What is the Evidence Base for Vitamin D Supplementation in Pregnant Women?
Many people are interested in how diet during pregnancy can affect health of both mother and infant. In fact, our fourth most popular blog post ever discusses how vitamin D supplementation influences gestational diabetes-related health measures. Recently, Harvey and co-workers published an exhaustive review on the effects of vitamin D supplements in pregnancy. Did these authors find a benefit to vitamin D supplementation?
Vitamin D is required together with calcium to build bone: calcium cannot be properly absorbed and mobilized in the blood unless vitamin D levels are adequate. This is important for the developing fetus as the entire 30 g of calcium in a newborn has to be acquired during pregnancy: around 80% of fetal calcium is acquired in the last trimester. While the fetus generally seems to be able to gain enough calcium from the mother despite low vitamin D levels, deficiency may cause skeletal abnormalities. The elevated parathyroid hormone levels associated with vitamin D deficiency increase risk of miscarriage and stillbirth. Vitamin D has other roles outside of bone health. The most relevant for pregnant women vitamin D’s role in blood pressure and type II diabetes risk.
The authors initially identified over 25,000 potential studies that looked at whether vitamin D supplements or vitamin D levels in mothers could affect maternal health during pregnancy or birth outcomes. Of these, 76 studies were eventually included in the review. The authors looked in great detail at various health outcomes, and the final report is around 200 pages long. Some key results were:
* An increase in birth weight equivalent to 5.6 g per 10% increase in maternal circulating vitamin D level
* Possible increase in birth length with vitamin D supplementation
* Greater infant head circumference in mothers taking supplements
* Positive relationships were found a positive relationship between maternal vitamin D level and infant bone mass
* Lower levels of vitamin D were association with increased risk of low birthweight, preterm birth and small-for-gestational-age offspring, with a non-significant trend for higher risk in non-supplemented mothers
* There was an inverse relationship between risk of gestational diabetes and pre-eclampsia and vitamin D levels
The authors were unable to make strong recommendations for widespread vitamin D supplementation based on the current evidence base. However, they note that the results from observational studies and the few small intervention trials suggest that large intervention studies should be performed to look at whether supplements can be effective in reducing birth-related complications and improving neonatal and maternal outcomes.
Harvey N, Holroyd C, Ntani G, Javaid M, Cooper P, Moon R, Cole Z, Tinati T, Godfrey K, Dennison E, Bishop N, Baird J, Cooper C. Vitamin D supplementation in pregnancy: a systematic review. Health Technology Assessment. Volume 18. Issue 45. July 2014. DOI: 10.3310/hta18450
Carbone F, Mach F, Vuilleumier N, Montecucco F. Potential pathophysiological role for the vitamin D deficiency in essential hypertension. World J Cardiol. 2014 May 26;6(5):260-76. doi: 10.4330/wjc.v6.i5.260. http://www.ncbi.nlm.nih.gov/pubmed/24944756
Kovacs CS. Chapter 8. Skeletal Physiology: Fetus and Neonate. 2006. American Society for Bone and Mineral Research. Pages 50 – 55. In : Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, Edition: 6th, Chapter: 8, Publisher: ASBMR Press, Editor: Favus MJ. http://www.homepages.ucl.ac.uk/~ucgatma/Anat3048/PAPERS%20etc/ASBMR%20Primer%20Ed%206/Ch%208-12%20-%20Skeletal%20Physiology.pdf
Mitri J, Muraru MD, Pittas AG. Vitamin D and type 2 diabetes: a systematic review. Eur J Clin Nutr. 2011 Sep;65(9):1005-15. doi: 10.1038/ejcn.2011.118. Epub 2011 Jul 6. http://www.ncbi.nlm.nih.gov/pubmed/21731035