However, the study also showed that people who want to lose weight actually tend to acquire heavier friends over time. This may occur because people who are overweight or obese experience less stigma and weight discrimination when they have heavier friends, the researchers said.
Still, although such changes in people's inner circles "may be consistent with managing weight stigma, they also tend to undercut weight loss," the researchers wrote in the July issue of the journal Obesity.
Previous research has found that people's chances of becoming obese increase when their friends become obese.
In the new study, researchers analyzed information from more than 9,300 people ages 18 to 65 in the United States who participated in a Gallup poll that asked questions about people's weight and social networks.
Participants were asked whether they wanted to lose weight, stay the same weight or gain weight. These people were also asked to give information about the four people who they spent the most social time with, which could be friends or relatives. Participants rated the body mass of these four people as being thinner, heavier or the same as their own, and they also reported how frequently they interacted with these contacts. One year later, the participants completed follow-up surveys with the same questions.
The findings showed that people who said they wanted to lose weight were more likely to have at least one contact who was heavier than themselves, and more likely to have a greater number of interactions with heavier people, than those who reported wanting to stay the same weight, the study said. People who wanted to lose weight were less likely to have a social contact who was thinner than themselves one year later, compared to people who wanted to stay the same weight.
However, people who did add a thinner friend to their social network during the year were more likely to lose weight. Adding a thinner friend was linked with a decrease in body mass index (BMI) of 0.08, which translates to about a 0.6-lb. (0.3 kilograms) reduction in weight for a person who is 5 foot 10 inches (178 centimeters) tall.
What's more, each additional 100 social interactions with a thinner person were linked with a 0.06-lb. (0.03 kg) weight loss, while each 100 interactions with a heavier person were linked with a 0.1-lb. (0.04 kg) increase in weight, the study found.
Still, the results don't mean that people should ditch their heavier friends, the researchers said. The study had a number of limitations that prevented the researchers from understanding the reasons behind the findings. For example, it's not clear if people who wanted to lose weight acquired heavier friends because they joined a weight-loss group, the researchers said.
"What we don't know is what respondents are doing with their social contacts," study researcher Matthew Andersson, an assistant professor of sociology at Baylor University in Waco, Texas, said in a statement. "They might be going out to eat. They might be going to the gym...we just don't know," said Andersson, who conducted the research while at Yale University.
In addition, the study relied on people's reports of their own weight and the body sizes of their friends, rather than actual measurements of weight and body size, which could have affected the results, the researchers said.
Future studies are needed to look into the reasons behind the findings, the researchers said.