How much your friends weigh could influence your own weight, according to a new study published in the journal PLoS ONE.

Researchers from Loyola University found that students were likely to gain weight if their friends were heavier than they were. However, if their friends were leaner, they were more likely to slim down, or gain weight at a slower pace.

In addition, David Shoham, Ph.D., an assistant professor in the Department of Preventive Medicine & Epidemiology of Loyola University Chicago Stritch School of Medicine, and colleagues also found that a student’s social network (face-to-face friends, not Facebook friends) influences how much they participate in sports.

Shoham explained: “These results can help us develop better interventions to prevent obesity. We should not be treating adolescents in isolation.”

The researchers set out to determine why obesity and related behaviors appear to cluster in social networks. Is it because teenagers make friends with people who look similar to themselves or is it because friends influence one another’s behavior?

The team analyzed data from two high schools:

  • Jefferson High – located in a rural area and has primarily white students
  • Sunshine High – located in an urban area with a substantial racial and ethnic diversity

Both schools participated in the National Longitudinal Study of Adolescent Health. Students filled out questionnaires regarding their weight, friendships, sports activities and screen time during the 1994-1995 school year and then again during the 1996-1997 school year. In addition, students body mass index (BMI) was also measured. A BMI of 25+ is considered overweight and a BMI of 30+ is considered obese.

The team analyzed data from 624 students at Jefferson High and 1,151 students at Sunshine Hight.

The researchers found a strong association between obesity and a student’s circle of friends. For instance, if a borderline overweight student at Jefferson High had lean friends there was a 40% chance the student’s BMI would decrease in the future. However, if they had obese friends their was only a 15% chance the would lose weight.

According to the researchers, their study shows 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.”

Limitations of the study included:

  • The data was collected over 10 years ago – prior to Facebook and when childhood obesity rates were significantly lower
  • The model makes assumptions about how friendships form, are maintained, and dissolve, which the researchers where not able to test directly
  • Social network studies are observational rather than experimental, which limits researchers’ ability to call the associations causal

Shoham concluded: “Our results support the operation of both homophily and influence. Of course, no one study should ever be taken as conclusive and our future work will attempt to address many of these limitations.”

Written by Grace Rattue