May 4, 2010 -- Women who take high doses of vitamin D during pregnancy have a greatly reduced risk of complications, including gestational diabetes, preterm birth, and infection, new research suggests.
Based on the findings, study researchers are recommending that pregnant women take 4,000 international units (IU) of vitamin D every day -- at least 10 times the amount recommended by various health groups.
Women in the study who took 4,000 IU of the vitamin daily in their second and third trimesters showed no evidence of harm, but they had half the rate of pregnancy-related complications as women who took 400 IU of vitamin D every day, says neonatologist and study co-researcher Carol L. Wagner, MD, of the Medical University of South Carolina.
Wagner acknowledges the recommendation may be controversial because very high doses of vitamin D have long been believed to cause birth defects.
"Any doctor who hasn't followed the literature may be wary of telling their patients to take 4,000 IU of vitamin D," she says. "But there is no evidence that vitamin D supplementation is toxic, even at levels above 10,000 IU."
Fewer Complications With High Vitamin D Doses
Most prenatal vitamins have around 400 IU of vitamin D, and most health groups recommend taking no more than 2,000 IU of the vitamin in supplement form daily. Wagner says it took months to get permission to do a study in which pregnant women were given doses of the vitamin that were twice as high as this.
The study included about 500 women in Charleston, S.C., who were in their third or fourth months of pregnancy. The women took 400 IU, 2,000 IU, or 4,000 IU of vitamin D daily until they delivered.
Not surprisingly, women who took the highest doses of vitamin D were the least likely to have deficient or insufficient blood levels of the vitamin, as were their babies.
These women also had the lowest rate of pregnancy-related complications.
Compared to women who took 400 IU of vitamin D daily, those who took 4,000 IU were half as likely to develop gestational diabetes, pregnancy-related high blood pressure, or preeclampsia, Wagner says. They were also less likely to give birth prematurely.
The research was presented over the weekend at the annual meeting of the Pediatric Academic Societies in Vancouver, British Columbia.
Infants with very low vitamin D levels are at increased risk for soft bones, or rickets -- a condition that is now rare in the U.S.
But over the last decade, more and more studies suggest that vitamin D also protects against immune system disorders and other diseases, Wagner says.
Fortified milk and fatty fish are common food sources of vitamin D, but most people get only a small fraction of the vitamin D they need through food, Wagner says. Instead, the body makes vitamin D from sunlight.
But even in sunny climates like Charleston, few people are now getting adequate levels of vitamin D from sun exposure.
At the start of the study, deficient or insufficient levels of vitamin D were seen in 94% of the African-American women, 66% of Hispanic women, and 50% of white women who participated.
Vitamin D and Pregnancy: Is More Better?
University of Rochester professor of pediatrics Ruth Lawrence, MD, has been recording vitamin D levels in new mothers and their infants for three years. She did not take part in the new study.
Lawrence, who chairs the breastfeeding committee of the American Academy of Pediatrics, says exclusively breastfed babies whose mothers have low vitamin D levels and who don't take vitamin supplements are most likely to be deficient.
"It is clear that both for mothers and their babies, vitamin D levels are low," she tells WebMD. "This is true in northern areas like Rochester and in sunny climates like Charleston."
Lawrence sees no problem with the recommendation that women take 4,000 IU of vitamin D daily during pregnancy, although she says the impact of high doses of vitamin D on pregnancy-related complications remains to be proven.
"Four thousand IU may sound outrageous to some, but I believe it is really not unreasonable," she says.
"We have been searching for the causes of preeclampsia and premature birth for many years. It is reassuring that the risk of these complications are lower for women taking extra vitamin D, but it is premature to say it is the cause."
The independent health policy group the Institute of Medicine recommends 200 IU to 400 IU of vitamin D a day for everyone, including pregnant women, but this recommendation is under review. Revised guidelines are expected late this summer.