Chapter 43 - Current Understanding of Vitamin D Metabolism, Nutritional Status, and Role in Disease Prevention

Abstract

Vitamin D is a nutrient that, until recently, was neglected by the nutrition community. Although recognized in the early 20th century as an essential nutrient, recommendations for intake were often qualified as being needed only in the 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. Early 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 “clinical osteomalacia appears to be rare in the United States.” What is known today, however, is that vitamin D deficiency and insufficiency are widespread, which was not identified when the Dietary Reference Intakes (DRIs) for vitamin D were first published in 1997. In 2011, new DRIs for vitamin D reflected the need for more dietary vitamin D. While the 2011 DRI report did not set recommendations based on functions other than bone health, there remains a growing body of evidence for vitamin D’s many roles in the body.