Researchers from King's College London found that babies born to mothers with a higher vitamin D level showed improved neonatal defences to infection.
Previous prenatal vitamin D studies have shown that deficiencies in pregnant mothers can lead to disease development in offspring.
Is the RDA enough?
The study demonstrated that increased vitamin D supplementation could strengthen the immune system in new-borns, thus preventing the risk of asthma in later childhood.
Asthma, and other respiratory conditions, are normally diagnosed in early childhood and have been linked to weak immune responses.
The researchers used a random, controlled testing method to analyse the effect different supplementation doses had on the immune systems of new-borns.
The doses measured were 4,400 IU/day of vitamin D3 versus the recommended daily amount (RDA) of 400 IU/day - the control regimen.
Mothers were randomised at 10-18 weeks of pregnancy to receive either the higher (sample size - 26) or lower (sample size - 25) dose of vitamin D3.
Umbilical cord blood was collected and tested to analyse the effect of vitamin D supplementation on cytokine production by cord blood mononuclear cells (CBMCs) in response to innate stimulation.
Women receiving the higher dose demonstrated that, when stimulated with pathogen imitators, the immune systems of their new-borns responded with greater cytokine responses and greater IL-17A production in response to T lymphocyte stimulation.
Both of these responses are thought to predict improved neonatal defence to infection.
"Vitamin D exposure during foetal development influences the immune system of the neonate, which can contribute to protection from asthma-related, including infectious, outcomes in early life," the study authors said.
The researchers say this is the first study to date that investigates links between vitamin D and immunity in the baby that has not been solely observational.
"Vitamin D is a promising area of research for asthma, however this study shows that vitamin D supplementation in pregnancy may involve immune responses, much more research is needed to prove whether this does in fact lead to reduced asthma rates later in life," said Dr Samantha Walker, director of research and policy at Asthma UK.
"Asthma affects 1 in 11 people in the UK, yet years of underfunding in research mean that we still do not understand what causes asthma, or have the ability to predict which babies will go on to develop asthma. This is urgently needed if we are to develop strategies to treat, and ultimately prevent, asthma in children".
Experts respond
Experts in the field have commented that the study is "comprehensive and interesting" but it fails to make a conclusive decision on the effect of vitamin D in pregnant women.
Martin Hewison, Professor of Molecular Endocrinology at the University of Birmingham, said "...this is a high quality sub-study of the largest and most comprehensive vitamin D supplementation in pregnancy trial... the authors are cautious in their conclusions, but their data demonstrate for the first time that vitamin D supplementation during pregnancy not only influences pregnancy itself, but may have lasting impact on the immune 'health' of children, presumably through effects that occur during pregnancy."
Hewison added that future studies should carry out a longer term assessment of the children to properly assess the health of participants.
Dr Sheena Cruickshank, senior lecturer in Immunology at the University of Manchester, agreed with Hewison that the study report was interesting yet also commented that it was very preliminary.
"It's a small group size and difference between the cord blood samples in low and high dose vitamin D supplementation were model and the results were very variable, making it very hard to draw a conclusion about differences in cytokine responses between groups," she said.
The authors of the study agreed with this sentiment, saying "the size of the current study precluded meaningful analyses."
