The work, by scientists in Toronto, says that children between the ages of two and five should be drinking half a litre or approximately two eight-ounce cups of milk a day.
Less than that and kids may not be getting enough vitamin D, the study suggests. But more than that, and the stores of iron in their blood — which are essential for a developing brain — may start to slip below acceptable levels.
The study was led by Dr. Jonathon Maguire, a pediatrician and researcher at St. Michael's Hospital in Toronto. It is published in this week's issue of the journal Pediatrics.
"Cow's milk is a very important staple in our western diet for children. I don't want to underestimate the importance of cow's milk," Maguire said in an interview about the study.
"Our question was really: Well, how much?"
It's a query pediatricians face all the time, Maguire said. And they haven't had a good answer to give because expert groups are divided on the issue.
Some organizations have argued that young children should consume a litre of milk a day to get the vitamin D they need to build strong bones and avoid rickets, a formerly common bone-softening condition. (Milk is fortified with vitamin D.)
But other groups have warned that children's consumption of cow's milk should be curtailed because some studies have shown that kids who drink a lot of milk can have low levels of iron in their blood.
Low iron can lead to anemia, where the body produces too few of the red blood cells that transport oxygen throughout the body.
"It looks like in children who have iron deficiency severe enough to cause them ... to have anemia, those children have difficulties with their cognitive development. Over time they're not quite as bright as other children," Maguire said.
Iron deficiency in young children isn't uncommon in Canada. While it's just a guestimate — Maguire said recent studies haven't been done — it is believed between 10 per cent and 20 per cent of young children in Canada may have low iron stores.
Given the confusing advice and the fact that milk consumption by preschoolers seems to involve a trade-off between vitamin D and iron, Maguire and some colleagues decided to try to find the sweet spot.
They enrolled 1,311 healthy Toronto children aged two to five in a study, evaluating samples of their blood for vitamin D and iron stores and gathering information from parents about the amount of milk the kids drank.
The researchers found that about 500 millilitres of milk a day for most children was the right amount to have adequate levels of vitamin D and iron.
There was an exception: during winter, children with dark skin didn't hit the vitamin D target with 500 ml daily. The study suggests in winter children with dark skin may need a vitamin D supplement as well as the milk.
The researchers also saw this previously reported inverse relationship, where more milk consumed meant higher vitamin D levels but lower iron stores.
What's behind the puzzling interplay? The director of the nutrition and metabolism research program at B.C. Children's and Women's Hospitals said little kids who drink a lot of milk often aren't eating enough solid foods to get the needed amount of iron. (There is little iron in milk.)
Dr. Sheila Innis explained that some young children have a hard time making the transition from breast or bottle to solids. They may be drinking more milk because they still prefer to suck and swallow than to chew.
Innis said that parents of children like this should figure out what's going on rather than cutting back on the milk.
"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," she said from Vancouver.
In fact, Innis warned that trying to reduce milk intake in a child like this may provoke resistance and other problems. She urged parents in this situation to get help.
"Go see a good public health dietitian or nutritionist and get guidance on how to increase the variety and quantity of solid food in the diet. And then the milk intake will come down."
A similar conundrum the study identified related to children over two who drank from a bottle. Analysis of their blood samples suggested they weren't getting enough iron or vitamin D.
Maguire said this confirms something pediatricians see — many kids over two who still drink from a bottle are iron deficient.
"Given that it doesn't seem to be much of a benefit from cow's milk in the bottle for vitamin D and it looks like it decreases children's iron source, it's probably a good idea not to be using a bottle in children who are over two years of age," he said.
Innis said bottle feeding over age two is also probably a sign of a child who is having a hard time making the transition to solid foods.
"Still drinking out of bottles over two goes hand in hand with not taking a good variety of solid foods. Not chewing well. Not liking textures. Still on sucking," she said.
"It's important when we talk about children still drinking from a bottle over two, what else are they doing?"
The study was funded by The Canadian Institutes of Health Research and the St. Michael's Hospital Foundation.