Children are urged to drink plenty of milk but a study published on Monday suggests that the more milk that kids drink, the fatter they grow -- and skim milk is a worse culprit than whole milk.

A survey of more than 12,000 children aged 9 to 14 showed that those who drank more milk weighed more than those who drank less.

"Children who drank the most milk gained more weight, but the added calories appeared responsible," the team at Brigham and Women's Hospital and Harvard University in Boston wrote in their report, published in the journal Archives of Pediatrics & Adolescent Medicine.

But there was a surprising finding.

"Contrary to our hypotheses, dietary calcium and skim and 1 percent milk were associated with weight gain, but dairy fat was not," they wrote.

It could be that the youngsters drink lower-fat milk more freely. Thus, it may not be milk itself but the calories in milk that are to blame, said biostatistician Catherine Berkey, who led the study, in a statement.

An eight-ounce (225-ml) serving of whole milk has 150 calories, one percent milk has 100 calories in an eight-ounce cup and skim milk has 85 calories.

"The take-home message is that children should not be drinking milk as a means of losing weight or trying to control weight," Berkey said.

An estimated 16 percent of U.S. children weigh too much.

SWITCHING TO WATER

Dr. Walter Willett of the Harvard School of Public Health, who worked on the study, said he was concerned about the heavy advertising of milk.

"The basic beverage should be water," Willett added. "We know that in many parts of the world, kids don't drink any milk at all and they end up with healthy bones."

In March a study in the journal Pediatrics showed that exercise was at least as important for building strong bones in children as eating calcium-rich foods was.

Willett said leafy green vegetables are rich in calcium and other nutrients that Americans lack in their diets, and are low in calories.

Willett's team followed 12,829 U.S. children, aged 9 to 14 in 1996, through 1999. The children, all offspring of nurses taking part in a big health study, filled out regular questionnaires on eating and lifestyle habits.

"Children who drank more than 3 servings a day of milk gained more in BMI than those who drank smaller amounts," the researchers wrote.

BMI stands for body mass index, a comparison of height to weight that is accepted as a good way to measure whether someone is overweight.

Children who drank more than three servings a day were 25 percent more likely to become overweight than those who drank two to three servings a day.

"The US Department of Agriculture Food Guide Pyramid recommends 2 to 3 servings per day from the milk, cheese, and yogurt group, primarily to promote adequate calcium intake for the prevention of osteoporosis in old age," the researchers noted.

"Given the high prevalence of lactose intolerance, the energy content and saturated fat in milk, and evidence that dairy products may promote both male (prostate) and female (ovarian) cancers, we should not assume that high intakes are beneficial," they added.

"Furthermore, these cancers may be linked to consumption during adolescence."

Nutrition Professor Dr. Marion Nestle of New York University said the study did not clearly show that milk causes weight gain.

"It's one more study. One way or another it's a very small effect. If it were a big effect, it would be consistent," Nestle said in a telephone interview.