What is vitamin d3 vitamin d3 is a fat soluble vitamin found naturally in oily fish and eggs it can also be synthesised in the body with the presence of ultraviolet beta rays sunlight why use vitamin d3 vitamin d3 provides 625mcg of a high strength natural source of vitamin d3 meeting your daily needs of the essential mineral vitamin d3 contributes to the normal functioning of the immune system and absorption and utilisation of calcium contributing to the maintenance of normal bone