The recipe for a healthy child should include no more than 2 cups of milk a day, Canadian researchers say.


But in the wintertime, milk alone might not be enough for kids with darker skin to get the Vitamin D they need, pediatricians from Toronto wrote in a study published in the journal Pediatrics on Monday.


In the study, researchers followed more than 1300 children between two and five years of age from December 2008 to December 2010. The kids' parents reported how much cow's milk they drank, and used blood samples to determine the childrens' Vitamin D and iron levels.


Though milk provides important nutrients like Vitamin D and iron, too much of a good thing isn't necessarily good for kids.


"We saw that two cups of cow's milk per day was enough to maintain adequate vitamin D levels for most children, while also maintaining iron stores. With additional cow's milk, there was a further reduction in iron stores without greater benefit from vitamin D," St. Michael's Hospital pediatrician Jonathan Maguire said in a statement Monday.


Since going beyond two cups of milk per day doesn't seem to help, Maguire suggests that kids with darker skin might need Vitamin D supplements in the winter months.


Keeping Vitamin D levels up in kids is essential, since deficiency can have wide-ranging effects on a child's bones, heart, immune system and metabolism. Vitamin D deficiency early in childhood can cause overt problems, like bone deformities and rickets. But because Vitamin D deficiency only rarely causes noticeable symptoms, it can often lurk undetected.


One large-scale study published in Pediatrics in 2009 estimated that about 1 in 10 American children are deficient in vitamin D, and about 60 percent of American children are thought to have suboptimal levels of the vitamin. Risks for childhood vitamin D deficiency include breast-fed infants, obese children, kids with certain conditions like cystic fibrosis, diabetes, and gastrointestinal disorders.


Darker skin raises a person's risk for vitamin D deficiency because higher melanin levels interfere with the interactions between sunlight and 7-dehydrolcholesterol, a naturally-occurring steroid molecule in human skin that can be converted into vitamin D by ultraviolet light. As a result, darker-skinned people need to stay in the sun much longer than fairer-skinned people to make adequate amounts of the vitamin.


“Vitamin D deficiency can be a problem year round, but because sun exposure is critical for vitamin D synthesis and production, the winter months further exacerbate what is a perennial problem,” Johns Hopkins Children’s Center endocrinologist Dominique Long said in a February statement.


SOURCE: Maguire et al. “The Relationship Between Cow's Milk and Stores of Vitamin D and Iron in Early Childhood.” Pediatrics published online 17 December 2012.