Vitamins are nutrients essential for life that cannot be produced within the body. They must be consumed in the diet. Identification of the substance in cod liver oil that cured rickets led to the classification of vitamin D as a "vitamin." In reality, however, vitamin D is a "prohormone." It is a substance produced in the body upon exposure to ultraviolet solar radiation, which is then converted by the liver and kidneys into an active hormone. With abundant sunlight exposure we need not consume vitamin D. In fact, few sources of human food contain vitamin D, and as a rule 80% or more of our vitamin D is produced from sunlight exposure.

Humans evolved in open sunlight and needed no other source of vitamin D. However, as if Nature anticipated the move into colder, darker climates, she provided for limited dietary sources of this vitamin D. While these dietary sources do not provide for optimal levels of vitamin D as does sunlight exposure, they can provide enough vitamin D to prevent bone-damaging deficiency disease.