Photo: Ryan Edwards-Crewe
A vitamin can be defined as a substance that is all of the following (a) soluble in either water or fat; (b) needed to maintain life but does not supply energy; and (c) a deficiency of which may result in various diseases.
An deficiency example would be scurvy, which results from a deficiency of vitamin C and was a common disease among sailors who had few fresh fruits and vegetables to eat.
There are five forms of vitamin D, of which two (D2 and D3) are especially important. In fact, the label “vitamin D” normally refers exclusively to them – unless otherwise specified – and it is those two that we will discuss.