About Vitamin D

History of Vitamin D

Historical Review

Man is reported to have been aware since early antiquity of the substance we now know as vitamin D. The first scientific description of a vitamin D-deficiency, namely rickets, was provided in the 17th century by both Dr. Daniel Whistler (1645) and Professor Francis Glisson (1650). The major breakthrough in understanding the causative factors of rickets was the development in the period 1910 - 1930 of nutrition as an experimental science and the appreciation of the existence of vitamins.

Considering the fact that now we accept that the biologically active form of vitamin D, namely 1a,25(OH)2-vitamin D3, is a steroid hormone, it is somewhat ironic that vitamin D, through a historical accident, became classified as a vitamin. It was in 1919/20 that Sir Edward Mellanby, working with dogs raised exclusively indoors (in the absence of sunlight or ultraviolet light), devised a diet that allowed him to unequivocally establish that the bone disease, rickets was caused by a deficiency of a trace component present in the diet. In 1921 he wrote, "The action of fats in rickets is due to a vitamin or accessory food factor which they contain, probably identical with the fat-soluble vitamin." Furthermore, he established that cod liver oil was an excellent antirachitic agent.

Shortly thereafter E.V. McCollum and associates observed that by bubbling oxygen through a preparation of the "fat-soluble vitamin" they were able to distinguish between vitamin A ( which was inactivated) and vitamin D (which retained activity). In 1923 Goldblatt and Soames clearly identified that when a precursor of vitamin D in the skin (7-dehydrocholesterol) was irradiated with sunlight or ultraviolet light, a substance equivalent to the fat-soluble vitamin was produced. Hess and Weinstock confirmed the dictum that "light equals vitamin D". They excised a small portion of skin, irradiated it with ultraviolet light, and then fed it to groups of rachitic rats. The skin that had been irradiated provided an absolute protection against rickets, whereas the unirradiated skin provided no protection whatsoever; clearly, these animals were able to produce by uv irradiation adequate quantities of "the fat-soluble vitamin", suggesting that it was not an essential dietary trace constituent. In parallel studies, Steenbock and Black at the Biochemistry Department of the University of Wisconsin found that rat food which was irradiated with ultra violet light also acquired the property of being antirachitic. However, because of the rapid rise of the science of nutrition -- and the discovery of the families of water-soluble and fat-soluble vitamins -- it rapidly became firmly established that the antirachitic factor was to be classified as a vitamin.

The chemical structures of the vitamins D were determined in the 1930s in the laboratory of Professor Adolf Otto Reinhold Windaus at the University of Göttingen in Germany. Professor Windaus had some 55 doctoral and postdoctoral chemists working on the 'vitamin D project'. Professor Windaus received a Nobel Prize in Chemistry in 1928 for his work on sterols and their relationship to vitamins.

Vitamin D2 which could be produced by ultraviolet irradiation of ergosterol was chemically characterized in 1932. Vitamin D3 was not chemically characterized until 1936 when it was shown to result from the ultraviolet irradiation of 7-dehydrocholesterol. Virtually simultaneously, the elusive antirachitic component of cod liver oil was shown to be identical to the newly characterized vitamin D3. These results clearly established that the antirachitic substance vitamin D was chemically a steroid, more specifically a seco-steroid.

Key reference citations:

Whistler, D. Morbo puerili Anglorum, quem patrio idiomate indigenae vocant The Rickets. Lugduni Batavorum 1-13 (1645).

Glisson, F. De Rachitide sive morbo puerili, qui vulgo The Rickets diciteur, London 1-416 (1650).

Glisson, F. A treatise of the rickets being a disease common to children. London 1-373 (1668).

Mellanby, E. and Cantag, M.D. Experimental investigation on rickets. Lancet 196:407-412 (1919).

Mellanby, E. Experimental rickets. Medical Research (G.B.), Special Report Series SRS-61:1-78 (1921).

Hess, A. Influence of light on the prevention of rickets. Lancet 2:1222 (1922).

McCollum, E.V., Simmonds, N., Becker, J.E. and Shipley, P.G. Studies on experimental rickets. XXI. An experimental demonstration of the existence of a vitamin which promotes calcium deposition. J. Biol. Chem. 53:293-312 (1922).

Goldblatt, H. and Soames, K.N. A study of rats on a normal diet irradiated daily by the mercury vapor quartz lamp or kept in darkness. Biochem. J. 17:294-297 (1923).

Steenbock, H. and Nelson, M. T. Fat-soluble vitamins. XIX. The induction of calcifying properties in a rickets-producing ration by radiant energy. Methods Enzymol. 62:209-216 (1924).

Steenbock, H. The induction of growth promoting and calcifying properties in a ration by exposure to light. Science 60:224-225 (1924).

Windaus, A., Linsert, O. Luttringhaus, A. and Weidlinch, G. Uber das krystallistierte Vitamin D2. Justis. Liebigs. Ann. Chem. 492:226-231 (1932).

Brockmann, H. Die Isolierung des antirachitischen Vitamins aus Thunfischleberol. H.-S.Zeit. Physiol. Chem. 241:104-115 (1936).

Crowfoot-Hodgkin, D., Webster, M.S. and Dunitz, J.D. Structure of calciferol. Chem. Industry 1148-1149 (1957).

Solecki, R.S. Shanidar: The Humanity of Neanderthal Man, New York: Knopf. pp. 1-252 (1971).

Article was Last Updated: November 2011.

Nutritional Aspects of Vitamin D

Introduction

A "vitamin" by definition is a substance regularly required by the body in small amounts but which the body cannot make and is, therefore, required to be supplied in the daily diet. Technically the molecular species classified as vitamin D3 is not really a vitamin because it can be produced by exposure of the skin to sunlight (see section on Chemistry). However, for nutritional and public health reasons, vitamin D3 continues to be classified officially as a vitamin (see section on History of Vitamin D).

Nutritional Aspects

The World Health Organization had responsibility for defining the "International Unit" of vitamin D3. Their most recent definition, provided in 1950 states that "the International Unit of vitamin D recommended for adoption is the vitamin D activity of 0.025 micrograms (25 nanograms) of the international standard preparation of crystalline vitamin D3". Thus, 1.0 IU of vitamin D3 is 25 nanograms, which is equivalent to 65.0 pmoles. With the discovery of the metabolism of vitamin D3 to other active seco-steroids, particularly 1α,25(OH)2D3, it was recommended that 1.0 unit of 1α,25(OH)2D3 be set equivalent in molar terms to that of the parent vitamin D3. Thus, 1.0 unit of 1α,25(OH)2D3 has been operationally defined to be equivalent to 65 pmoles.

The vitamin D requirement for healthy adults has never been precisely defined. Since vitamin D3 is produced in the skin after exposure to sunlight, the human does not have a requirement for vitamin D when sufficient sunlight is available. However, vitamin D does become an important nutritional factor in the absence of sunlight. It is known that a substantial proportion of the U.S. population is exposed to quite suboptimal levels of sunlight especially during the winter months; it is likely that during these intervals that a regular dietary supply of vitamin D3 should be provided. In addition to geographical and seasonal factors, ultraviolet light from the sun may also be blocked by air pollution. The tendency to wear clothes, to live in cities where tall buildings block adequate sunlight from reaching the ground, to live indoors, to use synthetic sunscreens that block ultraviolet rays, and to live in geographical regions of the world that do not receive adequate sunlight, all contribute to the inability of the skin to biosynthesize sufficient amounts of vitamin D3. Under these conditions vitamin D becomes a true vitamin in that it must be supplied in the diet on a regular basis.

Since vitamin D3 can be endogenously produced by the body and since it is retained for long periods of time by vertebrate tissue, it is difficult to determine with precision the minimum daily requirements for this seco-steroid. The requirement for vitamin D is also known to be dependent on the concentration of calcium and phosphorus in the diet, the physiological stage of development, age, sex, degree of exposure to the sun, and the amount of pigmentation in the skin.

In November of 2010, , the Institute of Medicine's (IOM) special committee of 15 experts from the US and Canada issued its report for the citizens of both countries defining the formal Dietary Reference Intakes (average daily doses) of vitamin D and calcium required for good health. Their recommendation for vitamin D is that from ages 1 to 70, people need to consume no more than 600 International Units (IU) per day. For individuals 70 or older, the recommendation is 800 IU/day to maintain strong bone. This is only a modest adjustment of the very conservative advice rendered by the 1997 IOM committee of 200 – 600 IU/day, depending upon age.

In the United States, adequate amounts of vitamin D can readily be obtained from the diet and from casual exposure to sunlight. However, in some parts of the world where food is not routinely fortified and sunlight is often limited during some periods of the year, obtaining adequate amounts of vitamin D becomes major problem. The 13th and 14th Vitamin D Workshops reported in white papers that two thirds of the world population has a vitamin D deficiency.

Vitamin D3 versus Vitamin D2

For decades since the determination of the chemical structures of vitamin D3 and vitamin D2 in the 1930's it has been assumed that both vitamins had equivalent biological activity in humans. This was based on biological determination in rats of their comparative antirachitic activity. However in 1997, the IOM vitamin D reference intake publication for vitamin D, serum 25-hydroxyvitamin D [25(OH)D], rather than antirachitic activity, was defined as the functional indicator of vitamin D status.

In a 2010 paper by R. Heaney and coworkers it was reported that vitamin D3 is approximately 87% more potent in raising and maintaining serum 25(OH)D levels than was vitamin D2. In addition, vitamin D3 produced a 2- to 3-fold greater storage of vitamin D than does equimolar D2. For

neither was there evidence of sequestration in fat, as had been postulated for doses in this range.

Thus the authors felt that given the greater potency and lower cost, vitamin D3 should be the preferred choice for correcting vitamin D deficiency in humans.

Food Sources

Animal products constitute the bulk source of vitamin D that occurs naturally in unfortified foods. Salt water fish such as herring, salmon, sardines, and fish liver oils are good sources of vitamin D3. Small quantities of vitamin D3 are also derived from eggs, veal, beef, butter, and vegetable oils while plants, fruits, and nuts are extremely poor sources of vitamin D. In the United States, artificial fortification of foods such as milk (both fresh and evaporated), margarine and butter, cereals, and chocolate mixes help in meeting the RDA recommendations.

References:

Heaney,R.P.; Recker,R.R.; Grote,J.; Horst,R.L.; Armas,L.A. Vitamin D3 is more potent than vitamin D2 in humans J.Clin.End.Metab.93: 447-452 (2011).

Vieth, R.., Why the minimum desirable serum 25-hydroxyvitamin D level should be 75 nmol (30 ng/ml), Best Practice & Research- Clinical endocrinology & Metabolism, 25:681-692 (2011).

Bouillon, R.., Why modest but widespread improvement of vitamin D status is the best strategy. Best Practice & Research- Clinical endocrinology & Metabolism, 25:693-692702 (2011).

Institute of Medicine (2011) Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press.

Norman, A.W. and Bouillon, R. Vitamin D nutritional policy needs a vision for the future. Exp. Biol. Med. 235:1034-1045 (2010).

Henry, H.L., Bouillon, R., Norman, A.W., Gallagher, J.C., Lips, P., Heaney, R.P., Vieth, R.,Pettifor, J.M., Dawson-Hughes, B., Lamberg-Allardt, C.J., and Ebeling, P.R. 14th Vitamin D Workshop consensus on vitamin D nutritional guidelines. J. Steroid Biochem. Mol. Biol. 121:4-6 (2010).

Norman, A.W. From Vitamin D to hormone D: Fundamentals of the vitamin D endocrine system essential for good health. Amer. J. Clin. Nutrition. 88(2):4915-4995 (2008).

Vieth, R., Bischoff-Ferrari, H., Boucher, B., Dawson-Hughes, B., Garland, C., Heaney, R., Holick, M., Hollis, B., Lamberg-Allardt, C., McGrath, J., Norman, A., Scragg, R., Whiting, S., Willett, W., and Zittermann, A. The urgent need to recommend an intake of vitamin D that is effective. Am. J. Clin. Nutr. 2007 85: 649-650 (2007).

Norman, A.W., Henry, H.L. Vitamin D In: Present Knowledge in Nutrition, 9th Edition, (Bownam, B.A. and Russell, R.M.), International Life Sciences Institute, Washington D.C. Chapter 12, pp 198-210 (2006).

Subcommittee on the Tenth Edition of the RDAs, Food & Nutrition Board, Commission on Life Sciences and National Research Council. Recommended dietary allowances, Washington, D.C.: National Academy Press. Ed. 10th pp. 1-285 (1989).

Dietary reference intakes for calcium, magnesium, phosphorus, vitamin D, and fluoride. Food and Nutrition Board, Institute of Medicine. Washington, DC: National Academy Press (1997).

Article Last Updated: November 2011.

Chemistry

Chemistry of Vitamin D

The structures of vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) chem1 and their provitamins are presented in Figure 1 on the right. Vitamin D is a generic term and indicates a molecule of the general structure shown for rings A, B, C, and D with differing side chain structures. The A, B, C, and D ring structure is derived from the cyclopentanoperhydrophenanthrene ring structure for steroids. Technically vitamin D is classified as a seco-steroid. Seco-steroids are those in which one of the rings has been broken; in vitamin D, the 9,10 carbon-carbon bond of ring B is broken, and it is indicated by the inclusion of "9,10-seco" in the official nomenclature.

Vitamin D (calciferol) is named according to the revised rules of the International Union of Pure and Applied Chemists (IUPAC). Because vitamin D is derived from a steroid, the structure retains its numbering from the parent compound cholesterol. Asymmetric centers are designated by using the R,S notation; the configuration of the double bonds are notated E for "entgegen" or trans, and Z for "zuzammen" or cis. Thus the official name of vitamin D3 is 9,10-seco(5Z,7E)-5,7,10(19)cholestatriene-3b-ol, and the official name of vitamin D2 is 9,10-seco(5Z,7E)-5,7,10(19), 22-ergostatetraene-3b-ol.

chem

Vitamin D3 can be produced photochemically by the action of sunlight or ultraviolet light from the precursor sterol 7-dehydrocholesterol which is present in the epidermis or skin of man and most higher animals. The chief structural prerequisite of a provitamin D is that it be a sterol with a D5,7 diene double bond system in ring B (Figure 2 to the left). The conjugated double bond system in this specific location of the molecule allows the absorption of light quanta at certain wavelengths in the UV range; this can readily be provided in most geographical locations by natural sunlight (or UV-B). This initiates a complex series of transformations ( partially summarized above in Fig. 1) that ultimately results in the appearance of vitamin D3. Thus, it is important to appreciate that vitamin D3 can be endogenously produced and that as long as the animal (or human) has access on a regular basis to sunlight there is no dietary requirement for this vitamin.

References

Crowfoot-Hodgkin, D., Webster, M.S. and Dunitz, J.D. Structure of Calciferol. Chem. Industry 1148-1149 (1957).

Calverley, M.J. and Jones, G. Vitamin D. In: Antitumor Steroids, edited by Blickenstaff, R.T. San Diego: Academic Press, pp. 193-270 (1992).

Ikekawa, N. and Ishizuka, S. Molecular structure and biological activity of vitamin D metabolites and their analogs. In: Molecular Structure and Biological Activity of Steroids. Boca Raton: CRC Press, pp. 293-316 (1993).

Zhu, G.-D and Okamura, W.H. Synthesis of vitamin D (calciferol). Chem. Rev. 95:1877-1952 (1995).

Ma,Y.; Khalifa,B.; Yee,Y.K.; Lu,J.; Memezawa,A.; Savkur,R.S.; Yamamoto,Y.; Chintalacharuvu,S.R.; Yamaoka,K.; Stayrook,K.R.; Bramlett,K.S.; Zeng,Q.Q.; Chandrasekhar,S.; Yu,X.P.; Linebarger,J.H.; Iturria,S.J.; Burris,T.P.; Kato,S.; Chin,W.W.; Nagpal,S. Identification and characterization of noncalcemic, tissue-selective, nonsecosteroidal vitamin D receptor modulators J. Clin.Invest.116:892-904 (2006).

Zhang,F.; Nunes,M.; Segmuller,B.; Dunphy,R.; Hesse,R.H.; Setty,S.K. Degradation chemistry of a Vitamin D analogue (ecalcidene) investigated by HPLC-MS, HPLC-NMR and chemical derivatization J. Pharm. Biomed. Anal. 40: 850-863 (2006).

Article Last Updated: November 2011.

Figure 1

Structural relationship of vitamin D3 (cholecalciferol) and vitamin D2(ergocalciferol) with their respective provitamins, cholesterol, and a classic steroid hormone, cortisol (see inset box). The two structural representations presented at the bottom for both vitamin D3 and vitamin D.2 are equivalent; these are simply different ways of drawing the same molecule. It is to be emphasized that vitamin D3 is the naturally occurring form of the vitamin; it is produced from 7-dehydrocholesterol, which is present in the skin, by the action of sunlight (see Figure 2). Vitamin D2 (which is equivalently potent to vitamin D3 in humans and many mammals, but not birds) is produced commercially by the irradiation of the plant sterol ergosterol with ultraviolet light.

Figure 2

Photochemical pathway of production of vitamin D3 (cholecalciferol) from 7-dehydrocholesterol. The starting point is the irradiation of a provitamin D, which contains the mandatoryD5,7-conjugated double bonds; in the skin this is 7-dehydrocholesterol. After absorption of a quantum of light from sunlight (UV-B), the activated molecule can return to the ground state and generate at least six distinct products. The four steroids that do not have a broken 9, 10-carbon bond (provitamin D, lumisterol, pyrocalciferol, and isopyrocalciferol) represent the four diastereomers with either an a- or b-orientation of the methyl group on carbon-10 and the hydrogen on carbon-9. The two secosteroid products, vitamin D3, previtamin D3, and tachysterol3 have differing positions of the three conjugated double bonds. In the skin, the principal product is previtamin D3, which then undergoes a 1,7-sigmatropic hydrogen transfer from C-19 to C-9, yielding the final vitamin D3: Vitamin D3 can be drawn as either a 6-s-trans representation or as 6-s-cis representation depending upon the state of rotation about the 6,7-single bond. The resulting vitamin D3, which is formed in the skin, is removed by binding to the plasma transport protein, the vitamin D-binding protein (DBP), present in the capillary bed of the dermis. The DBP-D3 then enters the general circulatory system.

Biochemistry and Physiology of Vitamin D

Biochemistry and Physiology of the Vitamin D Endocrine System

phys

A detailed study of the biochemical mode of action of the fat-soluble vitamin D was not possible until the availability in the 1960s of preparations of high specific activity radioactive vitamin D. As a consequence of efforts in several laboratories a new model emerged in the late 1960’s to describe the biological mechanisms of action of vitamin D3. This model is based on the concept that, in terms of its structure and mode of action, vitamin D is similar to the classic steroid hormones, e.g. aldosterone, testosterone, estradiol, progesterone, cortisol, and ecdysterone.

As summarized in the figure on the left, the existence of the vitamin D endocrine system is now firmly established.

The key elements of the vitamin D endocrine system include the following:

(a) In the skin, photoconversion of 7-dehydrocholesterol to vitamin D3 or dietary intake of vitamin D3.

(b) Metabolism of vitamin D3 by the liver to 25(OH)D3; this the major form of vitamin D circulating in the blood compartment.

(c) Functioning of the kidney as an endocrine gland, to metabolize 25(OH)D3 to produce the two principal dihydroxylated vitamin D metabolites, namely 1a,25(OH)2D3 and 24R,25(OH)2D3.

(d) Systemic transport of the dihydroxylated metabolites 1a,25(OH)2D3 and 24R,25(OH)2D3 to distal target organs by the plasma vitamin D binding protein (DBP).

(e) Binding of the dihydroxylated metabolites, particularly 1a,25(OH)2D3, to a receptor a that is localized in the nucleus and plasma membrane of t the target organs followed by the subsequent generation of appropriate biological responses (both genomic and rapid responses).

An additional key component in the operation of the vitamin D endocrine system is the plasma vitamin D binding protein (DBP) that carries the hydrophobic vitamin D3 and all of its metabolites through the circulatory system to their various target organs. A target organ, by definition will have the vitamin receptor, the VDR.

Since 1971, research efforts have largely focused upon understanding how 1a,25(OH)2D3 generates biological responses. From 1960 – 2021approximately 25,000 scientific papers were published that used the term vitamin D either in the title or abstract. By comparison, the biological actions of 24R,25(OH)2D3 have been relatively less studied. However, evidence has been presented to support the view that the combined presence of both 1a,25(OH)2D3 and 24R,25(OH)2D3 are required to generate the complete spectrum of biological responses attributable to the parent vitamin D.

Metabolism of Vitamin D

Thus, vitamin D3 is, in reality, a prohormone and is not known to have any intrinsic biological activity itself. It is only after vitamin D3 is metabolized, first into 25(OH)D3 in the liver, and then into 1a,25(OH)2D3 and 24R,25(OH)2D3 by the kidney, that biologically active molecules are produced. In toto some 37 vitamin D3 metabolites have been isolated and chemically characterized.

The key kidney enzymes, the 25(OH)D3-1-hydroxylase and the 25(OH)D3-24-hydroxylase, as well as the liver vitamin D3-25-hydroxylase, are all known to be cytochrome P-450 mixed-function oxidases. Both of the renal enzymes are localized in mitochondria of the proximal tubules of the kidney. Mixed-function oxidases use molecular oxygen as the oxygen source instead of water. Mitochondrial mixed-function oxidases are composed of three proteins that are integral components of the mitochondrial membrane; they are renal ferredoxin reductase, renal ferredoxin, and cytochrome P-450.

The most important point of regulation of the vitamin D endocrine system occurs through the stringent control of the activity of the renal 1-hydroxylase. In this way the production of the hormone 1a,25(OH)2D3 can be modulated according to the calcium and other endocrine needs of the organism. The chief regulatory factors are 1a,25(OH)2D3 itself, parathyroid hormone (PTH), and the serum concentrations of calcium and phosphate. The most important determinant of the 1-hydroxylase activity is the vitamin D status of the animal. When circulating concentrations of 1a,25(OH)2D3 are low, production of 1a,25(OH)2D3 by the kidney is high, and when circulating concentrations of 1a,25(OH)2D3 are high, the output of 1a,25(OH)2D3 by the kidney is sharply reduced.

Actions of the vitamin D receptor:

1a,25(OH)2D3working with its VDR is known to selectively activate ≈ 3% of the some 22,000 genes of the human genome. The regulation of gene transcription by 1a,25(OH)2D3 is known to be mediated by interaction of this ligand with its personal nuclear receptor protein, termed the VDR. The VDR is known to occur in over 35 different cell types. 1a,25(OH)2D3 when bound to the VDR regulates the transcription of numerous proteins. In addition, the VDR is known to localize with the plasma membrane of the target cell where it initiates rapid responses (e.g. opening of chloride or calcium channels or stimulating exocytosis). A number of excellent articles have appeared describing the current understanding of how the VDR regulates both gene transcription and rapid responses.

References

References for lay persons:

Norman, A.W., Henry, H.L. Vitamin D In: Present Knowledge in Nutrition, 9th Edition, (Bownam, B.A. and Russell, R.M.), International Life Sciences Institute, Washington D.C. Chapter 12, pp 198-210 (2006).

Comprehensive references to review articles covering all aspects of vitamin D with particular emphasis on 1a,25(OH)2D3:

Vitamin D, 3rd Ed. Edited by Feldman, D., Pike, J.W, Adams, J.S. San Diego, Academic Press, pp. 1-2081 (2011).

Henry, H.L., Regulation of vitamin D metabolism, Best Practice & Research- Clinical endocrinology & Metabolism, 25:531-541 (2011).

Haussler, M.R., Jurutka, P.W., Mizwicki, M., & Norman, A.W., Vitamin D receptor (VDR)-mediated actions of 1a,25(OH)2-vitamin D3: Genomic and non-genomic mechanisms. Best Practice & Research- Clinical endocrinology & Metabolism, 25:543-559 (2011).

Bouillon,R.; Carmeliet,G.; Verlinden,L.; van Etten,E.; Verstuyf,A.; Luderer,H.F.; Lieben,L.; Mathieu,C.; Demay,M., Vitamin D and human health: Lessons from vitamin D receptor null mice, Endocr.Rev.6: 726-776 (2008).

Pike,J.W.; Meyer,M.B.; Watanuki,M.; Kim,S.; Zella,L.A.; Fretz,J.A.; Yamazaki,M.; Shevde,N.K., Perspectives on mechanisms of gene regulation by 1,25-dihydroxyvitamin D3 and its receptor, J.Steroid Biochem.Mol.Biol., 103: 389-395 (2007).

Norman, A.W. Vitamin D Receptor (VDR): New assignments for an already busy receptor. Endocrinology 147: 5542-5548 (2006).

Bouillon, R., Okamura, W.H. and Norman, A.W. Structure-function relationships in the vitamin D endocrine system. Endocr. Rev. 16:200-257 (1995).

Article Last Updated: November 2011.

Figure 1

Summary of the vitamin D endocrine system. In addition, to production of 1a,25(OH)2D3 and 24R,25(OH)2D3 by the endocrine gland function of the kidney, small amounts of 1a,25(OH)2D3 are also produced in a paracrine fashion and by the placenta during pregnancy. Target organs and cells for 1a,25(OH)2D3 by definition contain nuclear receptors for 1a,25(OH)2D3 (VDRnuc). Also, 1a,25(OH)2D3 generates biological effects involving rapid signal transduction pathways utilizing a putative membrane receptor. The precise biological roles of 24,25(OH)2D3 are not yet defined although it is believed to function in bone and cartilage.

Disease and Vitamin D

contributions

The figure to the right summarizes the contributions of vitamin D to good health. Over the past decade, four lines of investigation have collectively yielded striking new insights into the many newly appreciated actions of vitamin D. These include the following: (i) a broad range of molecular and cellular effects of 1a,25(OH)2D3; (ii) experimental studies in the VDR-KO mouse model; (iii) several large observational epidemiological studies in subjects with variable nutritional vitamin D status ; and (iv) prospective randomized intervention studies with vitamin D.

Consequently, evidence has accumulated that beside the calcium homeostasis system (intestine, kidney, bone and the parathyroid gland) there are five additional physiological systems where VDR + 1α,25(OH)2D generates essential biological responses; see the first column under the header of Physiological Systems.. These include the immune system (both innate and adaptive), pancreas and glucose and fat metabolism, heart-cardiovascular, muscle and brain systems as well as the control of the cell cycle in virtually all cells and thus of the disease process of cancer

Acting through the VDR, 1a,25(OH)2D can produce a wide array of favorable biological effects that collectively are projected to contribute to the improvement of human health.; see the second column under the header Biological Responses.   The third column under the header VitaminD Deficiency Associated Diseases identifies for each physiological system some of the disease states that are associated with an inadequate vitamin D nutritional status. The supporting information for this figure have been published in Norman, A.W. and Bouillon, R. Vitamin D nutritional policy needs a vision for the future. Exp. Biol. Med. 235:1034-1045 (2010).

Conceptually, human clinical disorders related to vitamin D can be considered as those arising because of (a) altered availability of vitamin D; (b) altered conversion of vitamin D3 to 25(OH)D3; (c) altered conversion of 25(OH)D3 to 1a,25(OH)2D3 and/or 24R,25(OH)2D3; (d) variations in end organ responsiveness to 1a,25(OH)2D3 or possibly 24R,25(OH)2D3; and (e) other conditions of uncertain relation to vitamin D. Thus, the clinician/nutritionist/biochemist is faced with a problem, in a diagnostic sense, of identifying parameters of hypersensitivity, antagonism, or resistance (including genetic aberrations) to vitamin D or one of its metabolites as well as identifying perturbations of metabolism that result in problems in production and/or delivery of the hormonally active form, 1a,25(OH)2D3. A detailed consideration of this area is beyond the scope of this presentation. There are many scientific publications; a list of recent summary articles are available at the end of this presentation.

Drug Forms of 1a,25(OH)2D3

As a consequence of the significant scientific advances in the understanding of how vitamin D generates biological responses [principally via 1a,25(OH)2D3], a number of new drug forms of 1a,25(OH)2D3 have been generated by pharmaceutical companies. The table below summarizes these new drugs and the relevant pharmaceutical company, and identifies the clinical circumstance for which their use has been authorized.

Drug Forms of Vitamin D Analogs

Compound NameGeneric NameCommercial NamePharmaceutical CompanyEffective Daily Dose (micrograms)*Approved Use
1α,25(OH)2D3 CALCITRIOL ROCALTROL ROFFMAN-LA ROCHE 0.5-1.0 RO, HP, Ob
1α,25(OH)2D3 CALCITRIOL CALCIJEX ABBOTT 0.5 (i.v.) HC
1α,25(OH)2-19-nor-D2 PARICALCITOL ZEMPLAR ABBOTT 2.8-7 (eod) SHP
1α,24(OH)2D3 TACALCITOL BONALFA TEIJIN LTD.-JAPAN 40-80 (topical) PP
1α,24S(OH)2-22-ene-24-cyclopropyl-D3 CALCIPOTRIENE DOVONEX LEO-DENMARK 40-80 (topical) PP
1α,24S(OH)2-22-ene-24-cyclopropyl-D3 CALCIPOTRIENE DOVONEX WESTWOOD-SQUIBB 40-80 (topical) PP
1α-OH-D3 ALFACALCIDOL ONE-ALFA LEO-DENMARK 1-2 RO, HP, O, VDRR
1α-OH-D3 ALFACALCIDOL ALPHA-D3 TEVA-ISRAEL 0.25-1.0 RO, O, HC, HP
1α-OH-D3 ALFACALCIDOL ONEALFA TEIJIN LTD.-JAPAN 0.25-1.0 RO, O
1α-OH-D3 ALFACALCIDOL ONEALFA CHUGAI-JAPAN 0.25-1.0 RO, 0
1α-OH-D2 DOXERCALCIFEROL HECTOROL GENZYME 10 4x/WEEK (i.v.) SHP
25(OH)D3 CALCIFEDIOL CALDEROL ORGANON-USA 50-500 RO
25(OH)D3 CALCIFEDIOL DEDROGYL ROUSSEL-UCLAF-
FRANCE
50-500 RO
10,19-dihydrotachysterol3 DIHYDROTACHYSTEROL3 HYTAKEROL WINTHROP 200-1000 RO
1α,25(OH)2-22-oxa-D3 MAXACALCITOL OXAROL CHUGAI-JAPAN 5-10 3x/WEEK (i.v.) SHP
1α,25(OH)2-26,27-F6-D3 FALECALCITRIOL FULSTAN TABLETS SUMMITOMO PHARMACEUTICALS-
JAPAN
0.15-0.34 HC, SHP, RO
1a,25(OH)2-26,27-F6-D3 FALECALCITRIOL HORNEL TABLETS TAISHO PHARMACEUTICALS-
JAPAN
0.15-0.35 HC, SHP, RO

The key to the approved uses of the vitamin D analogs is as follows: RO = renal osteodystrophy, O = postmenopausal osteoporosis; PP = plaque psoriasis; HC = hypocalcemia (frequently present in patients with renal osteodystrophy who are subjected to hemodialysis); HP = hypoparathyroidism and associated hypocalcemia which may frequently be encountered in patients with hypoparathyroidism, pseudohypoparathyroidism or in circumstances of post-surgical hypoparathyroidism; SHP = secondary hyperparathyroidism associated with renal osteodystrophy; VDRR = vitamin D-resistant rickets.

a Oral dose unless otherwise indicated; eod = every other day

b The use of Rocaltrol for postmenopausalosteoporosis is approved in Argentina, Australia, Austria, Czech Republic, Columbia, India, Ireland, Italy, Japan, Malaysia, Mexico, New Zealand, Peru, Philippines, South Korea, South Africa, Switzerland, Turkey, and the United Kingdom.

NOTE: This Table 1 was published in Present Knowledge in Nutrition, 9th edition by A. W. Norman & H..L. Henry (Bohman, B. A and Russell, R. M.) International Life Sciences Institute, Washington D.C.; Chapter 12, pp. 198- 210 (2006).

References

Vitamin D, 3rd Ed. Edited by Feldman, D., Pike, J.W, Adams, J.S. San Diego, Academic Press, pp. 1-2081 (2011).

Holick, M.F., Binkley, N.C., Bischoff-Ferrari, H.A., Gordon, C.M., Hanley, D. A., Heaney, R. P., Murad, M. H., and Weaver, C. M., Evaluation, treatment and prevention of vitamin D deficiency: An Endocrine Society clinical practice guideline. J. Clin. Endoctinol. Metab. 96: 1911-1930 (2011).

Lappe, J.M., The role of vitamin D in human health: A paradigm shift, J. Evidence-Based Complementary & Alternative Medicine, 16:58-72 (2011).

Haussler, M.R., Jurutka, P.W., Mizwicki, M., & Norman, A.W., Vitamin D receptor (VDR)-mediated actions of 1a,25(OH)2-vitamin D3: Genomic and non-genomic mechanisms. Best Practice & Research- Clinical endocrinology & Metabolism, 25:543-559 (2011).

Henry, H.L., Regulation of vitamin D metabolism, Best Practice & Research- Clinical endocrinology & Metabolism, 25:531-541 (2011).Best Practice & Research- Clinical endocrinology & Metabolism, 25:543-559 (2011).

Bouillon,R.; Carmeliet,G.; Verlinden,L.; van Etten,E.; Verstuyf,A.; Luderer,H.F.; Lieben,L.; Mathieu,C.; Demay,M., Vitamin D and human health: Lessons from vitamin D receptor null mice, Endocr.Rev.6: 726-776 (2008).

Melamed, ML, Michos, ED, Post, W, Astor, B. 25-hydroxyvitamin D levels and the risk of mortality in the general population. 6 168: 1629-1637, 2008.

Pike,J.W.; Meyer,M.B.; Watanuki,M.; Kim,S.; Zella,L.A.; Fretz,J.A.; Yamazaki,M.; Shevde,N.K., Perspectives on mechanisms of gene regulation by 1,25-dihydroxyvitamin D3 and its receptor, J.Steroid Biochem.Mol.Biol., 103: 389-395 (2007).

Bouillon, R, Verstuyf, A, Mathieu, C, Van, CS, Masuyama, R, Dehaes, P, Carmeliet, G. Vitamin D resistance.6 20: 627-645, 2006,

H.P. and Norman, A.W. The role of the vitamin D endocrine system in health and disease. New Engl. J. Med. 320:980-991 (1989).

Aricle last updated on November 2011.

Vitamin D and Milk

Prepared by Professor Anthony W. Norman;
Department of Biochemistry & Biomedical Sciences
University of California, Riverside CA 92521
(December 12, 2000 & updated in 2011)

Purpose of this statement:

The objective of this presentation is to provide a brief descriptions of vitamin D's chemistry, nutritional importance, sources, production and presence in milk. This will first require a review of the biological and nutritional background on vitamin D, which is a precursor of a steroid hormone [1a,25(OH)2D3] in higher animals, including humans.

What is a vitamin?

A vitamin is a substance (a specific organic molecule) whose presence is crucial to the normal every day life and functions of animals. However vitamins can not be directly produced by the animal's body. Accordingly, the daily requirements for each vitamin must be met through regular dietary intake of appropriate quantities of the vitamin(s). There are two general chemical categories of vitamins based on their solubility: water soluble vitamins (the B vitamins and others) and fat soluble vitamins (A, D, E and K).

What is a hormone?

A hormone is a chemical messenger that is produced and secreted by specific glands and cells within the body of animals. After secretion of the hormone, it is transported through the bloodstream to designated target organs where the hormone by binding to its specific personal receptor delivers its "message" to that set of cells. These cells then promptly produce biological responses specific for that hormone.

What is vitamin D and why is it important:

Chemistry: There are two chemical forms of vitamin D, namely vitamin D2 (sometimes referred to as ergocalciferol) and vitamin D3 (sometimes referred to a cholecalciferol). The natural form of vitamin D for animals and man is vitamin D3; it can be produced in their bodies from cholesterol and 7-dehydrocholesterol. An alternative vitamin D2 is commercially prepared from ergosterol that is present in yeast.

The molecular structure of vitamin D is closely allied to that of the classical steroid hormones, e.g. cortisol, estradiol, progesterone, aldosterone, and testosterone (3). All steroid hormones and vitamin D3 are chemically related to the well known sterol cholesterol. Cholesterol in animals and man is a precursor substance for all steroid hormones and as well vitamin D3.

Technically the molecule called vitamin D3 is not really a vitamin because it can be produced by exposure of the skin (higher animals and humans) to ultraviolet light or sunlight. The skin of many animals and man has a high concentration of the sterol cholesterol which is converted by enzymes in the skin to the sterol 7-dehydrocholesterol. Exposure of skin (including human skin) to sunlight for regular intervals results in the photochemical conversion of 7-dehydrocholesterol into vitamin D3. This sunlight- generated vitamin D3 is a precursor of the steroid hormone 1a,25(OH)2D3. Under these circumstances vitamin D3 is not a vitamin because it has been produced by the body (with the assistance of sunlight). However, if the animal or man lives in the absence of sunlight (e.g., Alaska in the winter) or exclusively indoors, then there is indeed an absolute regular requirement for the fat soluble vitamin D, that must be met through proper dietary intake.

Therefore for nutritional and public health reasons, vitamin D3 continues to be classified even today in 2012 officially as a vitamin. Thus many vitamin capsules and food sources including cows milk are supplemented with vitamin D3 to improve their nutritional value. In the 1940's this milk supplementation process reduced the incidence rate of juvenile rickets by 85% in the United States.

Importance: Vitamin D3 is essential for life in higher animals. Classically vitamin D3 has been shown to be one of the most important biological regulators of calcium metabolism through stimulating the absorption of calcium from food across the intestine and participating in the incorporation of the absorbed calcium into the skeleton (2). These important biological effects are only achieved as a consequence of the metabolism of vitamin D into a family of daughter metabolites, including 1a,25(OH)2-vitamin D3 [1a,25(OH)2D3]. 1a,25(OH)2D3, is considered to be a steroid hormone because the general mechanism by which it produces the biological responses attributed to vitamin D is similar to those of steroid hormones (3;4).

It has become increasingly apparent since the 1980s that 1a,25(OH)2D3 also plays an important multidisciplinary role in tissues not primarily related to mineral metabolism, e.g. activation of the immune system, both innate and adaptive, in the pancreas where it facilitates insulin secretion, in muscle where it improves muscle strength, and in the heart and cardiovascular systems, where it is concerned with heart muscle function and blood pressure regulation.,

Vitamin D Deficiency: The classic deficiency state resulting from a dietary absence of vitamin D3 or lack of ultraviolet (sunlight) exposure is the bone disease called rickets in children or osteomalacia in adults. The clinical features of rickets and osteomalacia depend upon the age of onset. The classical skeletal disorder of rickets includes deformity of the bones, especially in the knees, wrists, and ankles, as well as associated changes in the rib joint functions, which have been termed by some as the rachitic rosary (1). A regular access to vitamin D3 throughout life is important to facilitate the normal absorption into the body of dietary calcium which, in turn, is essential for normal bone health and may diminish or prevent the onset in the elderly of the bone disease osteoporosis.

Requirements for vitamin D:

Since vitamin D3 is produced in the skin after exposure of 7-dehydrocholesterol to sunlight, the human does not have a requirement for vitamin D when sufficient sunlight is available. Man's tendency to wear clothes, to live in cities where tall buildings block adequate sunlight from reaching the ground, to live indoors, to use synthetic sunscreens that block ultraviolet rays, and to live in geographical regions of the world that do not receive adequate sunlight, all contribute to the inability of the skin to biosynthesize sufficient amounts of vitamin D3 (5). Thus, vitamin D3 does become an important nutritional factor in the absence of sunlight. It is known that a substantial proportion of the U.S. population is exposed to suboptimal levels of sunlight. This is particularly true during winter months (6;7). Under these conditions, vitamin D becomes a true vitamin which dictates that it must be supplied in the diet on a regular basis.

Since vitamin D3 can be produced by the body and since it is retained for long periods of time by animal tissues, it has been difficult to determine with precision the minimum daily requirements for this fat soluble vitamin. The requirement for vitamin D3 is also known to be dependent on the age, sex, degree of exposure to the sun, season, and the amount of pigmentation in the skin (8).

The current "adequate intake" allowance of vitamin D recommended in 2010 by the Food and Nutrition Board of the US Institute of Medicine is 600 IU/day (15 µgrams/day) for children and adult males and females up to age 70 (9). For adults greater than 70 years, the recommended intake is 800 IU (20 µgrams/day).The adequate allowance for pregnancy and lactation is set at 600 IU/day (15 µg/day). These recommendations are all summarized in a 2010 publication from the Food and Nutrition Board of the Institute of Medicine (9).

In the United States adequate amounts of vitamin D3 can readily be obtained from the diet and/or from casual exposure to sunlight. The ultraviolet exposure can be as little as 3 X per week exposure of the face and hands to ambient sunlight for 20 minutes (10). However, in some parts of the world where food is not routinely fortified and sunlight is often limited during some periods of the year, obtaining adequate amounts of vitamin D becomes more of a problem. As a result, the incidence of rickets in these countries is higher than in the United States.

What are the sources of vitamin D for humans?

Animal products constitute the primary source of vitamin D that occurs naturally in unfortified foods. Salt water fish such as herring, salmon, sardines, and fish liver oils are good sources of vitamin D3. Small quantities of vitamin D3 are also found in eggs, veal, beef, butter, and vegetable oils while plants, fruits, and nuts are extremely poor sources of vitamin D. In the United States, fortification of foods such as milk (both fresh and evaporated), margarine and butter, cereals, and chocolate mixes help in meeting the adequate intake (RDA) recommendations (11). Because only fluid milk is fortified with vitamin D, other dairy products (cheese, yogurt, etc.) only provide the vitamin that was produced by the animal itself.

How is vitamin D produced commercially for food supplementation>

When the critical importance to human health of a regular dietary access to vitamin D3 was understood (in the 1930's), milk suppliers realized it would be advantageous to their customers' health to market milk which had been supplemented with vitamin D3. Thus there developed in the 1940's, and continues to the present, a large business of industrial production of vitamin D3 used for the supplementation of foods for human consumption: milk (both homogenized and evaporated), some margarine and breads. Since the 1960's vitamin D3 has been used also for the supplementation of farm animal and poultry food.

In 1973 in the United States some 290 trillion (290 x 10-12) International Units of vitamin D3 was manufactured and sold for over 3 million dollars. This vitamin D3 is the equivalent of approximately 8 tons.

The commercial production of vitamin D3 is completely dependent on the availability of either 7-dehydrocholesterol or cholesterol. 7-Dehydrocholesterol can be obtained via organic solvent extraction of animal skins (cow, pig or sheep) followed by an extensive purification. Cholesterol typically is extracted from the lanolin of sheep wool and after thorough purification and crystallization can be converted via a laborious chemical synthesis into 7-dehydrocholesterol. It should be appreciated that once chemically pure, crystalline 7-dehydrocholesterol has been obtained, it is impossible to use any chemical or biological tests or procedures to determine the original source (sheep lanolin, pig skin, cow skin, etc.) of the cholesterol or 7-dehydrocholesterol.

Next the crystalline 7-dehydrocholesterol is dissolved in an organic solvent and irradiated with ultraviolet light to carry out the transformation (similar to that which occurs in human and animal skin) to produce vitamin D3. This vitamin D3 is then purified and crystallized further before it is formulated for use in dairy milk and animal feed supplementation. The exact details of the chemical conversion of cholesterol to 7-dehydrocholesterol and the method of large-scale ultraviolet light conversion into vitamin D3 and subsequent purification are closely held topics for which there have been many patents issued (3).

Historically, the major producers of vitamin D3 used for milk and other food supplementation were the companies F. Hoffman La Roche, Ltd (Switzerland) and BASF (Germany). Today much of the commercially produced vitamin D2 is manufactured in China.

What is the source of vitamin D in milk?

Milk from all lactating animals, including humans, contains vitamin D3 that has been produced photochemically from 7-dehydrocholesterol present in the skin. In cow's milk it has been determined that the concentration of vitamin D3 in milk provided by the cow is roughly 35-70 International Units per quart as determined via biological assay (12) and approximately 50-80 International Units as determined by modern chemical mass spectrometric procedures (13). However these are rather low levels of vitamin D3 from the perspective of providing the 600 IU per day as recommended by the Food and Nutrition Board of the Institute of Medicine in 2010.. Accordingly, as discussed above, the business practice of supplementing cows milk with chemically synthesized vitamin D3 was initiated. At the present time almost all milk sold commercially in the United States has 400 IU of chemically synthesized vitamin D3 added per quart. Any vendor of milk for human consumption containing added vitamin D3 is required by the US Food and Drug Administration (FDA) to include a notice on the milk carton label. Usually this label states "400 IU of added vitamin D3". However it is not required by law to indicate either the manufacturer of the added vitamin D3 or the sources of the cholesterol and 7-dehydrocholesterol used for its production.

It is a fact that most milk sold in the US will contain vitamin D3 with two origins. (a) That vitamin D3 made by the cow using sunlight to irradiate 7-dehydrocholesterol present in her skin. (b) That vitamin D3 made by a chemical process and then added to the cow milk as a nutritional supplement. It is simply not possible to distinguish the origins of the two vitamin D3 preparations by any biological or chemical procedure, because they are the same molecular structure. Further, there is no legal requirement for the manufacturer of the vitamin D3 formulated for human food supplementation to specify the animal sources of the precursor molecules that were employed in the synthesis of the D vitamin.

If a "food product" is construed to include a chemically pure substance that is the same in all animal species, then those individuals with strict religious reasons for avoiding food products from a particular species have, in the instance of milk and vitamin D3, a dilemma.

Selected references:

Also the WEB sites for the Vitamin D Workshop and provides other general information related to vitamin D.

Reference List

  1. Norman,A.W. and Litwack,G.L. Hormones, Academic Press, San Diego, CA.(1997).
  2. Norman,A.W. Vitamin D: The calcium homeostatic steroid hormone., Academic Press, New York.(1979).
  3. # Bouillon,R., Okamura,W.H., and Norman,A.W. Structure-function relationships in the vitamin D endocrine system. Endocr.Rev. 16 (1995) 200-257.
  4. Norman,A.W.: Vitamin D. In Present knowledge in nutrution (PKN7). Ziegler,E.E. and Filer,L.J., Eds., International Life Sciences Institute, Washington (1996) pp. 120-129 .
  5. Holick,M.F. Environmental factors that influence the cutaneous production of vitamin D. Am.J.Clin.Nutr. 61 Suppl. (1995) 638S-645S.
  6. Webb,A.R. and Holick,M.F. The role of sunlight in the cutaneous production of vitamin D3. Ann.Rev.Nutr. 8 (1988) 375-399.
  7. Webb,A.R., Pilbeam,C., Hanafin,N., and Holick,M.F. An evaluation of the relative contributions of exposure to sunlight and of diet to the circulating concentrations of 25-hydroxyvitamin D in an elderly nursing home population in Boston. Am.J.Clin.Nutr. 51(6) (1990) 1075-1081.
  8. Harris,S.S. and Dawson-Hughes,B. Seasonal changes in plasma 25-hydroxyvitamin D concentrations of young American black and white women. Am.J.Clin.Nutr. 67 (1998) 1232-1236.
  9. Food and Nutrition Board. Dietary reference intakes: A risk assessment model for establishing upper intake levels for nutrients. 1998) , 1-71. Washington, D.C., National Academy Press, Institute of Medicine.
  10. Adams,J.S., Clemens,T.L., Parrish,J.A., and Holick,M.F. Vitamin-D synthesis and metabolism after ultraviolet irradiation of normal and vitamin-D-deficient subjects. New Engl.J.Med. 306 (1982) 722-725.
  11. Collins,E.D. and Norman,A.W.: Vitamin D In Handbook of vitamins. Machlin,L.J., Ed., Marcel Dekker, New York (1990) pp. 59-98 .
  12. Hollis,B.W., Roos,B.A., and Lambert,P.W.: Vitamin D compounds in human and bovine milk In Advances in nutritional research. Draper,H.H., Ed., Plenum Press, New York (1994) pp. 59-75 .
  13. Adachi,A. and Kobayashi,T. Identification of vitamin D3 and 7-dehydrocholesterol in cow's milk by gas chromatography-mass spectrometry and their quantitation by high-performance liquid chromatography. J.Nutr.Sci.Vitaminol. 25 (1979) 67-78.

Article Last Updated: November 2011.

More Information 

General Campus Information

University of California, Riverside
900 University Ave.
Riverside, CA 92521
Tel: (951) 827-1012

Workshop Information

Dr. Anthony W. Norman
Department of Biochemistry
University of California-Riverside
Riverside, CA 92521
E-mail: vitamind@ucr.edu

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