Vitamin D and health – Current perspectives and future directions
Nearly 100 years has passed since the discovery of vitamin D as the active component of cod-liver oil which cured the bone disease rickets. Since then our knowledge of vitamin D has expanded tremendously and has included recognition of the importance of UV radiation as a source of the vitamin as well as the discovery of the vitamin as a nutrient, a pro-hormone and a potent steroid hormone with a major role in calcium and bone metabolism. In the last 20 years or so, the discovery of the vitamin D receptor in over 30 different body tissues together with the existence of the alpha-1-hydroxylase enzyme in these tissues provided evidence of a pleiotropic role of vitamin D outside its classical role in the skeleton. These important discoveries have provided the basis for the increasing interest in vitamin D among the scientific community. The recent publication of the Dietary Reference Intake report on vitamin D and calcium by the North American Institute of Medicine (IOM) is the most comprehensive report on vitamin D to date and should serve as a vital resource for all scientists with an interest in vitamin D. Notwithstanding these very timely and exciting developments which have had a clear and lasting positive impact in the field of vitamin D nutrition, numerous questions still remain in many aspects of vitamin D. This short review will summarize the health outcomes associated with vitamin D and will outline the rationale used by the IOM in setting the new vitamin D Dietary Reference Intakes. Brief mention will also be made to the current problem of vitamin D under-nutrition across Europe. The review will also identify some key research needs for future studies in the area of vitamin D and health including a discussion of the potential for using genotyping in such studies.
OVERVIEW OF THE SYNTHESIS AND METABOLISM OF VITAMIN D
Vitamin D belongs to the group of lipid-soluble vitamins, together with vitamins A, E and K. The term ‘vitamin D’ was given during the early 1920’s to a group of closely- related secosteroids with antirachitic properties. Two of the most important nutritional forms of vitamin D are cholecalciferol (vitamin D3, derived from animal origin) and ergocalciferol (vitamin D2, derived from plant origin). However, natural dietary sources of vitamin D are limited with oily fish, egg yolk and meat contributing up to 90% of vitamin D intake from non-fortified food sources. Vitamin D3 and D2 can also be derived by photoirradiation from their precursors 7-dehydrocholesterol and ergosterol, respectively. In vertebrates, the cholesterol-like precursor, 7-dehydrocholesterol, present in the skin epidermis, undergoes photolysis when exposed to UV-B-light of wavelengths 290-315 nm to yield a variety of photoirradiation products including tachysterol, lumisterol and previtamin D3. Previtamin D3 then undergoes spontaneous thermal rearrangement to vitamin D3. Because of the skin’s ability to synthesise the vitamin u ...