The concept of supplementary food substances made its appearance earlier in this century. It was then clearly established that both fat- and water-soluble substances can prevent deficiency disease (DeLuca, 1981; DeLuca & Schnoes, 1983). A fatsoluble substance with antirachitic activity termed ‘vitamin D’ was identified in cod liver oil. Furthermore, it was demonstrated that ultraviolet light could heal rickets (DeLuca, 1981; DeLuca & Schnoes, 1983) by inducing antirachitic activity in the sterol fraction of food (DeLuca, 1981; DeLuca & Schnoes, 1983). This pioneering work provided the means for the prevention and elimination of rickets as a major medical problem.

It is now recognized that the role of vitamin D is much broader than regulation of calcium metabolism as reflected by its effects on the proliferation and differentiation of a variety of cells, including normal, malignant and immune cells (Holick, 1995).

Classical actions of vitamin D

A deficiency of vitamin D results in one of two diseases: either rickets in children or osteomalacia in the adult. The major target tissues of vitamin D in the regulation of calcium metabolism are the intestine, bone and kidney. Parathyroid hormone (PTH) is required for calcium mobilization from bone and for renal conservation of calcium, but vitamin D itself, stimulates intestinal calcium absorption.