Decades ago when I was diagnosed with MS, they saw the pattern of living further away from the equator increased the incidence of MS. They thought however, it might have something to do with getting ill more often and being exposed to a particular viral agent (the mystery trigger for MS). That may still be a part of it, but the following study, Vitamin D Supplements Reduce Relapses in Multiple Sclerosis, illustrates that Vitamin D plays a huge role. When I first saw my functional medicine MD, my level was 41.6 ng/ml. I still take 5000 IU daily.