3 Even if regularly exposed to sunlight, elderly people produce 75% less cutaneous D 3 than young adults. 2 Even those who normally reside in sunny climates are commonly found to be deficient in vitamin D, probably due to cultural habits and/or dress. In Minnesota in 2008, less than half of days provided enough solar UVB radiation at noon to effect cutaneous vitamin D production. These include time of day, season, latitude, altitude, clothing, sunscreen use, pigmentation, and age. However, many variables influence the amount of UVB from sunlight that reaches the skin and its effectiveness. Therefore, for most people, vitamin D is primarily obtained by cutaneous production from sun exposure. Worldwide, naturally occurring dietary sources of vitamin D are limited, and food fortification is optional, inconsistent, inadequate, or nonexistent. How Prevalent Is Vitamin D Deficiency And Who Is At Risk? The predominant effects of vitamin D are exerted through the endocrine and autocrine actions of calcitriol via activation of the vitamin D receptor in cells.
Once absorbed from the intestine, they are metabolized in the liver to 25-hydroxyvitamin D, composed of 25(OH)D 2 and 25(OH)D 3 25(OH)D (also called calcidiol) is subsequently converted to 1,25-dihydroxyvitamin D, also known as calcitriol, in the kidney and select other tissues by the action of the 1α-hydroxylase enzyme. This value varies greatly by region of the world because fortification markedly improves the availability of vitamin D through diet. Aside from rich sources such as oily fish, the vitamin D content of most foods is between 50 and 200 IU per serving. Vitamin D 3 (D 3), also known as cholecalciferol, is obtained primarily from skin exposure to ultraviolet B (UVB) radiation in sunlight, ingestion of food sources such as oily fish and variably fortified foods (milk, juices, margarines, yogurts, cereals, and soy), and oral supplements. Vitamin D 2 (D 2), also known as ergocalciferol, is obtained from dietary vegetable sources and oral supplements. Vitamin D consists of 2 bioequivalent forms. Recent advances in biochemical assessment, therapeutic goals for vitamin D nutrition for optimal bone health, and the association of vitamin D deficiency with nonskeletal disease have revived interest in this hormone. 1 Even so, 25% to 50% or more of patients commonly encountered in clinical practice are deficient in vitamin D. Vitamin D has been appreciated for its role in calcium homeostasis and bone health since its identification in 1921. AI = adequate intake CKD = chronic kidney disease D2 = vitamin D 2 D3 = vitamin D 3 1,25(OH)2D = 1,25-dihydroxyvitamin D HPT = hyperparathyroidism 25(OH)D = 25-hydroxyvitamin D PTH = parathyroid hormone UVB = ultraviolet B
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