"We started to research the question because professional recommendations around milk intake were unclear and doctors and parents were seeking answers," said Dr. Jonathon Maguire, a pediatrician at St. Michael's Hospital and the lead author of the study.

Dr. Maguire and his team looked at how cow's milk affected body stores of iron and vitamin D -- two of the most important nutrients in milk -- in more than 1,300 children aged two to five years.

The results of the study appeared online in Pediatrics December 17.

They found that children who drank more cow's milk had higher Vitamin D stores but lower iron stores.

"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," Dr. Maguire said.

The researchers recruited healthy children during routine doctor's appointments between 2008 and 2010. Parents were asked to fill out an extensive questionnaire about their children's milk drinking habits and other factors that could affect iron and Vitamin D stores. A blood sample was obtained from each child to determine body stores of iron and Vitamin D.

The children were participating in TARGet Kids!, a unique collaboration between children's doctors and researchers from St. Michael's Hospital and The Hospital for Sick Children. The program follows children from birth with the aim of understanding and preventing common nutrition problems in the early years and their impact on health and disease later in life.

The study also suggested that children with darker skin pigmentation may not have enough vitamin D stores during the winter months. Dr. Maguire suggested that instead of consuming more milk to increase these levels, wintertime vitamin D supplementation may be a more appropriate way of increasing vitamin D stores while preserving iron stores.

"Vitamin D deficiency in children has been linked to bone health issues and iron deficiency has been linked to anemia and delays in cognitive development," Dr. Maguire said. "Being able to answer parent's questions about healthy cow's milk intake is important to avoiding these potentially serious complications of low vitamin D and iron stores."

The Canadian Paediatric Society recommends that cow's milk not be started before one year of age. The study was supported in part by the Canadian Institutes of Health Research and the St. Michael's Hospital Foundation.