Vitamin D Supplements Halve The Risk of Severe Asthma

If you have asthma, adding an oral vitamin D supplement to your usual asthma medication may reduce your chance of experiencing a severe asthma attack, and halve the need for emergency hospital treatment.

That’s the finding from a review of data from nine trials, involving 435 children and 658 adults across the UK, US, Canada, India, Japan and Poland. Most were classed as having mild to moderate asthma, while a few were diagnosed with severe asthma.

Researchers looked at the effects of adding oral vitamin D supplements to usual treatment for follow-up periods of between four months to one year. They found that taking vitamin D tablets reduced the risk of experiencing at least one severe asthma attack requiring hospital admission or emergency department attendance by 61% (from 6% down to around 3%) compared with inactive placebo. Vitamin D supplements also reduced the need for rescue therapy with oral steroids, such as prednisolone, by 37%.

Why does vitamin D protect against asthma?

Vitamin D has antimicrobial and antiviral actions and boosts immunity against respiratory infections which can worsen asthma symptoms. Researchers also recently discovered that vitamin D suppresses inflammation, and that the enzyme needed to activate vitamin D increases in the lungs when inflammation or infection are present. What’s more, vitamin D enhances the response to inhaled corticosteroids. All in all a win-win situation.

Will it work for everyone?

Whether or not vitamin D supplements can reduce the risk of severe asthma attacks in all people with asthma, or just those with low vitamin D status, remains uncertain. However, vitamin D deficiency becomes increasingly common during winter months when UV levels are too low to trigger its synthesis in the skin.

Who is at risk?

An estimated 1 billion people worldwide have inadequate vitamin D status, largely due to the so-called vitamin D winter that occurs at northern latitudes. The following UV map of the world, from Scientific America, suggests that people living in Northern Europe, the UK and Canada are at risk of insufficient UV intensity for making vitamin D during most of the year.

UV map of world from Sci Am


According to Public Health England (PHE) 22% of children aged 11 to 18 years, 23% of adults aged 19 to 64 years and 21% of adults aged 65 years and above have low blood levels of vitamin D.

People who are most at risk including those taking a statin medication, the elderly, and those who are housebound or who cover up in the sun and always apply high-factor UV protection.

Home Vitamin D Test Kits

If you want to know your vitamin D status, accurate home test kits are available which involve sending a fingerprick sample of blood to a laboratory. These results will tell you if you need a vitamin D3 supplement, and usually suggest a dose to take based on your readings.

What vitamin D  dose do you need?

The doses used in the asthma studies reviewed were typically between 10mcg (400 IU) to 100 mcg (4000 IU) per day, although some used higher doses given by injection.

The EU upper safe level for long-term use from supplements is 100 mcg (4000 IU) per day.

Only take vitamin D doses higher than the upper safe level under medical supervision. Excess can lead to side effects associated with raised calcium levels, such as bone demineralisation, kidney stones, headache and weakness.

If in doubt, consult your doctor.

The best vitamin D supplements

If you decide to take a vitamin D supplement, it’s important to obtain one made to pharmaceutical standards as this ensures you get a consistent daily dose.

Select a supplement that provides vitamin D3 (cholecalciferol) which is two and a half times more effective at maintaining blood vitamin D levels than the plant-based form, vitamin D2 (ergocalciferol).

The following brands all offer excellent quality and value, whether you decide to take a capsule, oral spray, or a vitamin D3 topical cream that is absorbed via the skin.

Image credits: photousvp77/depositphotos; Scientific American/


About DrSarahBrewer

Dr Sarah Brewer MSc (Nutr Med), MA (Cantab), MB, BChir, RNutr, MBANT qualified from Cambridge University with degrees in Natural Sciences, Medicine and Surgery. After working in general practice, she gained a Master's degree in Nutritional Medicine from the University of Surrey. Sarah is a licensed Medical Doctor, a Registered Nutritionist, a Registered Nutritional Therapist and the award winning author of over 60 popular self-help books. Sarah's other websites are and

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