SAN FRANCISCO – Vitamin D supplementation during pregnancy may reduce the incidence of asthma or allergies in children at high risk for atopic disease, a study showed.

At age 3 years, asthma or recurrent wheeze occurred in 24% of children born to mothers with substantial vitamin D₃ supplementation in pregnancy, compared with 30% of children whose mothers took a placebo during pregnancy.

Previous research already has suggested that vitamin D deficiency, particularly during pregnancy, may contribute to increasing incidence of asthma through a variety of potential mechanisms, explained lead author Augusto Litonjua, MD, of the Harvard Medical School and Brigham and Women’s Hospital in Boston. These mechanisms include possible effects from insufficient vitamin D on lung growth, the microbiome, and immune cell development and regulations.

However, observational study findings on vitamin D deficiency in pregnancy and asthma risk have been mixed, with no effect seen in research using measurements of 25OHD levels, despite protective effects seen in studies estimating vitamin D intake based on diet. Dr. Litonjua, therefore, led a randomized, controlled trial at three clinical centers to test whether vitamin D supplementation in pregnancy could prevent children at high risk of asthma from developing the condition. High-risk status was based on the presence of maternal and/or paternal asthma, allergic rhinitis, or eczema.

The 881 initial participants, enrolled between October 2009 and January 2015, were randomized to receive either 4,000 IU daily of vitamin D₃ (440 women) or a placebo daily (436 women). Both groups took a multivitamin that contained 400 IU of vitamin D₃. The participants included 43% black women, 26% white women, 14% Hispanic women, and 17% of other races/ethnicities.

The researchers collected maternal blood at the start of the study, between 32 and 38 weeks’ post partum, and at 1-year post partum. They collected cord blood at birth and then children’s blood at 1, 3, and 6 years old. At the third trimester, 87% of the women in the intervention group and 72% of the control group women had at least 50 nmol/L of 25OHD. Levels of at least 75 nmol/L were present in 75% of the intervention group and 35% of the control group in the third trimester.

Primary follow-up occurred at 3 years old with continuing follow-up through 6 years old, but data also were collected every 3 months regarding asthma and allergy symptoms and environmental exposures and diet. Stool was collected for microbiome analysis at 6 months, 1 year, and 3 years, and then annually after age 3 years. Children’s lung function was assessed with impulse oscillometry annually starting at age 4 years, with spirometry annually starting at age 5 years, and with bronchodilator response at age 6 years, Dr. Litonjua reported at the Pediatric Academic Societies meeting.

Just over one-quarter (27%) of the 806 children included in the final analysis had parental report of either an asthma diagnosis or recurrent wheeze, defined using any of five criteria involving multiple wheeze reports and/or use of an asthma controller. Rates of asthma or wheeze were significantly lower in children of women supplemented with 4,400 IU of vitamin D than in those born to women in the control group.

At age 1 year, the rate of asthma or wheeze among children from the intervention group was 9 percentage points lower than that of children from the control group, a 36% reduction. By age 2 years, the rate difference was 7%, a 25% reduced risk for intervention children, compared with control children. Children from the intervention group had a 20% lower risk of asthma or wheeze at age 3 years than those from the control group, with a rate difference of 6% (P = .051).

“Both maternal baseline 25OHD levels and third-trimester 25OHD levels were inversely associated with asthma/recurrent wheeze by age 3 years,” Dr. Litonjua reported, and relative risk at age 3 years was identical in black and white children. Maternal levels of 25OHD in the first trimester, however, “modified the effects of supplementation such that children born to women with higher levels and [who] were in the treatment arm had lowest risks for asthma/recurrent wheeze,” he said.

The greater reduction among women with higher baseline levels and supplementation suggests that higher levels than 30 ng/mL may be necessary for the prevention of asthma or allergies, Dr. Litonjua said. Personalized dosing or earlier supplementation may, therefore, be needed, he said.

“One of the main findings from our trial was that there were no serious adverse effects,” he said. Other trials, however, have found concerns with vitamin D toxicity. One challenge in the study was very low adherence: Only about half the women regularly took their vitamin D supplements at first, Dr. Litonjua said, although they were eventually able to raise adherence to around 80%.

The research was funded by the National Heart, Lung, and Blood Institute. Dr. Litonjua reported royalties from UpToDate and Springer Humana Press and consultation fees from AstraZeneca.

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Children’s asthma risk reduced with prenatal vitamin D supplementation