If you want teenagers to lose weight, whatever you do, don't tell them to go on a diet. That most likely will make matters worse, according to a new study. University of Minnesota researchers found that parents who perceived their kids as overweight tended to use only one strategy—advising them to diet. But five years later, those kids were far more likely to be heavier than kids whose parents said nothing. In short, it's a technique that seems certain to backfire, said Dianne Neumark-Sztainer, a professor of epidemiology at the university and the lead author of the study. "My concern is that if parents know their kids are overweight, they are going to do things that lead to further weight gain over time," she said.

Neumark-Sztainer said she decided to research the issue because of the growing practice of schools evaluating kids' weight and sending the results home to parents. In some places it's called an "obesity report card." It began because parents often don't know whether their kids are at a healthy weight, and some experts believe telling them will help prevent childhood obesity. However, the results show that this is not the case.

Researchers looked at survey results for 300 adolescents and some of their parents. The kids, all from Minnesota middle and high schools, reported heights and weights that put them in the overweight category. They found that 46 percent of girls' parents and 60 percent of boys' parents incorrectly thought their kids' weights were about right. Of the parents who thought their kids were too heavy, about 60 percent encouraged them to diet.

Five years later, about 200 of the kids were surveyed again. Those who had been encouraged to diet were much more likely to be heavy—about 74 percent of boys compared with 52 percent of those boys not encouraged to diet. For girls, the difference was 66 and 44 percent, respectively. Both groups reported about the same eating patterns, including the frequency of fast food meals, and the quantity of fruits and vegetables at home.

Neumark-Sztainer said public health experts have known for years that adolescents and teenagers who say they diet are the ones who are most likely to have weight and eating disorder problems that can last for years. This study shows that just informing parents that their kids are overweight is counterproductive, she said.

"If you are going to talk with parents about their children's weight, you need to specifically help them make positive changes at home," said Neumark-Sztainer. One of the worst things parents could do was pressure their children. "There seems to be a fine line between helpful and harmful parenting," the researchers explained in the study.

The best thing parents can do to be role models, said Neumark-Sztainer, is to provide and eat healthful food, have regular family meals, and do physically active things with their kids. "Do more. Talk less," she said.