(TIME.com) -- Got milk? It turns out that low-fat versions may not be the answer to helping kids maintain a healthy weight.
Long a staple of childhood nutrition, milk is a good source of calcium and vitamin D, which can help to build bone, and experts believed that lower-fat versions could help children to avoid the extra calories that came with the fat in whole milk.
But in a study published in the Archives of Disease in Childhood, scientists found that skim and low-fat milks may not be as useful for weight loss as experts had hoped.
Since 2005, both the American Academy of Pediatrics (AAP) and the American Heart Association (AHA) recommended that children drink skim or low-fat milk after age 2.
Because whole milk has more calories from fat than skim, 1% fat or 2% fat versions, the thinking was that the lighter varieties would help youngsters avoid weight gain and curb the growing problem of overweight and obesity in childhood.
Not all of the studies supported this idea, however; some found no relationship between the type of milk preschoolers drank and their body weight, while others found that skim milk drinkers were heavier than their whole milk-drinking counterparts.
So Dr. Mark Daniel DeBoer, an associate professor of pediatric endocrinology at the University of Virginia School of Medicine, and his colleagues turned to a large database of 10,700 children in the Early Childhood Longitudinal Survey, Birth Cohort to investigate the relationship between the type of milk children drank and their body mass index (BMI).
The survey tracked the long-term health of a population of American kids born in 2001, and the researchers interviewed the caregivers and parents of the kids when they were 2 and again when they were 4 about what kind of milk the kids drank: skim, 1% semi-skimmed milk, 2% milk, full fat milk or soy. When the children were 4 years old, the scientists also asked the parents about how often the children consumed other beverages, including fruit juices, sports drink and sugared sodas.
The findings surprised DeBoer and his team. Overweight or obesity rates were relatively high among the children at both ages; 30% of the 2-year-olds and 32% of the 4-year-olds had BMIs that qualified them as overweight or obese.
But it wasn't the normal-weight children who were drinking the most skim or 1% fat milk. In fact, after adjusting for the effect of the other beverages, 14% of the heavier 2-year-olds drank 1% milk compared to 9% of normal-weight youngsters, and 16% of the overweight or obese 4-year-olds drank the 2% milk compared to 13% of the normal-weight 4-year-olds. Overall, children who drank the 2% milk showed lower BMI scores than those drinking the 1% milk.
Even more puzzling, when DeBoer analyzed how choice of milk might be affecting weight over time, he found that children who were normal weight at the start of the study and consistently drank the 1% milk showed a 57% increased chance of becoming overweight or obese by the they were 4.
"Our original hypothesis was that children who drank high-fat milk, either whole milk or 2%, would be heavier because they were consuming more saturated fat calories. We were really surprised when we looked at the data and it was very clear that within every ethnicity and every socioeconomic strata, that it was actually the opposite, that children who drank skim milk and 1% were heavier than those who drank 2% and whole," says DeBoer, who is also the chair-elect for the AAP Committee on Nutrition.
DeBoer says when they broke down the data into the different types of milk with increasing fat content, the findings were even more striking. As BMI scores went up among the kids, the amount of fat in the milk they were drinking went down. "So the ones drinking skim were by far the heaviest, and those drinking whole milk were the lightest," he says.
One explanation for the results might be that parents of heavier children may be more likely to switch their youngsters to 1% or skim milk in an effort to lower the amount of calories they drink. But the data on normal-weight 2-year-olds who drank 1% milk and still gained weight contradicts this idea.
"In that sense it was a little deflating that skim milk and 1% milk didn't restrain the amount of weight gain in the groups over time," says DeBoer.
It's also possible that the skim milk drinkers were more likely to consume more calories overall; DeBoer and his colleagues were not able to incorporate data on other foods that the children ate. The study also did not account for genetic factors and family history of obesity.
"I think it's an interesting paper, but not necessarily conclusive," says Dr. Linda Van Horn, a nutritional spokesperson for AHA and a professor of Preventive Medicine for the Northwestern University Comprehensive Center on Obesity. "It is important to remember that this study was not directly observed. It was 100% dependent on parental reports, and that gets harder as the kids get older. When they are away at daycare and grandma's, you just don't know what they eating."
Dr. Stephen Daniels, the chairman of the department of pediatrics at the University of Colorado, agrees.
"We do not know enough about the rest of their diet or their activity levels," he says. "So, one possibility is that the kids on skim would be on an even steeper rise for weight had they been on whole milk. The investigators point out that the rate of gain between the two groups between ages 2 and 4 were not significantly different, making it less likely that skim milk was a cause of increased weight gain."
The fact that whole milk drinkers may still end up weighing less than skim milk drinkers despite the added calories from fat actually does make some sense, however. It's possible, for example, that the fat in whole milk may help kids to feel more full, and therefore eat fewer calories in a day than those who drink skim milk.
Based on the results, DeBoer and his colleagues suggest that current recommendations on milk be reconsidered, especially since the relationship between milk and the calories that children consume may be more complicated that previously thought.
They write: "The logic behind these AAP recommendations is that if children drink reduced-fat milk, this results in overall fewer calories consumed. It has been well established that as compared to traditional plant-based diets, Western diets high in saturated fat are associated with increased weight gain.
"In both children and adults, key contributors to the current obesity epidemic are the high-fat diets increasingly consumed worldwide. Encouraging consumption of low-fat/skim milk instead of high-fat milk promotes a reduction of 5.5-22.5 grams of fat (50-202 calories) daily among children drinking one to three cups of milk per day."
The added calories in milk may not end up being such an important contributor to weight gain, especially among young children. And there are other, better-documented ways to helping youngsters to keep weight down.
"There are other interventions that pediatricians can advocate for that have a better evidence basis than this one," says DeBoer. "With the limited amount of time a pediatrician or family practitioner has with a patient, emphasizing other points that are more likely to be efficacious may be a better approach than focusing on this one, which logic would dictate would be effective, but this observation suggests it may not be."
Pediatricians could be focusing on recommending less television viewing, for instance, and more physical activity, as well as advising parents to limit children's consumption of juices and sugar-sweetened beverages.
DeBoer agrees that studies designed to look directly at the link are needed. "The big caveat here is that if you were able to keep the rest of your diet exactly the same, switching from whole milk to skim milk should be able to decrease your total number of calories, but those don't exist in isolation. I do think that going back and thinking which strategies to fight weight are going to be effective is an important step. I do think we should look at this again, certainly with more detailed randomized studies."
This article was originally published on TIME.com.