The King's College London team say most asthma cases are diagnosed in early childhood, suggesting that the origin of the disease is in early life or when the foetus is still in the womb.
Previous studies have been observational, drawing on third-party data to draw conclusions. The latest investigation is based on a study of 51 women who were randomly assigned at between 10 and 18 weeks of pregnancy to take high or low doses of vitamin D supplements.
Around half the women were given the recommended daily intake of 400 IU of vitamin D during the second and third trimesters of pregnancy and the remainder took a high dose of 4,400 IU.
Umbilical cord blood
Samples of all the women's umbilical cord blood were analysed to assess the effectiveness of the newborn babies' immune systems.
They found that blood samples from babies born to mothers who had taken the higher doses of vitamin D responded better when exposed to simulations of pathogens.
The researchers say future studies should examine the long-term impact on the immunity of the baby.
'Promising' area of research
Dr Samantha Walker, director of research and policy at Asthma UK, comments in a statement: "Vitamin D is a promising area of research for asthma; however, this study is just the first step of many needed to explore this topic. Although this study shows that vitamin D supplementation in pregnancy may improve immune responses, much more research is needed to prove whether this does in fact lead to reduced asthma rates later in life.
"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."
Vitamin D supplementation during pregnancy: Effect on the neonatal immune system in a randomized controlled trial, Hornsby E et al, Journal of Allergy and Clinical Immunology
King's College London
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