BMJ: There is no strong evidence that vitamin D protects against pregnancy-induced high blood pressure (hypertension) or pre-eclampsia, conclude researchers in The BMJ today. The findings support current World Health Organization guidance that evidence recommending vitamin D supplements for women during pregnancy to reduce adverse pregnancy outcomes is insufficient. However, in many countries, including the UK and the US, pregnant women are advised to take a daily dose of vitamin D.
It is common for pregnant women to have low levels of vitamin D,
which can suppress the hormone that regulates blood pressure which may
increase the risk of both hypertension and pre-eclampsia during
Previous population-based studies have found that women with lower
levels of vitamin D are at greater risk of pre-eclampsia and some trials
of vitamin D supplementation in pregnancy suggest a potential benefit.
But it remains unclear whether vitamin D is a cause of pre-eclampsia.
So an international team led by Maria Magnus at the University of
Bristol, set out to investigate whether vitamin D has a effect on
pregnancy induced hypertension or pre-eclampsia.
Using a technique called Mendelian randomisation, they examined
whether genetic variants associated with vitamin D production and
metabolism also influenced the risk of pregnancy-induced hypertension
and pre-eclampsia for 7,389 women (751 with gestational hypertension and
135 with pre-eclampsia) from two large European studies (Avon
Longitudinal Study of Parents and Children, also known as Children of
the 90s, and Generation R Study).
They also performed another (two sample) Mendelian randomisation analysis of 3,388 pre-eclampsia cases and 6,059 controls.
Analysing genetic information as proxies for the exposure of interest
in this way avoids some of the problems that afflict traditional
observational studies, making the results less prone to unmeasured
(confounding) factors. An association that is observed using Mendelian
randomisation therefore strengthens the inference of a causal
Mendelian randomisation analysis showed no evidence to support a
direct (causal) effect of vitamin D levels on risk of gestational
hypertension or pre-eclampsia.
The researchers point to some study limitations. For example, the
analyses were restricted to pregnant women. If vitamin D affects
fertility, this may have resulted in selection bias.
In light of the uncertainty, they suggest further studies with a
larger number of women with preeclampsia or more genetic variants that
would increase the predictive power of vitamin D levels are needed.
“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 conclude.
Research: Vitamin D and risk of pregnancy related hypertensive disorders:
mendelian randomisation study
Journal: The BMJ
For a primer on Mendelian randomisation, see: https://www.youtube.com/watch?v=LoTgfGotaQ4