That may come as a surprise to parents who pride themselves on seeking out fruit drinks with no added sugar.

"Juice is definitely a part of this," said lead researcher Jean Welsh of the Centers for Disease Control and Prevention.

While fruit juice does have vitamins, nutritionists say it's inferior to fresh fruit. The new U.S. dietary guidelines, for example, urge consumers away from juice, suggesting they eat whole fruit instead.

The bottom line, though, is that "children need very few calories in their day," Welsh said.

"Sweet drinks are a source of added sugar in the diet."

She said preschoolers were better off snacking on fruit or drinking water or milk.

Welsh's research, published in the February issue of Pediatrics, found that for three-and four-year-olds already on the heavy side, drinking something sweet once or twice a day doubled their risk of becoming seriously overweight a year later.

The sweet drinks seemed to have little effect, however, on children of normal weight.

The American Academy of Pediatrics recommends limiting preschoolers to four to six ounces of juice per day. Some parents and schools are paying attention.

One Chicago Head Start program banned juice last year as part of an anti-obesity effort after finding that one out of five of its students was obese.

Monica Dillion, community health nurse for the Howard Area Family Center, said the preschool also added more fruits and vegetables to meals and more exercise to the daily schedule. The preschool has never served soft drinks.

The juice ban drew no complaints, Dillion said. "The kids didn't notice at all."

The Pediatrics study followed 10,904 Missouri children in a nutrition program for low-income families. Researchers looked at the effect of sweet drinks in three groups: normal and underweight children, those at risk of becoming overweight, and those who already were overweight.

The researchers compared the children's heights and weights, approximately one year apart. They also looked at parents' reports of what their children ate and drank during a four-week period at the beginning of the first year. Fruit drinks like Kool-Aid and Hi-C were included as sweet drinks, along with juice and soda.

The link between sweet drinks and being overweight showed up for all three weight categories, although it wasn't statistically significant for the normal and underweight children.

Taking into account other differences, such as ethnicity, birth weight and a high-fat diet, didn't erase the effect of sweet drinks.

The children in the study drank, on average, more fruit juice than soft drinks or sweetened fruit drinks.

Sweet drinks are high in calories and low in fibre. Nutritionists believe that calorie-dense, low-fibre foods may lead to overeating because those foods are quickly consumed but less filling than foods higher in fiber.

The authors suggest that limiting sweet drinks may help solve the growing problem of childhood obesity. One in five American children is overweight, according to the National Institutes of Health.

The study defined at-risk children as those whose size put them in the 85th to 95th per centile in growth charts. A child in the 85th per centile would be heavier than 85 per cent of children of the same gender and age.

Richard H. Adamson, vice president for scientific and technical affairs at the American Beverage Association, questioned the study's methods, saying it didn't take into account television viewing, overweight parents and the children's activity levels.

But Dr. Rebecca Unger, who evaluates overweight children in private practice and at Children's Memorial Hospital in Chicago, said the study backs up what she sees in the real world.

"We do see kids do well when we cut out juice," she said. "Sometimes that's all they need to do."

Source: Associated Press