CIE 219

Abstract

Scope: Introduction Vitamin D is widely accepted as necessary for the regulation of calcium homeostasis and thus a healthy skeleton. Other potential health effects of vitamin D are discussed in Clause 3. The active metabolite of the vitamin is 1,25-dihydroxyvitamin D (1,25(OH)2D), formed after hydroxylations of vitamin D in first the liver and then the kidney, though both these hydroxylations also occur in other tissues. Production of 1,25(OH)2D by the kidney is tightly controlled and it is the intermediate 25-hydroxyvitamin D (25(OH)D) that is observed to rise and fall with supply of the vitamin, and its concentration in the blood is used as a measure of vitamin D status. Vitamin D (strictly a pro-hormone) is unique amongst vitamins in having a second source pathway that is independent of oral intake. A major source of the vitamin is through synthesis in the skin. This is initiated when the precursor 7-dehydrocholesterol (7DHC) is transformed into previtamin D3 (pre-D3) by the action of ultraviolet radiation (naturally occurring in sunlight). The photochemistry of vitamin D3 is complex as several other isomers of pre-D3 are also formed in sunlight, influencing the eventual formation, via a slow heat isomerization, of vitamin D3. For those unfamiliar with the processes that lead from UV radiation falling on the skin to the active 1,25(OH)2D3, a more detailed description is provided in Annex A. For those living at middle to high latitudes there is a period of the year (the length increases with latitude) when it is impractical to synthesize any appreciable vitamin D3 in the skin through exposure to solar UV radiation. The primary reason is that the solar elevation is so low that there is very little radiation of the appropriate wavelengths (essentially UV-B radiation) in sunlight reaching these positions on the Earth’s surface. In addition, day length decreases in winter so there is less time available for sun exposure, and the temperature is often not conducive to exposing significant areas of skin to sunlight. During this period, dubbed the “vitamin D winter”, circulating levels of 25(OH)D are observed to decrease. If the vitamin D winter is prolonged, and/or the summer end levels of circulating 25(OH)D are themselves low, then vitamin D status is likely to fall to undesirable levels before the next summer. See Clause 9 for discussion of target vitamin D status. To prevent this drop in circulating 25(OH)D either UV exposure must be increased (the possibilities of using artificial sources are the subject of this report), or oral intake of vitamin D must replace synthesis in the skin. Note that there are two forms of vitamin D, vitamin D3 that is synthesized in skin, and vitamin D2 which is a plant form of the vitamin. Dietary intake may be in either form, and vitamin D (no subscript) can refer to either or both forms. Measurements of circulating 25(OH)D generally refer to the sum of the hydroxylated version of both forms of the vitamin, though some assays can distinguish between the two forms if necessary.