A new study finds that women who develop a severe form of pregnancy-related high blood pressure tend to have lower blood levels of vitamin D than healthy pregnant women — raising the possibility that the vitamin plays a role in the complication.
The condition is known as early-onset severe preeclampsia, and while it arises in about 2 to 3 percent of pregnancies, it contributes to about 15 percent of preterm births in the U.S. each year.
Preeclampsia is a syndrome marked by a sudden increase in blood pressure and a buildup of protein in the urine due to stress on the kidneys. Early-onset severe preeclampsia is a particularly serious form that arises before the 34th week of pregnancy.
In the current study, researchers found that vitamin D levels were generally lower among 50 women with early severe preeclampsia compared with those of 100 healthy pregnant women. The average vitamin D level in the former group was 18 nanograms per milliliter (ng/mL), versus 32 ng/mL in the latter group.
There is debate about what constitutes an adequate level of vitamin D in the blood. But many experts say that at least 32 ng/mL is needed for overall health.
The findings, reported in the American Journal of Obstetrics & Gynecology, do not prove that lower vitamin D levels contribute to early-onset severe preeclampsia.
Lack of vitamin D and other risks
They do, however, add to a spate of recent research finding connections between vitamin D levels in the blood, or vitamin D intake, and the risks of a host of health problems.
Studies have, for example, linked relatively low vitamin D levels to higher risks of type 1 diabetes and severe asthma attacks in children and, in adults, heart disease, certain cancers and depression.
But whether vitamin D is the reason for the excess risks — and whether taking supplements can curb those risks — has yet to be shown.
A few past studies have found an association between vitamin D and preeclampsia in general. Now more work is needed to see whether pregnant women's vitamin D levels predict the odds of preeclampsia developing — and whether raising those levels with vitamin D supplements lowers women's risk of the complication, according to Dr. Christopher J. Robinson, of the Medical University of South Carolina in Charleston.
If vitamin D is involved in preeclampsia risk, Robinson told Reuters Health, then it might help explain why African American women are at greater risk of the complication than other racial groups — even when factors like income and healthcare access are taken into account.
Vitamin D is naturally synthesized in the skin when it is exposed to sunlight. This process is less efficient in people with darker skin, and studies have found that African-Americans commonly have low levels of vitamin D in their blood; a recent study of U.S. teenagers, for example, found that while 14 percent overall had vitamin D deficiency — defined as less than 20 ng/mL — the same was true of half of black teens.
The current findings are based on 50 pregnant women with early-onset preeclampsia seen at the Medical University of South Carolina, along with 100 women with healthy pregnancies.
Of the preeclampsia group, 54 percent were deemed to have vitamin D deficiency (less than 20 ng/mL), versus 27 percent of the healthy group. Only 24 percent of women with preeclampsia had vitamin D levels greater than 32 ng/mL, compared with 47 percent of their healthy counterparts.
When Robinson and his colleagues accounted for a number of factors in preeclampsia risk — including older age, heavier body weight and African-American race — vitamin D levels were independently related to the odds of early preeclampsia.
A 10 ng/mL increase in vitamin D was linked to a 63 percent reduction in the odds of the complication.
It is biologically plausible, Robinson said, that the vitamin could affect preeclampsia risk. Vitamin D acts as a hormone, and lab research has found that it may affect the regulation and function of proteins in the placenta; problems in the development of the placenta are believed to be at the roots of preeclampsia.
Recommended levels in question
Right now, it's generally recommended that pregnant women get anywhere from 200 to 400 IU of vitamin D per day; prenatal vitamins contain 400 IU.
But there is much debate over that recommendation, with many researchers arguing that pregnant women — and everyone else — need more vitamin D than is officially advised.
Current U.S. guidelines call for adults age 50 and younger to get 200 IU of vitamin D each day, while older adults should get 400 to 600 IU. The upper intake limit is set at 2,000 IU; it is thought that the risk of vitamin D toxicity — with symptoms such as nausea, vomiting and weight loss — may go up with intakes beyond that level.
However, research in recent years has been challenging those ideas on what is enough and what is too much vitamin D, and the U.S. guidelines are currently under review.
For now, though, Robinson said, the 400 IU in prenatal vitamins remains the recommended daily intake for pregnant women.