Young children who are obese have an increased risk for becoming socially isolated in elementary school.
Researchers in Australia followed more than 3,000 children for four years.
They measured their height and weight at the start of the study, and interviewed parents about their children's' mental health.
They found children who were obese at the start of the study were 20% more likely to have difficulty with their peers four years later.
For example, they were subject to teasing and had trouble making friends.
Researchers say parents should work hard to help their children follow a healthy lifestyle, and encourage them to exercise and participate in group activities.