NEW YORK (Reuters Health) - An "extremely high" proportion of pregnant women living in the northern United States, and their newborns, have insufficient vitamin D levels, and taking prenatal vitamins may not increase vitamin D levels adequately.

That's according to University of Pittsburgh researchers who assessed the vitamin D status of 200 black and 200 white pregnant women and their newborns living in Pittsburgh.

"In our study, more than 80 percent of African-American women and nearly half of the white women tested at delivery had levels of vitamin D that were too low, even though more than 90 percent of them used prenatal vitamins during pregnancy," Dr. Lisa M. Bodnar said in a university statement.

"The numbers also were striking for their newborns," Bodnar said, with 92 percent of African-American babies and 66 percent of white infants found to have inadequate vitamin D concentrations in their blood at birth.

"While black women and their newborns clearly carried the burden of vitamin D deficiency, white women were not immune to this problem," Bodnar told Reuters Health.

"My colleagues and I expected vitamin D insufficiency to occur frequently in our population, but we had no idea the rates would be this staggering," she added.

Writing in The Journal of Nutrition, the researchers say the finding of widespread vitamin D insufficiency in spite of prenatal vitamins is troubling and suggests that higher doses, different vitamin formulations, or a moderate increase in sunlight exposure in northern latitudes might be needed to boost vitamin D to healthier levels.

In a commentary, Marjorie L. McCullough of the American Cancer Society, Atlanta, says the Pittsburgh study provides "compelling evidence" that current formulations of prenatal vitamins fail to achieve adequate vitamin D levels.

This study is timely, McCullough adds, due to the reemergence in the US of rickets -- a condition marked by a softening and malformation of the bones that arises from deficiency of vitamin D.

"Very healthy people may have poor vitamin D status," Bodnar said. "This condition is extremely common -- even in non-pregnant adults and older children -- and is associated with serious health outcomes like cancer, heart disease, skeletal problems, asthma, schizophrenia, and type 1 diabetes."

She concluded: "Most of the food that we eat contains relatively modest amounts of vitamin D, so the best way to get enough is by taking 1000 international units of vitamin D3 per day in the form of a supplement."