Scientists have suggested that the amount of time that asthma patients spend in the sun may have an impact on their illness.
A research team at King's College London said low levels of vitamin D - made by the body in sunlight - has been linked to a worsening of symptoms.
Its latest research shows that the vitamin calms an over-active part of the immune system in asthma, the BBC reported.
However, treatment of patients with vitamin D has not yet been tested.
Prof Catherine Hawrylowicz and her group researched the impact that vitamin D has on a chemical in the body, interleukin-17 - a vital part of the immune system, which helps to ward off infections.
But the chemical can cause problems when levels are too high, which has been strongly implicated in asthma.
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In the study, levels of interleukin-17 were brought down by vitamin D when it was added to blood samples taken from 28 patients.
The clinical trials are now being conducted by the researchers to see if soaking up the sun can ease the symptoms of the patient.
The study has been published in the Journal of Allergy and Clinical Immunology.