tissue in the body. Because of this storage, it takes quite a while for vitamin D levels measured in the blood to come up after regular sun exposure, or after taking an oral dose of vitamin D. So, for a person without MS, if the level is low some doctors would simply start a supplement, and the blood level would eventually come up to normal over a period of months. For PwMS, however—particularly those just diagnosed or having a relapse—it is critical to get the blood level up quickly if it is low,