Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 2/1/2008
Publication Date: 3/1/2008
Citation: Whiting, S.J., Calvo, M.S., Stephensen, C.B. 2008. Current Understanding of Vitamin D Metabolism, Nutritional Status, and Role in Disease Prevention. Book Chapter. ISBN-13:978-0-12-374118-9, publ.date March 2008, Chapter 43:807-832.
Technical Abstract: INTRODUCTION: Vitamin D is a nutrient that, until recently, was neglected by the nutrition community. Although it was recognized in the early twentieth century as an essential nutrient, recommendations for intake were often qualified as being needed only in absence of sunlight. In theory (and in ancient times when early humans all lived closer to the equator), all vitamin D needs could be met by exposure to sunlight that provided ultraviolet (UV)B radiation, but only recently have we come to understand how UVB acts and what other factors—particularly environmental— mitigate cutaneous vitamin D synthesis. Studying vitamin D requirements is difficult. Previous dietary recommendations for vitamin D, such as the 1989 Recommended Dietary Allowance , indicated a ‘‘relative paucity of recent controlled studies [and] . . . lack of data’’ on which to base requirements. It further stated that ‘‘[c]linical osteomalacia appears to be rare in the United States.’’ What is known today, however, is that vitamin D deficiency and insufficiency are more widespread  than imaged even a decade ago when the Dietary Reference Intakes (DRIs) were first published for vitamin D . What has occurred in the past 10 years is a new understanding of vitamin D’s roles in the body, especially for functions unrelated to calcium absorption, which has been the long-standing primary recognized function of vitamin D [4–7]. The role of vitamin D in preventing rickets was discovered early in the nineteenth century, but it was not until the 1970s that the sequence of steps from skin precursors to active metabolite was understood. Despite the interest generated in solving the puzzle of how vitamin D increased intestinal calcium absorption, there were several reasons why progress toward a better understanding of vitamin D requirements was not made. There were technical challenges in analyzing vitamin D and its metabolites. There was, beginning in the 1980s, a greater focus on dietary calcium as the major ‘‘bone’’ nutrient, leaving vitamin D with only a minor role in osteoporosis research. And finally, the important contribution of sun exposure to vitamin D status was not fully realized until recently. Indeed, estimations for dietary recommendations in the complete yearround absence of sun exposure give values that are 5 to 8 times higher than what is needed to maintain vitamin D status through the winter. It has been shown that globally there is greater prevalence of chronic diseases such as cancer and immune disorders at extremes of latitudes where sun exposure for skin synthesis of vitamin D is limited . Vitamin D affects people starting with fetal development and continuing to old age, functioning at both the genomic and nongenomic level in the regulation of key protein synthesis or in the intracellular metabolic pathways in virtually all tissues [5–7]. Growth, development, and The opinions expressed in this chapter are those of the authors and do not reflect those of the U.S. Food and Drug Administration. Font: Times and FrizQuadrata Size:10/12pt Margins:Top:3pc Gutter:5pc T.Area:42pc_54pc6.6 1 Color Lines: 55 Chap. Open: Fresh Recto Els US HAYA Ch43-P374118 18-12-2007 Page: 801 22:4 Trim:8.5in_11in Floats: Top or Bottom TS: Integra, India Nutrition in the Prevention and Treatment of Disease, 2 Ed. Copyright 2008, Elsevier, Inc. All rights of reproduction in any form reserved. 801 maintenance of health are all affected, and in many regards, quality of life is as well. This chapter, although acknowledging vitamin D’s contribution through the lifespan, focuses on vitamin D needs for maintenance of health, and on vitamin D’s specific actions in selected clinical conditions. Because research is ongoing, the reader can expect to learn enough about vitamin D’s roles to be able to understand and apply the research as it unfolds.