Multiple randomized trials have suggested that vitamin D supplementation might improve asthma control and reduce severity of asthma attacks. A new meta-analysis bolsters that hypothesis, and may encourage more physicians and people with asthma to consider vitamin D supplements for low vitamin D levels.

In a study in Lancet Respiratory Medicine, authors analyzed the experience of 955 asthma patients previously enrolled in seven randomized controlled trials testing vitamin D versus placebo.

Supplementation with vitamin D was associated with a 50% reduction in asthma attacks severe enough to require hospitalization or emergency department visit (3% vs 6%), compared to placebo. Vitamin D use was also associated with a 30% reduction in asthma attacks necessitating use of systemic corticosteroids.

Importantly, patients with low baseline vitamin D levels (<25 nmol/L serum 25(OH)D ) experienced statistically significant benefits, while those with higher baseline levels did not--suggesting vitamin D supplementation might be more beneficial in patients with asthma and low baseline vitamin D levels.

The treatment effect was strong enough to be seen in the overall cohort, with an adjusted incidence ratio of 0.74 for a severe asthma exacerbation among patients taking vitamin D (irrespective of baseline vitamin D level).

The study echoes a previous finding in a 2016 Cochrane meta-analysis that concluded supplementing vitamin D resulted in a 37% reduction in asthma exacerbations requiring systemic corticosteroids. That analysis could not stratify patients by baseline vitamin D levels, however.

The meta-analysis was underpowered to definitively conclude that patients with lower vitamin D levels experienced a reduction in asthma exacerbation severity. Editorialists called for a large, adequately powered randomized trial to answer the question:

Certainly there is a case to answer and definitive well powered randomized placebo controlled trials need to be done as a priority because the magnitude of the reduction in risk of severe exacerbations with vitamin D reported in this meta-analysis is substantial.

Given the relative benignity of vitamin D supplementation, and the magnitude of a potential benefit, the analysis will likely lead many physicians to test vitamin D levels in patients with a history of severe asthma exacerbations, and to advise supplementation in patients with low levels.

Sources: Lancet Resp Med, Cochrane Reviews

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