TOM R. HILL
School of Agriculture, Food and Rural Development, Human Nutrition Research Centre, Room 4.03 Agriculture Building, Newcastle University, NE1 7RU, United Kingdom

Abstract

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 ...