May 2, 2006

Women with low vitamin D levels have small infants

By Amy Norton

NEW YORK - Pregnant women with relatively low amounts of vitamin D in their diets tend to give birth to smaller infants, a new study suggests.

Canadian researchers found that pregnant women who drank little milk or had a lower vitamin D intake tended to have smaller babies than women with higher intakes.

Fortified milk is a primary source of vitamin D in Western diets, and the nutrient may be the main reason why women's milk consumption was linked to birthweight, according to the researchers.

"Absence of vitamin D seems to be what's driving this," study co-author Dr. Kristine G. Koski, of McGill University in Montreal, told Reuters Health.

What's more, the study suggests, vitamin D deficiency is not required to potentially affect fetal growth.

Although vitamin D intake was lower among women who avoided milk, it was still, on average, within the recommended range. This suggests that pregnant women need more than the current "adequate" intake level for vitamin D - 5 micrograms for adults age 50 and younger, Koski said.

She and her colleagues report the findings in the current issue of the Canadian Medical Association Journal.

Vitamin D may be best known for its role in bone health, but it's also believed to be important in immune system function and growth and cell maturation, among other jobs.

There has been little research, however, into women's vitamin D needs during pregnancy. The adequate level set for all younger adults is based on decades-old research on the amount of vitamin D necessary to prevent the bone disease rickets.

The current study included 279 pregnant women who were interviewed several times about their diet and use of vitamin and mineral supplements. Seventy-two women restricted milk intake, while the rest did not restrict their consumption.

On average, the researchers found, infants born to women who drank less than one cup of milk per day were more than 4 ounces lighter than newborns whose mothers drank more milk.

Neither protein nor calcium -- two key nutrients in milk -- predicted birthweight. But vitamin D intake did, with each extra microgram translating into a small gain in newborn weight.

Women who don't drink milk, Koski said, should be sure to get vitamin D from supplements and other foods that are fortified with the vitamin, such as certain brands of breakfast cereal, soymilk and margarine.

This is especially important, she noted, for women who live in colder climates where the skin gets limited sun exposure, which triggers the synthesis of vitamin D in the skin.

The findings also support calls from some experts that North American officials reconsider their definition of adequate vitamin D intake -- as has been done in Europe, according to Koski and her colleagues.

"The reality is that we do not know what the actual vitamin D requirement during pregnancy is," write Drs. Bruce Hollis and Carol Wagner of the Medical University of South Carolina, in an accompanying editorial.

"For that matter," they add, "we do not know the requirement for the general population, either."

Hollis and Wagner are currently studying the effects of giving pregnant women vitamin D doses that are much higher than what is currently thought to be an adequate level.

SOURCE: Canadian Medical Association Journal, April 25, 2006.