Having low levels of vitamin D worsens asthma symptoms and makes asthmatics less likely to respond to steroid treatment, a new study has found.

Researchers at National Jewish Health in Denver took the vitamin D levels of 54 asthmatics and tested them for lung function, constriction of the airways and responsiveness to steroid treatment.

They found that those with low vitamin D levels did worse on every measure and airway constriction almost doubled in those with vitamin D levels below 30 nanograms/ml.