Taking vitamin D supplements could significantly reduce the risk of severe asthma attacks for some asthmatics, health experts have found.
Asthma patients who suffer from severe attacks, or exacerbations, were at a lower risk of having an attack and less likely to need hospital treatment if they had been taking regular doses of the vitamin, trials carried out by a Cochrane Review showed.
They were also less likely to need treatment with steroid tablets.
Asthma is a chronic respiratory disease that affects around 300 million people worldwide, causing wheezing, coughing, chest tightness and shortage of breath.
Vitamin D in children and adults has been linked to lowering the risk of asthma attacks. Evidence suggests it may help prevent respiratory infections, such as the common cold, which can lead to exacerbations in patients.
The trials were organised by Cochrane, a not-for-profit network of health professionals, after experts noticed a link between low levels of the vitamin in asthmatics and an increased risk of attacks.
Speaking at a review of the research at London's Science Media Centre on Monday, lead research author Professor Adrian Martineau said: "Asthma is a rising problem in the UK, with one out of 11 people receiving treatment for it every day.
"Vitamin D, or the sunshine vitamin, as well as enhancing bone development, helps the development of at least 35 other tissues and white blood cells. This can boost immunity against other illnesses and dampen down inflammation."
The investigation involved nine trials with 1,093 people (including 435 children and 658 adults) with different levels of asthma. The patients, from a range of ethnic backgrounds, were given different dosages over six to 12-month periods.
Dosages ranged from one to 2,000 units (the equivalent of 50 micrograms) per day - five times the 400-unit dosage recently recommended by Public Health England for all people, with or without asthma.
The results also showed that the vitamin supplements did not increase the risk of side effects, but did not improve lung function or day-to-day asthma symptoms in patients either.
Prof Martineau said: "While the vitamin supplements reduced the number of attacks needing hospital treatment from 6% to 3%, one size may not fit all.
"We don't know whether these heightened dosages will benefit all asthma patients or just those who already have low levels of vitamin D. Also, about three quarters of asthma patients do not suffer from exacerbations, and we need to do further trials to discover whether the supplements can benefit other groups."
Asked if taking additional vitamin D supplements is the most effective way to boost levels in the blood, he said: "The absorption of the quantity of the vitamin into the bloodstream is less controlled when taken as pills, but emerging evidence suggests it is more effective to take a regular dose than just the strong and widely spaced apart bursts we get from exposure to the sun."
GP Dr Rebecca Normansell said: "We would recommend anybody to take the simple blood test to determine their vitamin D levels, and to talk to a GP or pharmacist for advice."
Prof Martineau added: "We would not encourage any asthma patients to replace their regular medication with vitamin D supplements, but to consider taking them in addition to it."
Dr Imran Rafi, chair of clinical innovation and research at the Royal College of GPs, shared his support for the research and the impact that further investigation could have for treating asthma.
He said: "More work still needs to be done in gathering the evidence, particularly around effectiveness for young people and children - especially as it currently affects as many as one in 11 children.
"It is still too early to make general recommendations on prescribing vitamin D to patients with asthma, but we look forward to seeing the results of further rigorous clinical trials both in adults and children so that we gain a better understanding of this potential method of treatment."