NEW ORLEANS — Young adults might want to change their wedding vows to say they are taking each other "for better or girth."

Many married and single people in their late teens and early 20s gain a significant amount of weight — an average of 15 to 30 pounds — over a five-year period.

But newly married men and women in this age group gain six to nine pounds more than their peers who are single and dating.

These are among the findings of a new study released here Monday at the annual meeting of the Obesity Society, a group of weight-loss scientists and professionals.

The study confirms other research that shows young adults are especially vulnerable to putting on pounds, and gaining weight is socially contagious.

"The weight gain in this age group is frightening," says Penny Gordon-Larsen, an assistant professor of nutrition in the school of public health at the University of North Carolina-Chapel Hill.

She has done other research on this age group and found the number of people who become obese — 30 or more pounds over a healthy weight — increases significantly from the teens to young adulthood.

Gordon-Larsen and her colleagues followed almost 8,000 people, ages 12 to 28, over five years and a sub-sample of 1,200 couples. Among their findings:

• Women in their teens and early 20s who continued to date during early adulthood but didn't cohabitate gained an average of 15 pounds over five years; their male counterparts added about 24 pounds.

• Newly married women in this age group packed on 24 pounds in five years; newly married men, 30 pounds.

This degree of gain wasn't seen in couples who were living together but not married. Woman gained three pounds more than their single peers —18 pounds — and men gained the same 24 pounds.

"When people are dating, there may be more incentive to be thin," Gordon-Larsen says.

Single young adults tend to be the most active, watch the least amount of TV and are the least likely to be obese, says Natalie The, also a researcher at the University of North Carolina.

She says many factors likely contribute to couples' weight, including post-pregnancy pounds, having children and less time to exercise and eating out more or cooking bigger meals.

A study released this summer showed one person's obesity can significantly increase the chance that his or her friends, siblings and spouse also will become heavy, and if a person slims down, the people around him or her also may lose weight.

"Ideally, we'd like to shift the influence couples can have on each other to positive behaviors so they can support each other to get to a healthy weight and maintain it," Gordon-Larsen says.