Vitamin D has so much good literature comming out.  Helps with Calcium absorption and helps prevent fractures and falls by increasing muscle strength by 30% in elderly in nursing homes.  Vitamin D is needed for muscle contraction.  The sun is an excellent free source of D3 (the best D) and you need 20min/day to forarms and face.  1000-2000mg is recommended for a daily dose.  800mg is ok too if that is easier to find (2x 400mg). Most people up north have low levels especially in the winter and if they work inside most of the time.