The new study suggests that it can also increase attacks for asthmatics.

One question scientists hope to answer in the next few months is whether supplements only benefit asthma patients with low levels of vitamin D.

"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", said lead author Dr. Adrian Martineau.

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 United Kingdom, with one out of 11 people receiving treatment for it every day".

Patients who suffer from severe attacks were at a lower risk of 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.

About 300 million people across the world experience symptoms of asthma, including chest tightness, wheezing, coughing and shortness of breath. Further studies are needed to determine whether patients who have severe asthma could get any benefits from taking vitamin D supplements. There has been a growing interest in the potential role of vitamin D in asthma management because it might help to reduce upper respiratory infections, such as the common cold, that can lead to exacerbations of asthma.

The Cochrane review looked at seven trials on the subject involving 435 children and two studies involving 658 adults. Most of the participants had mild to restrained asthma with a smaller number having severe asthma.

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"Vitamin D supplements could halve risk of serious asthma attacks", The Guardian reports. The body makes vitamin D when sunlight falls on the skin, but people who cover up outside, and those who are housebound can struggle to make enough.

No large studies of vitamin D supplement use in the treatment of asthma have been conducted.

"Going to see your GP is a key part of the message we want to give - I don't think it would be appropriate to just start taking vitamin D without knowing whether you have vitamin D deficiency or not and we don't yet know what the threshold of vitamin D is below which you will have a benefit". The majority of patients had mild to moderate asthma, and a minority had severe asthma.

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.

They also found that vitamin D did not improve lung function or day-to-day asthma symptoms, and that it did not increase the risk of side effects at the doses that were tested.

The recommendation to take supplements usually comes as a result of a blood test that shows low levels of a certain vitamin. Martineau and colleagues analyzed the data of nine studies - lasting between 6-12 months - that assessed how vitamin D supplementation influences asthma symptoms and asthma attacks.