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Clinical trial data suggests prenatal vitamin D reduces a child's risk of asthma

by Brigham and Women’s Hospital

Vitamin D

"Based on our findings, we would recommend that all pregnant women consider a daily intake of at least 4400 IU vitamin D3 throughout their pregnancy, starting at the time of conception."


Supplementing pregnant women with Vitamin D could halve the rates of asthma and wheezing in their children - according to a new, detailed review of data from randomised controlled trials (the highest level of clinical evidence) collected over 15 years.

Adjusting properly for baseline vitamin D levels showed that Vitamin D supplementation of pregnant mothers led to a 50% reduction in asthma and wheezing in their children - which is exactly the effect long suggested by observational studies. 

Previous published reports - which failed to make this adjustment - significantly underestimated the benefits of Vitamin D supplementation in pregnancy. 

And NB - this same omission - along with numerous other methodological issues - also affects most clinical trials of Vitamin D for the numerous physical and mental health problems firmly associated with low Vitamin D levels.

The study authors also strongly recommend that:

  • ALL pregnant women should take Vitamin D supplements. 
  • They also recommend around 4000 i.u daily during pregnancy - which is the EU Upper Safe Level - but is 10 times more than the current extremely low level that is 'suggested' for UK adults - but barely if ever publicised by public health authorities.

Vitamin D deficiency is widespread - particularly during winter months, when none can be made in the UK and countries at similar latitudes.

Those at highest deficiency risk include individuals with darker skin, infants and older adults, and anyone whose exposure to bright sunshine on the skin in spring and summer may be limited for any reason (which is many if not most people, given modern 'indoor' lifestyles).

Impaired immunity, and weakened bones and teeth, are classic effects of Vitamin D deficiency - although importantly: 

Vitamin D also affects brain development - and deficiency during pregnancy is linked with increased risks for ADHD, Autism Spectrum Disorders and related conditions - most notably schizophrenia, as well as with poorer behaviour and cognition

Underlying research:

See also:

And for further information on Vitamin D, please see the following lists of articles, which are frequently updated

09/11/23 - Medical Xpress


A review of 15 years' worth of data from the Vitamin D Antenatal Asthma Reduction Trial (VDAART) found that vitamin D supplementation during pregnancy reduced rates of asthma and wheezing in children compared to standard prenatal multivitamin
A new review paper from Brigham and Women's Hospital investigators strengthens the link between vitamin D levels during pregnancy and childhood wheezing and asthma in offspring. The researchers published their review paper in the Journal of Allergy and Clinical Immunology.
"Vitamin D deficiency is very common, especially in pregnant women who are not taking supplements," study first author Scott T Weiss, associate director of the Channing Division of Network Medicine at Brigham Women's Hospital and professor at Harvard Medical School, said.

"Based on our findings, we would recommend that all pregnant women consider a daily intake of at least 4400 IU vitamin D3 throughout their pregnancy, starting at the time of conception."
Vitamin D is a nutrient from sunlight exposure, diet, or supplements. It is commonly considered essential to bone health but also has a role in autoimmune and other illnesses. The review links vitamin D deficiency to childhood asthma and wheezing, a major cause of illness in young children. About 40% of kids report daily wheezing at age three. By age 6, 20% are diagnosed with asthma.
The link between childhood asthma and vitamin D has been contentious. Observational studies suggest that higher vitamin D levels during pregnancy can be protective against asthma. However, a clinical trial of vitamin D supplementation in pregnancy, called the Vitamin D Antenatal Asthma Reduction Trial (VDAART), was inconclusive when comparing the supplemented group to the non-supplement group.
"In general, the observational studies show an effect, but the clinical trials don't because nutrient trials are very different from drug trials," Weiss said. "In a drug trial, you're comparing giving a drug to giving no drug. In a nutrient trial, you're comparing more of a nutrient to less, but that baseline amount in the control group is variable."
Understanding the role of a nutrient during pregnancy requires consideration of the nutrient dose, the timing of when dosing starts, and the baseline levels in the control group. Weiss said the original VDAART trial and analysis and other meta-analyses of vitamin D supplements during pregnancy do not consider this.
Pregnant women with a family history of allergy or asthma enrolled in the original VDAART study between 10 and 18 weeks of pregnancy. Half of the women were given a dose of 4400 IU of vitamin D in addition to the 400 IU of vitamin D in their prenatal vitamin. The other half got placebos alongside their prenatal vitamins.
The VDAART findings at age three, published in JAMA in 2016, showed a 20% reduction of asthma in the treatment group, with borderline statistical significance. The results were even less significant at age six, published in the NEJM in 2020.
"But, when we stratified the results by the vitamin D level in the control group, both of those analyses became significant," Weiss said. "When you adjust for baseline vitamin D levels, we see exactly the effect in the observational studies—a 50% reduction in asthma and wheezing."
Weiss's group published the age three reanalysis in PLoS One in 2017 and the age six data in the American Journal of Clinical Nutrition in 2023. The latest review article summarizes these studies as well as genetic findings that further strengthen the possibility of a causal relationship between vitamin D and asthma and suggests several considerations for planning a follow-up study.
"Based on the insights gained from VDAART, we recommend that a follow-up clinical trial should start as early as possible in pregnancy and supplement with 6000 IU vitamin D and seek a very high enrollment of women of color," said Weiss.

"Such trials could deepen our understanding of the potential impact of vitamin D on pregnancy outcomes and early-life asthma."