Drinking two cups of milk is sufficient to maintain levels of vitamin D and iron in children, says a new study from Canada.

Researchers say that drinking more than 16 oz of milk may actually be bad for a child's health as more milk increases vitamin D levels, but decreases levels of iron in the body.

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

The study included 1,311 children aged between 2 and 5 years. Researchers assessed children's milk intake and the level of vitamin D and iron in their body.

Researchers found that children who drank two cups of cow's milk per day had the required amount of vitamin D and iron, but children who drank more than two cups of milk a day had lower levels of vitamin D and iron.

According to Medline Plus, cow's milk is a common cause of iron deficiency as it contains very little amount of iron and it also makes it difficult for the body to absorb iron from other foods. Consequently, children who drink a lot of cow's milk are at an increased risk of developing iron deficiency. Cow's milk can even cause the intestine to lose small amounts of blood.

Researchers added that children with darker skin may require additional vitamin D supplementation during winter months.

The study is published in the journal Pediatrics.