Dr. Jerry Krishnan -- professor of medicine, pulmonary, critical care, sleep and allergy -- said the researchers are looking for participants who are age 18 or older and are using medications to control their asthma.
The researchers are looking for volunteers whose baseline vitamin D levels are low and whose asthma is not well controlled, Krishnan said.
"A number of people with asthma have low vitamin D levels," Krishnan said in a statement. "Patients with asthma and low vitamin D levels tend to have worse lung function, and tend to have more asthma attacks."
Study participants will receive vitamin D or a placebo and will continue on asthma medications. Over the course of nine months, the volunteers will have their lung function monitored.
"We want to understand if taking vitamin D allows their asthma to get better," Krishnan said.
The researchers said supplements might make it possible for people with low levels of vitamin D to use less asthma medication. However, taking vitamin D may result in side effects, so Krishnan warned against taking vitamin D for asthma outside of a study.