Vitamin D3 (as cholecalciferol) is a fat-soluble vitamin that's naturally present in very few food sources. It has many functions in the body from contributing to bone health to supporting immune function. NutraBio Vitamin D3 is the same form of Vitamin D3 the body manufactures when skin is exposed to ultraviolet (UV) sunlight. Vitamin D3 may be more than three times as effective than other forms of Vitamin D3 in raising concentrations in the bloodstream and maintaining those levels for a longer time.