Vitamin D Supplements Effect Fetal Bone Health Only in Winter

The primary outcome was neonatal whole-body BMC, assessed within two weeks of birth by dual-energy X-ray absorptiometry (DXA), analyzed in all randomly assigned neonates who had a usable DXA scan. Safety outcomes were assessed in all randomly assigned participants. The results showed that there was no significant difference in BMC and bone mass between the babies born to women supplemented with vitamin D and those who had taken placebo.

Further analysis that took into account the season of birth showed that babies born during winter months to mothers who had taken vitamin D supplements had greater BMC than winter babies born to mothers who received the placebo. Furthermore, in mothers who gave birth in winter, vitamin D concentrations fell from 14 to 34 weeks’ gestation in the placebo group, but rose in the treatment group, suggesting that the supplementation may counteract the drop in the body's normal levels of vitamin D caused by lack of sunlight when late pregnancy occurs in the winter months. The study was published on March 1, 2016, in the Lancet Diabetes & Endocrinology.

“Since sunlight is our most important source of vitamin D, mothers' levels of vitamin D tend to drop from summer to winter, and babies born in the winter months tend to have lower bone density than those born during the summer,” said study coauthor Prof. Nicholas Harvey, MD, of the University of Southampton. “Supplementing mothers with vitamin D during pregnancy counteracts the seasonal drop in maternal vitamin D levels and may help to ensure good bone development in these winter births.”

Vitamin D is a group of fat-soluble secosteroids found in many dietary sources, such as fish, eggs, fortified milk, and cod liver oil. Although vitamin D is commonly called a vitamin, it is not actually an essential dietary vitamin in the strict sense, as it can be synthesized in adequate amounts by all mammals from sunlight. The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus.

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