"For each one microgram increase of dietary vitamin D (food plus supplements), birth weight increased by 11 grams," wrote lead author Cynthia Mannion, from the University of Calgary.
Increased intakes of the vitamin have been linked to a wide range of benefits including reduced incidence of certain cancers, and lower risk of osteoporosis. A recent study from the University of Southampton reported that higher intake of vitamin D during late-stage pregnancy was linked to stronger bones in children.
The new study, published in the Canadian Medical Association Journal (Vol. 174, pp. 1273-1277), reports that women with low milk consumption during pregnancy had lighter babies, an association linked to the vitamin D content of the milk.
Most of the body's vitamin D is made on exposure to sunshine, but small amounts can be consumed from milk, fish, liver and egg yolk. In Canada, where the new study was performed, milk is fortified with the vitamin, with a 250 ml cup providing about 90 IU. In the UK, where milk is not fortified, a 250 ml cup contains only a trace of the vitamin.
Mannion, and colleagues, Katherine Gray-Donald and Kristine Koski from McGill University, screened the milk consumption of 279 pregnant women. Seventy-two of the women consumed less than 250 ml of milk every day.
"Restricting fortified milk or vitamin D intake during pregnancy lowered infant birth weight in otherwise healthy, non-smoking, well-educated mothers," reported Mannion.
The researchers found that by drinking an extra 250 ml cup of milk every day could increase the weight of their baby by 41 grams.
"This is an important finding because increasing numbers of women are restricting milk consumption during pregnancy, believing that it will lower fat intake, minimize weight gain, treat self-diagnosed lactose intolerance or prevent their children from developing allergies," said the researchers.
A 250 ml cup of milk also contained significant amounts of protein, riboflavin and calcium, but the researchers reported that none of these nutrients were associated with an effect on birth weight.
The mechanism involved is not clear, but Mannion suggests that the vitamin may be more important for faetal development than previously thought.
The authors called for doctors to question expectant mothers about their milk consumption and multivitamin supplements in order to avoid low birth weight babies, particularly in northern climates when and where exposure to sunlight is limited.
Deficiency of the vitamin is estimated to affect approximately 60 per cent of the UK population. This is reflected across northern Europe where sunshine levels are so weak for six months of the year that the skin makes no vitamin D.
In an accompanying editorial, Bruce Hollis and Carol Wagner from the Medical University of South Carolina, said that Mannion's work was a "very intriguing and important observation."
"The reality is that we do not know what the actual vitamin D requirement during pregnancy is. For that matter, we do not know the requirement for the general population, either," they said.
The European and US recommended daily intake (RDI) of vitamin D is 5 micrograms (200 International Units). A recent study in the American Journal of Public Health called for a change of the RDI to 25 micrograms (1000 IU).
For further clarity concerning the requirements and tolerable limits, we may have to wait over two years. Hollis and Wagner are currently conducting a multiyear, double-blind placebo-controlled clinical trial to examine the effect of a daily supplement of 4000 IU during pregnancy.
"All we can say so far is that some mothers attain a robust nutritional vitamin D status with no observable adverse side-effects," said Hollis and Wagner.