Vitamin D-3 is a fat-soluble vitamin that has long been studied as an essential nutrient for bone health. It is also a critical nutrient for immune system function. Deficiency in vitamin D is associated with increased autoimmunity as well as increased susceptibility to infection. Vitamin D may also have an effect on mental health, as deficiency has also been associated with low mood and impaired cognitive performance. Although vitamin D can be synthesized in the skin upon exposure to sunlight, many environmental factors result in a significant portion of the population having sub-optimal vitamin D levels.