Originally published: February 2018
The active metabolite of vitamin D, 25-hydroxyvitamin D (25(OH)D), is essential for calcium homeostasis and bone health. It is produced mainly as a result of ultraviolet B radiation, from sunlight, on 7-dehydrocholesterol in the skin. A small amount of vitamin D can also be found in the diet. Low levels of vitamin D have been linked to disorders of a number of body systems including the immune system and the cardiovascular system. In additions, low levels of vitamin D have been implicated in respiratory illnesses such as infection with tuberculosis and influenza, and in inflammatory conditions including asthma and chronic obstructive pulmonary disease.1 Supplementation with vitamin D is cheap and easily administered orally.
In Lancet Respiratory Medicine, David Jolliffe and colleagues undertook a meta-analysis of vitamin D supplementation and the effect on asthma attacks.2 They reviewed individual participant data from the eight randomised controlled trials which met their criteria, with a total of 1,078 participants. The authors found that overall, vitamin D supplementation reduced the rate of asthma attacks requiring oral corticosteroid treatment by 26% (95% confidence interval [CI] 3–44%). Subgroup analysis showed that this effect was limited to participants with low vitamin D levels (25(OH)D <25 nmol/L), with a 67% asthma attack rate reduction (95% CI 2–89%). Participants with a 25(OH)D level of ≥25 nmol/L did not show a significant reduction in asthma attack rate with vitamin D supplementation.
There are some cautions required when interpreting this type of meta-analysis, not all participants or trials met all the criteria for the various analyses and so the results need to be interpreted in context. This study does, however, support a role for the testing and supplementation of vitamin D in asthma, adding to the findings of a 2016 Cochrane review.3 The benefits of vitamin D supplementation in patients with asthma and low vitamin D levels now need to be proven in well designed, randomised controlled trials.
Dr Rahul Shrimanker is a clinical research fellow, University of Oxford.
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