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.
This project was initiated and funded by Teva Respiratory. Teva have had no influence over content. Topics and content have been selected and written by independent experts.