(HealthScoutNews) -- As early as age 11, girls are more worried than boys about their weight.

Even worse, while boys shed their concerns about being overweight as they mature, girls become more even more worried that they're too fat, new research shows.

The study looked at more than 2,000 students in Glasgow, Scotland, at ages 11, 13 and 15. The researchers found the percentage of boys worried about being overweight dropped from 30 percent to 23 percent from the ages of 11 to 15. Meanwhile, the percentage of girls worried about their weight jumped from 40 percent to 70 percent over the same period.

Yet during this time, the prevalence of being overweight among all the students went up only one percentage point.

"People have known this anecdotally, but it is interesting to see it in the data," says University of Glasgow researcher Helen Sweeting, lead author of the study. "And to have tracked so many children through three ages adds weight to the results."

The study appears in today's issue of the Journal of Epidemiology and Community Health.

"This seems to be a significant validation of previous, smaller studies on attitudes towards weight," adds Connie Diekman, head of the nutrition department at Washington University in St. Louis, who treats college women with eating disorders.

"It is clear that the college women exhibit effects of eating disorders that started well before they came here," Diekman says. "This study provides a lot of insight to see how their processes of thought begin."

For the study, the researchers used data from a large health survey of schoolchildren in the Glasgow area. School nurses examined the students at ages 11, 13 and 15 to determine their body mass index (BMI), a measure of body fat based on height and weight. The children also filled out questionnaires that asked them if they were worried about putting on weight and if they were on a diet.

Predictably, the average BMIs increased as the children got older. The girls' BMIs increased slightly more than the boys' because girls gain more fat during adolescence. However, the girls' worries about weight mushroomed while the boys' concerns diminished.

At age 11, for instance, 30 percent of both boys and girls who were overweight said they were dieting. But by age 15, only 16 percent of the overweight boys were on a diet, while 48 percent of the overweight girls were dieting.

Similarly, even those girls in the "medium-weight" brackets were much more likely to be dieting than the boys as they got older. At age 11, 8 percent of medium-weight girls were on diets, while 4 percent of medium-weight boys reported dieting. By age 15, 26 percent of medium-weight girls were on diets, compared to only 3 percent of medium-weight boys.

Sweeting says the data show the need for more education about appropriate weights, especially for girls. "I wonder if we've lost sight of what's OK, and what's normal for weight," she says.

"Girls need to get the message as to what's healthy. They do need more body fat than boys. It's important," agrees Diekman, who's also a spokeswoman for the American Dietetic Association.

What To Do

For information about eating disorders, visit the National Eating Disorders Association. To calculate your body mass index, see the Centers for Disease Control and Prevention.

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