To determine if high-dose vitamin D supplementation given during pregnancy reduced the risk of the development of asthma and related disorders in offspring by age 6 years.

This study included 545 children (of the 581 children included in the original study) whose mothers had been randomly assigned to receive high-dose vitamin D or placebo during pregnancy.

The initial study was performed in 2009 to 2010, in which 623 women were enrolled. These women were randomly assigned to receive an additional 2400 IU per day of vitamin D or placebo in addition to the recommended intake of 400 IU per day at the 24th week of pregnancy. Five hundred eighty-one of the offspring of these women were managed at 12 scheduled visits through age 3 years. For this study, follow-up was performed again at age 6 years. The primary outcome was defined as a current diagnosis of asthma at age 6, still requiring the use of inhaled corticosteroids for control. Secondary outcomes analyzed were lung function, bronchial reactivity, airway inflammation, allergic sensitization, and rhinitis.

Of the 545 children analyzed at the 6-month follow-up, 274 were in the high-dose vitamin D group and 268 children were in the placebo group. Asthma was diagnosed in 23 of the 274 (8%) of those whose mothers received high-dose vitamin D supplementation. In the placebo group, 18 of 268 (7%) received a diagnosis of asthma. This difference was not significant (P = .57), with an adjusted odds ratio of 1.21 (95% confidence interval: 0.63–2.32). Analysis of the secondary outcomes showed no significant difference.

Researchers in this study found that there was no significant reduction of risk by age 6 years in children whose mothers received high-dose vitamin D during pregnancy compared with the placebo group.

Evidence has previously been reported suggesting that low in utero vitamin D levels are associated with a risk of developing asthma in offspring. This study, however, demonstrated no association between high-dose vitamin D administration during pregnancy and the development of asthma by age 6 in offspring. The initial study did suggest a possible clinically important protective effect of vitamin D on persistent wheeze by age 3 years; however, this effect was not seen at age 6, when a diagnosis of asthma could be established. Investigators in this study did not examine whether the effects of prenatal vitamin D supplementation was modified by other factors (dietary, genetic, or environmental) but highlight the variable clinical effects of vitamin D supplementation at early stages of development. Clinicians should be aware of these impacts as caregivers continue to seek and offer nutritional supplements to improve children’s health.