The guidance is based on studies that found children who consumed low-fat milk as part of a reduced-saturated-fat diet had lower concentrations of LDL cholesterol. Given the body of evidence in adults linking high cholesterol to increased risk of heart disease, it makes sense to keep an eye on cholesterol, beginning in childhood.

And if you take fat out of milk, you've also reduced calories, which should help protect kids against becoming overweight. At least, that's been the assumption.

So here's where things gets confusing. A new study of preschool-aged children published in the Archives of Disease in Childhood, a sister publication of the British Medical Journal, finds that low-fat milk was associated with higher weight.

That's right, kids drinking low-fat milk tended to be heavier.

"We were quite surprised" by the findings, Dr. Mark DeBoer told me in an email. He and his co-author, Dr. Rebecca Scharf, both of the University of Virginia, had hypothesized just the opposite.

But they found the relationship between skim-milk drinkers and higher body weights held up across all racial/ethnic and socioeconomic groups. DeBoer says their data also show that low-fat milk did not restrain weight gain in preschoolers over time.

The study included about 10,700 children in the United States. Parents were interviewed about their child's beverage consumption on two occasions: once when the children were 2 years old and again at 4 years. Direct measurements of height and weight (to calculate body mass index) were taken by researchers.

Interestingly, this is not the first study to point in this direction.

In a 2005 study, researchers at Brigham and Women's Hospital reported that skim and 1 percent milk were associated with weight gain among 9-to-14-year-olds.

And a 2010 study by researchers at Children's Hospital in Boston, which also looked at preschool-aged children, found that higher intake of whole milk at age 2 was associated with a slightly lower BMI (body mass index). The researchers concluded that switching from whole milk to reduced-fat milk at 2 years did not appear to prevent weight problems in early childhood.

When you look at these studies together, DeBoer's findings become more intriguing, though it's unclear how higher fat could lead to lower weight.

One theory: It's possible that whole milk gives us a greater sense of satiety.

"This is speculative," says DeBoer, but if you feel fuller after drinking whole-fat milk, "it may be protective if the other food options are high in calories." In other words, if whole-fat milk saves a kid from eating an extra cookie or a second serving of mashed potatoes, he or she may end up eating fewer calories overall.

As the authors acknowledge, one of the shortcomings of the new study is that the researchers did not know how many calories the children were consuming overall or what types of foods they were eating.

So is it time to think anew about switching toddlers to low-fat milk?

"I don't think there is harm in rethinking a recommendation, particularly if there weren't rigorous data behind it," says DeBoer. He says he hopes his results lead to further, more definitive studies.

But not everyone is convinced. "I do think that the recommendation to give low-fat milk at age 2 is sound advice," says Dr. Stephen Daniels, a pediatrician at the University of Colorado and member of the AAP's Committee on Nutrition.

"I don't think the link between low-fat milk and higher weight makes much sense from a biological perspective," he says.

Some of the earliest studies evaluating diets low in saturated fat and cholesterol did find a link to less obesity among girls. And Daniels points out that in the new study, the toddlers who were on low-fat milk were already heavier.

"This leaves open the real chance that parents may have been choosing low-fat milk as a weight-management strategy for those who were already overweight," Daniels says.

Parents, if this leaves you confused, one thing to keep in mind is that — whether it's whole, 2 percent or skim — milk is probably not a major driver when it comes to childhood weight problems. Many studies have shown that sugar-sweetened beverages play a much bigger role.

Copyright 2013 NPR.

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Whole Milk Or Skim? Study Links Fattier Milk To Slimmer Kids