There has been some debate in recent times about the association between vitamin D levels and the risk of developing certain conditions such as diabetes, colon cancer, arthritis and infections. Studies show conflicting results. But when it comes to asthma, vitamin D may indeed protect against severe bouts of the illness.
A Cochrane review published today shows that asthmatics given vitamin D had fewer severe asthma attacks that needed treatment with anti-inflammatory tablets, known as oral corticosteroids.
For those taking vitamin D, the average number of attacks per person per year went down from 0.44 to 0.22. This is a clinically relevant reduction, meaning it is significant enough to justify a change in treatment.
The Cochrane Library contains in-depth reviews, known as meta-analyses, that independently study the best available evidence generated through previous research in the field.
The Cochrane review also showed vitamin D reduced the likelihood of attending hospital for an acute asthma attack from six per 100 patients to around three per 100. However, vitamin D had little or no effect on day-to-day asthma symptoms or breathing tests.
No serious side-effects of vitamin D occurred at the doses tested and the evidence reviewed was graded as of high quality.
Previous trials
The meta-analysis included a comprehensive review of published and unpublished trials up to January 2016. The studies included compared children or adults with asthma who were randomly chosen to receive either vitamin D or identical dummy tablets (placebo) for at least 12 weeks.
Seven trials involving 435 children and two trials involving 658 adults contributed to a pooled analysis of the results. Most of the patients included had mild or moderate asthma.
Vitamin D comes in two forms in the body. Vitamin D3 (cholecalciferol) is made in human skin when exposed to sunlight or can be taken in the diet from eating oily fish or foods with added vitamin D or vitamin D tablets. The second form is vitamin D2 (ergocalciferol), which can be ingested in the diet, mainly by eating mushrooms.
Both forms are changed in the body to a chemical called 25-hydroxy-vitamin D, which can be measured in blood samples. Levels lower than 50 units (nmol/L) are widely accepted to indicate vitamin D deficiency. This is common in many populations, especially where sunlight levels are low or where people cover up.
Previous studies in both children and adults have found a link between low vitamin D levels and increased risk of asthma attacks. Individual studies have, however, used different definitions of asthma attacks and found differing results.