Some pediatricians are advising that parents only give their toddlers two cups of milk a day, because too much of it could deplete them of necessary nutrients.

A new study, published online on Dec. 17 in Pediatrics, found that 2.1 cups of milk a day gave young children the sufficient amount of vitamin D without lowering levels of iron, a potential side effect of drinking too much milk.

“For each additional cup of milk, it reduces the iron stores by a little bit,” said study author Dr. Jonathon Maguire, a pediatrician at St. Michael’s Hospital and the Hospital for Sick Children in Toronto, said to CBC News. “But in children who are at risk for iron deficiency for example, that little bit actually is very important.”

The American Heart Association recommends fat-free and low-fat dairy foods should be part of a balanced diet, including two cups of milk or its equivalent for children 1 through 8 and three cups of milk or its equivalent for children 9 through 18. The 2010 Dietary Guidelines put out by the United States Department of Agriculture recommends three cups of milk for adolescents between 9 and 18, 2.5 cups for children 4 to 8 and two cups for children 2 to 3.

Milk and other dairy products have been known to increase protein, calcium, magnesium, folate, B1, B2, B6, B12, and vitamins A, D, and E intake, according to WebMD. However, they can increase the amount of calories a person eats.

Cow’s milk can cause iron deficiency because it contains less iron than other foods it may replace, and also makes it harder for the body to absorb iron from other foods. Cow’s milk can also make the intestines lose a small amount of blood, further depleting iron. The National Institutes of Health said that the resulting condition, iron deficiency anemia, is more common in infants younger than 12 months who drink cow’s milk and young children who drink a lot of cow’s milk instead of eating foods with more iron. Dr. Emilia Baczek, a pediatric nutritionist at La Rabida Children’s Hospital in Chicago, told HealthDay that the calcium in milk is what affects iron levels.

“The calcium makes iron absorption a little bit harder,” Baczek said. “Toddlers often aren’t getting enough iron in their diet in the first place.”

Researchers looked at blood samples from 1,311 children between the ages of two to five who went to the dotor for a routine visit. They were asked how many cups of cow’s milk their child drank daily or if they used a bottle.

On average, children drank 1.9 cups of milk a day. Each cup of milk increased their vitamin D levels by 6.5 percent, but lowered their iron levels by 3.6 percent. Drinking from a bottle did not increase vitamin D levels, but lowered iron levels.

Baczek said the two-cups-per-day recommendation was a good call.

“It’s enough to promote adequate vitamin D with minimal decreases in [iron],” she said.

Dr. Sheila Innis, director of the nutrition and metabolism research program at B.C. Children’s and Women’s Hospitals, added to the Vancouver Sun that while milk sometimes takes the place of other iron-rich foods, the dairy product could help supplement the diet for some toddlers who picky eaters and are having a hard time transitioning from breast or bottle milk to solid food. Cutting milk for a kid who doesn’t eat much to begin with may make things worse, she said.

“It’s a complicated problem when you’re dealing with, say, a three-year-old child who is ... not a good eater. Stopping him drinking milk is not going to make that child a better eater,” said Innis.

She also warned that reducing milk intake may increase lactose resistance and other problems. She suggested in cases like these seeing a nutritionist or dietician can help parents find a balanced diet.

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Michelle Castillo
Michelle Castillo is an associate producer for CBSNews.com