There are two basic types of vitamin D (note that the same food sources for D are those for Omega 3). Ergosterol is the basic building block of vitamin D in plants. Cholesterol is the basic building block of vitamin D in humans. When ultraviolet light from the sun hits the leaf of a plant, ergosterol is converted into ergocalciferol, or vitamin D2.

In just the same way, when ultraviolet light hits the cells of our skin, cholesterol is converted into cholecalciferol, or vitamin D3. Both of these substances - ergocalciferol in plants and cholecalciferol in humans - are usually referred to as "provitamin D," and both can be converted by the liver, and then the kidneys, into fully active vitamin D (called calcitriol).

Most of the time, however, when plant-based vitamin D (vitamin D2) is added to foods or supplements, the company only uses the plant food (like soybean) to get the ergosterol building block, and then irradiates this substance in a chemistry lab to produce vitamin D2.