Is weight loss "contagious"? According to a new study published online in the journal Obesity, teammates in a team-based weight loss competition significantly influenced each other's weight loss, suggesting that shedding pounds can have a ripple effect.

Researchers from The Miriam Hospital's Weight Control and Diabetes Research Center and The Warren Alpert Medical School of Brown University found that team members not only achieved similar weight loss outcomes, but participants who said their teammates played a large role in their weight loss actually lost the most weight.

"We know that obesity can be socially contagious, but now we know that social networks play a significant role in weight loss as well, particularly team-based weight loss competitions," said lead author Tricia Leahey, Ph.D., of The Miriam Hospital and Alpert Medical School. "In our study, weight loss clearly clustered within teams, which suggests that teammates influenced each other, perhaps by providing accountability, setting expectations of weight loss, and providing encouragement and support."

Obesity remains a common, serious and costly disease in the United States. About one-third of American adults are obese, according to the Centers for Disease Control and Prevention, and no state has met the nation's Healthy People 2010 goal to lower obesity prevalence to 15 percent. Obesity and its associated health problems, including heart disease and diabetes, continue to have a significant economic impact on the U.S. health care system, costing the nation hundreds of billions of dollars each year.

To promote cost-effective weight loss initiatives, online team-based weight loss interventions are increasing in popularity as a way to encourage weight loss in large groups of people. The current study is the first to examine the effects of teammates and social influence on individual weight loss during one of these weight loss competitions.

The findings are based on the results of the 2009 Shape Up Rhode Island (SURI) campaign, a 12-week statewide online weight loss competition designed by study co-author Rajiv Kumar, M.D. Participants joined with a team and could compete against other teams in three divisions: weight loss, physical activity and pedometer steps. The weight loss competition included 3,330 overweight or obese individuals (BMI of 31.2 or greater), representing 987 teams averaging between 5 and 11 members each. The majority of these individuals enrolled in all three divisions.

Weight loss outcomes were clearly determined by which team an individual was on. Participants who lost clinically significant amounts of weight (at least 5 percent of their initial body weight) tended to be on the same teams, and being on a team with more teammates in the weight loss division was also associated with a greater weight loss. Individuals who reported higher levels of teammate social influence increased their odds of achieving a clinically significant weight loss by 20 percent. This effect was stronger than any other team characteristic, Leahey said.

"This is the first study to show that in these team-based campaigns, who's on your team really matters," she added. "Being surrounded by others with similar health goals all working to achieve the same thing may have really helped people with their weight loss efforts."

However, Leahey noted that individual characteristics were also associated with weight outcomes. Obese individuals had a greater percentage of weight loss than overweight participants. Team captains also lost more weight than team members, possibly due to their increased motivation and engagement in the campaign. Leahey says that future weight loss team competitions may consider requiring team members to share the leadership role.

"We're all influenced by the people around us, so if we can harness this positive peer pressure and these positive social influences, we can create a social environment to help encourage additional weight loss," she said.

Leahey is a researcher with The Miriam Hospital's Weight Control and Diabetes Research Center and assistant professor of psychiatry and human behavior (research) at Alpert Medical School. Additional co-authors of the study include Rena R. Wing, Ph.D., director of The Miriam Hospital's Weight Control and Diabetes Research Center and professor of psychiatry and human behavior at Alpert Medical School; and Brad M. Weinberg, M.D. and Rajiv Kumar, M.D. co-founders of ShapeUp, Inc.