New evidence has shown that vitamin D supplements may actually help reduce the risk of severe asthma attacks, in addition to helping sufferers cut back on steroid treatments.
In this recently-published research, it was found that using vitamin D supplements decreased the occurrence of asthma attacks requiring visits to the hospital, going from 6% to 3%. Furthermore, taking a vitamin D supplement does not cause any noticeable side effects, making it a great choice for treatment if it does indeed work as well as suggested. The down side: researchers noted that the vitamin D supplements did not improve day-to-day lung function of asthma sufferers. 
Vitamin D supplements also did not seem to have any effect on the time people took off of work or school due to their asthma. As no one died during the study, it is also not possible to tell if the supplement reduced mortality risk.
Dr. Erika Kennington, from the charity Asthma UK, stated:
“While this research shows promise, more evidence is needed to conclusively show whether Vitamin D can reduce asthma attacks and symptoms. With so many different types of asthma it could be that Vitamin D may benefit some people with the condition but not others.
Asthma UK’s research centres are working hard to discover how and why Vitamin D affects asthma symptoms and if it could be a potential treatment in the future.”
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The studies, however, do not differentiate between those who have asthma and are vitamin D deficient. Researchers have recognized that the benefit may only be available for those who are low in the vitamin. Neither do the studies give asthma sufferers a specific dosage to take in order to help reduce their asthma attacks.
The NHS or National Health Service in the UK recommends that each person take 10 micrograms of vitamin D, especially during the winter months. However, they note that in this study, some people were taking doses that were much higher. Many individuals taking vitamin D supplements take 4,000-10,000 IUs daily.  
Researchers recommend that those who suffer with asthma see their doctor in light of these recent results to discuss vitamin D deficiency and anything else that can be done to help manage asthma.
The authors of the study emphasize that vitamin D was not used to replace asthma medication, but was used in conjunction.
 IFL Science!
 BBC News