Humans obtain vitamin D from diet and sunlight. There are two main forms of vitamin D: cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). The main source of vitamin D3 is from exposure to sunlight, which accounts for more than 90% of the body’s vitamin D requirement. Vitamin D3 is synthesized in the human skin by a photochemical conversion via ultraviolet B (UVB) exposure of provitamin D3 (7-dehydrocholesterol). Vitamin D2 is sourced from the UV irradiation of ergosterol, which is a steroid found in some plants, but largely fungi. Humans have a combination of vitamin D3 and vitamin D2 available to them from sunlight UV exposure (vitamin D3), egg yolks and oily fish (vitamin D3), fortified margarine, milk and breakfast cereals (fortified vitamin D2) and dietary supplements (vitamin D2 and vitamin D3). Please note that we do not recommend foods fortified with vitamin D.
Both vitamin D2 and vitamin D3 undergo similar enzymatic hydroxylation processes which convert them to the active metabolite 1,25-dihydroxy vitamin D2 or D3 (calcitriol). The liver and kidneys participate to make this active form of the vitamin that the body can use. However, there may be differences in their respective efficacies in raising serum 25(OH)D, which is a marker of vitamin D status. Differences may be due to their differing affinities for the vitamin D receptor. It is also thought that vitamin D3 is the preferred substrate for hepatic 25-hydroxylase.
Some studies indicate that vitamin D3 is more efficacious at raising serum 25(OH)D concentrations than is vitamin D2; and that vitamin D2 is 30% to 50% less effective than vitamin D3 in maintaining serum 25(OH)D levels.