effort supporting and conducting research
on aging and the medical, social and behavioral issues of older
people. For information on research and aging, go to www.nia.nih.gov.
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on health and aging can be viewed and ordered by visiting the NIA
website or can be ordered by calling toll-free 1-800-222-2225.
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Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.