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Women who restrict their intake of milk during pregnancy may deliver smaller babies, primarily because they are not getting the vitamin D contained in the beverage, a Canadian study suggests.

A collaborative study between Montreal's McGill University and the University of Calgary found that pregnant women who drank less than 250 millilitres of milk a day -- about a cup -- gave birth to infants who weighed less than babies of women whose daily consumption was higher.

In fact, every additional 250 millilitres of milk a pregnant woman consumed per day was associated with 41 grams of extra birth weight in newborns, say the researchers, whose work appears in today's issue of the Canadian Medical Association Journal.

Principal investigator Kristine Koski, director of McGill's School of Dietetics and Human Nutrition, said studies have shown that less than 15 per cent of all Canadian women drink cow's milk. Many say they have been advised by their doctors to avoid the beverage because of concerns such as food allergies and gastrointestinal upset due to lactose intolerance.

"Since the majority of women don't drink milk, I'm concerned that pregnant women" also don't, Dr. Koski said, noting that Canada's population is known to be deficient in vitamin D. "I think we, as health professionals, need to ask women if they're restricting their milk intake. If they are, we have to talk about the need to have a source of vitamin D in their diet, particularly in the winter months."

Vitamin D plays a role in many aspects of health, including the absorption of calcium for strong bones and teeth. Without adequate vitamin D, children can get the soft-bone disease, rickets, and studies have shown they are at greater risk for osteoporosis later in life. The vitamin is also believed to help ward off some cancers.

But there are few natural sources of vitamin D, which is made by the body through skin exposure to sunlight -- something that is often lacking in many parts of Canada during the winter. Some foods, such as salmon and other ocean fish, contain vitamin D, but few Canadians make them part of their daily diets.

Dairy milk is required by federal guidelines to be fortified with vitamin D, and "that still stands as a basically good recommendation," said Dr. Koski, whose study was funded in part by the Dairy Farmers of Canada. "It doesn't mean that other foods couldn't be fortified. . . . We're not trying to advertise for the dairy farmer, we're trying to advertise for the need to look at the quality of the maternal diet," she said, noting that she approached the agricultural group for funding. It was initially reluctant to finance the research, she said.

The study involved 279 pregnant women, 19 to 45, who were enrolled through three Calgary hospitals, between 1997 and 1999. Almost a third of participants drank less than 250 millilitres of milk daily; the others consumed varying amounts above that level. Almost 90 per cent were non-smokers (tobacco is the major cause of low birth weights). The women reported their intake through regular in-home questionnaires by researchers.

Dr. Vyta Senikas, a vice-president of the Society of Obstetricians and Gynecologists of Canada, said the organization follows the Canada Food Guide in recommending a minimum of three servings of dairy products daily for pregnant women. Among calcium-rich milk, cheese and yogurt, only milk is fortified with vitamin D, she said.

Getting vitamin D through sunlight can also be problematic, said Dr. Senikas. Pregnant women are usually advised to avoid sun exposure due to chloasma -- or the mask of pregnancy -- in which the skin turns splotchy, a condition worsened by sunlight and one that is sometimes permanent.

"I think what is clear is that the daily requirement of vitamin D may have to be looked at because the original assessments (400 IU a day for pregnant women) under which they were done were quite arbitrary," she said.

"The issue at the end of the day in Canada is: Do we have to consider a higher vitamin D content for pregnancy, or a supplement for women -- in addition to the prenatal supplement -- who are not drinking milk? The answer from this article appears to be yes."

In a CMAJ commentary, Drs. Bruce Hollis and Carol Wagner of the Medical University of South Carolina called the study's findings "a very intriguing and important observation." They added it is "important to determine the true vitamin D requirement during pregnancy, not only for maternal skeletal preservation and fetal skeletal formation, but also for fetal 'imprinting' that may affect neurodevelopment, immune function and chronic disease susceptibility later in life as well as soon after birth."