There are vitamin D receptors in nearly every cell in the human body which indicates just how important this nutrient is. The two most important forms of vitamin D are, D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D3 is the form that is synthesised in the skin from cholesterol when we are exposed to sufficient amounts of sunlight. Vitamin D from sunlight is an effective vitamin D source but is only produced when the sun is high in the sky. In Ireland, this can only happen during the summer period, and therefore we cannot produce sufficient levels of vitamin D during the rest of the year.
Vitamin D2 Is only available from the diet. It is produced by certain fungi and plants when they are exposed to ultraviolet light. Both vitamin D2 and D3 require subsequent activation in the liver and kidneys in order to become biologically active. Science used to believe that both forms of vitamin D were equally effective in the body. However, studies indicate that vitamin D3 is 56-87% more effective than vitamin D2 when it comes to raising blood levels of vitamin D. Moreover, D3 is stored in fat tissue more than three times as effectively as D2.*
*Heaney RP, et al. Vitamin D3 Is More Potent Than Vitamin D2 in Humans. J Clin Endocrinol Metab 2010.