Some children are allergic to milk, so they drink milk substitutes such as soy or rice. And almond milk has become a well-marketed fad to due health claims.

But there may be negatives: though many of those products are fortified, children who drink them have lower levels of Vitamin D in their blood than those who drink cow's milk, according to a paper in the Canadian Medical Association Journal. 

But why? The odd correlation is odd because there is actually very little vitamin D in cow's milk naturally. Milk sold in stores is instead fortified just like almond milk or any other substitute, because Vitamin D is important in maintaining calcium and phosphate levels - without it, the body naturally releases calcium from bones, which can lead to health issues for developing bodies, including rickets.

Milk is fortified with Vitamin D because bone development is so important in kids and the best natural source, sunlight, is not available to everyone throughout the year. 

There shouldn't be any reason that kids who drink a milk substitute have less Vitamin D. There could be a psychological factor in play - a child with a milk allergy or lactose intolerance could associate any milk with illness and not drink it - but the deficit was also found in kids who drink goat's milk.

The difference, say the authors of a new study, is that only cow's milk is required to be fortified by law, 40 units per 100 milliliters. Adding vitamin D to non-cow's milk is voluntary.  

"Children drinking only non-cow's milk were more than twice as likely to be vitamin D deficient as children drinking only cow's milk," said Dr. Jonathon Maguire, a pediatrician and researcher with St. Michael's Hospital. "Among children who drank non-cow's milk, every additional cup of non-cow's milk was associated with a five per cent drop in vitamin D levels per month.  

"It is difficult for consumers to tell how much vitamin D is in non-cow's milk. Caregivers need to be aware of the amount of vitamin D, calcium and other nutrients in alternative milk beverages so they can make informed choices for their children."

The study involved 3,821 healthy children ages one to six. Researchers looked at differences in blood levels of vitamin D associated with drinking cow's milk and non-cow's milk. The children were recruited from seven Toronto pediatric or family medicine practices that are part of a research network called TARGet Kids!. 87 percent of children involved in the study drank predominantly cow's milk and 13 per cent drank non-cow's milk.