Vitamin D for mothers-to-be doesn't strengthen babies' bones--except in winter

Mothers who took vitamin D supplements during pregnancy saw no significant increase in their babies’ bone density, according to the first randomized controlled trial of its kind, published online March 1, 2016 in the journal The Lancet Diabetes & Endocrinology.

However, in a subgroup analysis of births occurring in winter, vitamin D supplementation increased neonatal bone mass by almost 10% and fat mass by 17%. The findings suggest vitamin D supplements may be beneficial for babies born in winter months, potentially counteracting the seasonal drop in mothers’ vitamin D levels caused by a lack of sunlight, researchers concluded.

Previous observational studies have associated higher vitamin D levels during pregnancy with an increase in bone mass in babies. Current nutritional guidelines in some countries, including the United Kingdom and Canada, recommend that all pregnant women take daily vitamin D supplements to improve their babies’ bone health.

But the link is controversial because not all studies have shown that higher maternal vitamin D levels in pregnancy provide benefits to childhood skeletal health, the researchers noted; stronger evidence is needed from randomized trials.

This study—the Maternal Vitamin D Osteoporosis Study (MAVIDOS)—is the first randomized, placebo-controlled trial designed to measure the impact of maternal vitamin D supplementation on the bone health of babies. The investigators used bone density scanning to measure the babies’ whole-body bone mineral content.

Between 2008 and 2014, MAVIDOS researchers recruited 1,134 pregnant women in the UK who had low to normal levels of vitamin D. From 14 weeks gestation until delivery, half of the women took an oral placebo daily and half took 1,000 IU cholecalciferol (vitamin D3).

Overall, the researchers found that babies born to women who took vitamin D had no difference in average bone mass (61.6 g) compared with that of babies born to women who took placebo (60.5 g).

However, when researchers took the season of birth into account, they found that babies born during winter months to mothers on vitamin D averaged greater bone mass (63.0 g) than winter babies born to mothers on placebo (57.5 g). This is an intriguing finding because bone mineral concentration accelerates during the last trimester of pregnancy, researchers noted.

Also, among all mothers who gave birth in winter, vitamin D levels rose from 14 to 34 weeks gestation in the treatment group but fell in the placebo group.

“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 co-author Nicholas Harvey, PhD, MB BChir, Professor of Rheumatology and Clinical Epidemiology at the University of Southampton, in Southampton, UK.

He added, “The MAVIDOS Trial has given us the first evidence that 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.”

“Although biologically plausible, this intriguing finding clearly needs to be replicated in further studies before it can provide a basis for alterations to clinical care,” the authors concluded.