Please explain importance of vitamin d? doc says my level is low. normal is 30-200 and mine is a 17

Low D causes illness: Low vitamin d is associated with depression, chronic back/musculoskeletal pain, poor immune function, increased risk of heart disease, many cancers, autoimmune disease and lower life expectancy. Everyone in northern latitudes (like ma) who does not take d in fall ; winter becomes deficient. One should take 5-8000 iu/day to achieve optimal levels of 50-80. See http://bit.Ly/1ekkvkv.