The amount of time asthma patients spend soaking up the sun may have an impact on the illness, researchers have suggested.
A team at King's College London said low levels of vitamin D, which is made by the body in sunlight, was linked to a worsening of symptoms.
Its latest research shows the vitamin calms an over-active part of the immune system in asthma.
However, treating patients with vitamin D has not yet been tested.
People with asthma can find it hard to breathe when their airways become inflamed, swollen and narrowed.
Most people are treated with steroids, but the drugs do not work for all.
"We know people with high levels of vitamin D are better able to control their asthma - that connection is quite striking," said researcher Prof Catherine Hawrylowicz.
Her group investigated the impact of the vitamin on a chemical in the body, interleukin-17. It is a vital part of the immune system and helps to fight off infections.