A study published recently in the New England Journal of Medicine found that vitamin D supplementation during the prenatal period alone did not influence the six-year incidence of asthma among at-risk children. The placebo-controlled trial, funded by the National Heart, Lung, and Blood Institute, provided 4,400 International Units (IU) of vitamin D3 every day to one group of pregnant women and 400 IU to another group. While early data suggested that prenatal vitamin D3 supplementation provided a protective effect at age three, no difference was seen between children in the two groups at age six.
Vitamin D deficiency has been hypothesized to have contributed to the increase in asthma and allergies in westernized societies. Observational studies have suggested that a higher maternal vitamin D level is associated with a lower risk of asthma-related outcomes among children. This trial did not bear out this association.