(PhysOrg.com) -- A new University of Michigan study finds that college women with roommates who weigh more than average gain less weight during their freshman year than women with slimmer roommates: half a pound versus 2.5 pounds.
That compares to the typical freshman weight gain of 2.5-to-6 pounds—much less than the mythical "Freshman 15."
"This finding seems counterintuitive, but there are some good explanations for why it may be happening," said Kandice Kapinos, an assistant research scientist at the U-M Institute for Social Research.
According to Kapinos, a labor and health economist, heavier roommates are more likely than average-weight women to diet. They also exercise more often and are more likely to use weight loss supplements and purchase college meal plans that limit access to food.
"It's not really the weight of your roommate that's important, but the behaviors your roommate engages in," Kapinos said. "These behaviors are what may really be 'contagious.'"
Kapinos conducted the study with Marquette University economist Olga Yakusheva. The study is the first to assess college weight gain using a natural experiment occurring on most college campuses in the United States—randomized roommate assignments.
"Previous studies have suggested that having an obese spouse, friend or sibling increases one's likelihood of becoming obese," Kapinos said. "But these relationships are obviously not random. People pick their friends and spouses, and they often select people who are similar to themselves. And even though we don't pick our siblings, we share a genetic inheritance and an early environment that may influence adult weight."
For the current study, which was presented this summer at the annual meeting of the American Society of Health Economists, the researchers assessed 144 female college students randomly assigned to share a living situation during their freshman year. At the start of the fall semester, the researchers obtained the women's weight and height, and asked about weight management behaviors. These included whether they had tried to lose weight at any time during the previous year, the average number of times per week they went to the gym and exercised outside, and whether they had signed up for an unlimited college meal plan.
The topic of peer influences on weight gain and weight management is important since obesity prevalence in young adults, aged 18 to 29, increased by 96 percent from 1988 to 2006—the largest percentage increase for all age groups.
Another study Kapinos and Yakusheva conducted found that freshmen assigned to dormitories with onsite dining halls gained more weight than those who had to venture outside of their dorms for food. Later this fall, the researchers will expand their study of the issue by analyzing a larger sample of students at a public university to see if roommate weight patterns persist. They will also examine other environmental influences and see if the findings vary with race, ethnicity or socioeconomic status.
"Our hope is that this line of research will have practical implications for university administrators and more generally for public health efforts aimed at reducing obesity," Kapinos said.
Explore further: Study: 'Freshman 15' is a myth -- almost