A study has revealed that adding vitamin D to asthma treatment to improve breathing only appears to benefit patients who achieve sufficient levels of the supplement in the blood.

According to new research at Washington University School of Medicine in St. Louis, overall, the ability to control asthma did not differ between a study group that received vitamin D supplements and a group that received placebo.

In the study, all patients took an inhaled steroid daily to control their asthma, and all had rescue inhalers in the event of an asthma attack.

The patients were randomly assigned to one of two groups. The treatment group received a loading dose of 100,000 international units of vitamin D3 followed by daily doses of 4,000 units, and the placebo group received identical looking but inactive capsules.

The investigators found no differences between the two groups in all major measures of asthma control. The groups showed no significant differences in the number of treatment failures requiring patients to take more medication, no difference in the number of asthma attacks and no difference in their need for emergency care. Patients taking vitamin D did not report improved quality of life, based on questionnaires.

One way the groups differed, however, was in how successfully they were able to reduce their daily dosages of inhaled steroids. After the first 12 weeks of the study, if the patient's asthma was well-controlled, the investigators cut in half the daily dose of inhaled steroid, reducing it from 320 micrograms per day to 160. Then, after eight more weeks, if the disease remained controlled, they cut the dose in half again. While both groups were able to taper off their doses of inhaled steroid, the vitamin D group was able to reduce its medication more.

By the end of the 28-week study, the vitamin D group was taking an average of 111 micrograms per day, and the placebo group was taking an average of 126.

Compared with placebo, the patients in the treatment group that achieved vitamin D sufficiency in the blood (with an average of 42 nanograms of vitamin D per milliliter) did show improved asthma control. They had 40 percent fewer treatment failures that required more medication and half the number of asthma attacks.

The study has been published online in JAMA.