As many will know, although Asthma and COPD are different diseases, what helps with one will often help with the other. Also, a number of Asthmatics have Asthma / COPD overlap syndrome.
The link I’ve included below leads to an Article that discusses Vitamin D in the context of COPD, but will likely also have relevance for Asthma Suffers.
I also saw an interesting Article a while back on Medical News Today, discussing Vitamin D in the context of another serious respiratory disease called Interstitial Lung Disease. I mention this because Vitamin D benefits are now being discussed in the context of three different lung diseases (Asthma, COPD and Interstitial Lung Disease) coincidence, unlikely.
By the way, the article linked to below is interrupted in places by ad boxes that can give the impression that the article finishes at that point, whereas scrolling down you find that the article continues.