There is not enough evidence to recommend women take vitamin D supplements during pregnancy to help avoid high blood pressure or pre-eclampsia, according to a major new study.
The study, led by researchers at the University of Bristol and published in the BMJ today, raises fresh questions about standard practice in the UK, where pregnant women are generally advised to take a daily dose of vitamin D.
Previous studies have found women with lower levels of vitamin D are at greater risk of pre-eclampsia and some trials suggest taking vitamin D supplements during pregnancy has benefits.
However, it is not clear whether lower levels of vitamin D, which can suppress the hormone that regulates blood pressure, actually cause pre-eclampsia and hypertension.
In order to explore this relationship, the researchers used a technique called Mendelian randomisation, which looks at variations in genes with a known function to examine the effect on a specific disease.
By focusing on genetic information, the technique is believed to produce robust results because it avoids some of the complicating factors that can skew the outcome of more traditional observational studies.
In this case, the research team set out to examine whether genetic variants associated with vitamin D production and metabolism also influenced the risk of high blood pressure in pregnancy and pre-eclampsia. They looked at data on nearly 7,400 pregnant women from two large European study cohorts, including about 750 with gestational hypertension and 135 with pre-eclampsia. They also conducted another Mendelian randomisation analysis of nearly 3,400 pre-eclampsia cases and more than 6,000 control cases.
The result show there is “no strong evidence” to suggest vitamin D levels have a direct effect on the risk of high blood pressure during pregnancy or pre-eclampsia.
The findings are a fresh twist in ongoing debate about vitamin D intake in pregnancy, suggested the paper, with vitamin D supplements recommended by the NHS, in the US and countries like Norway and The Netherlands, while the World Health Organisation [WHO] says there is insufficient evidence to support recommending vitamin D supplementation for women during pregnancy.
“Our findings support the WHO position,” said the researchers, who added that more research was needed with a larger number of women with pre-eclampsia or more genetic variants. “In combination with adequately powered clinical trials, this could help finally establish whether vitamin D status has a role in pregnancy related hypertensive disorders,” they concluded.