Helps Maintain Strong Bones. With Calcium Citrate. Vitamin D is normally obtained from the diet or produced by the skin from the ultraviolet energy of the sun. However, it is not abundant in food. As more people avoid sun exposure, Vitamin D supplementation becomes even more necessary to ensure that your body receives an adequate supply. NOW Vitamin D-3 Powder supplies this key vitamin along with Calcium Citrate to further support bone health.