Vitamin D Metabolism and Physiology

Background. Vitamin D insufficiency may lead to various health problems. By encouraging sun protection to prevent skin cancer, physicians may be limiting a major source of vitamin D for many individuals. Methods. We did a literature review on issues related to vitamin D inadequacy, particularly those relevant to various racial/ethnic groups. Results. Vitamin D is an important hormone with many physiologic roles beyond those related to bones, including blood pressure regulation and acting as a tumor suppressant. The recommended amount of vitamin D has recently increased for adults over age 50. Moreover, some researchers are urging higher doses for the entire population to prevent osteoporotic fractures. Conclusions. Previous studies that have found serum levels of vitamin D in their sunprotected subjects to be in the normal range may need to be reevaluated. In concert with counseling sun protection, physicians should consider discussing vitamin D intake, typically in the form of supplements. From the Center for the Study of Race and Ethnicity in Medicine and the Department of Internal Medicine (Division of Dermatology), University of Kansas School of Medicine, Kansas City. Reprint requests to Kathleen E. Fuller, PhD, University of Kansas School of Medicine, Center for the Study of Race and Ethnicity in Medicine, Center of Excellence, 3901 Rainbow Blvd, Kansas City, KS 66160-7121. 58 January 2001 • SOUTHERN MEDICAL JOURNAL • Vol. 94, No. 1 Fuller and Casparian • VITAMIN D SUPPLEMENTATION 59 hydroxycholecalciferol (25-hydroxyvitamin D, calcidiol). It is the serum level of this metabolite that is reflective of current vitamin D status. 25-Hydroxycholecalciferol (25-hydroxyvitamin D) travels to the kidneys, where it undergoes further conversion into the most active hormonal form of vitamin D, 1,25-dihydroxycholecalciferol (1,25-dihydroxyvitamin D, calcitriol), which enhances absorption of calcium and phosphorus from the intestine and may cause increased reabsorption of calcium in the kidneys. This serves to elevate the plasma concentrations of calcium and phosphorus and in a feedback loop with the parathyroid glands, prevents parathyroid hormone (PTH) from increasing osteoclastic action, which would lead to the release of calcium from the bones to the serum. Through these mechanisms, vitamin D sufficiency results in normal bone mineralization during development and growth in children and maintains bone strength in adults. If an individual obtaining inadequate amounts of vitamin D from dietary sources also receives insufficient levels of UVB radiation, the resulting vitamin D insufficiency is clinically manifested as rickets in children and as osteomalacia in adults. While dietary/supplemental calcium is also important, in the absence of adequate serum vitamin D, excess calcium will simply be excreted. Conversely, adequate serum vitamin D can allow the body to adapt to extremely low levels of calcium intake. VITAMIN D AND DIET: OFTEN NOT ENOUGH The name ascribed to cholecalciferol and the related metabolites is a misnomer by having the label “vitamin” attached to “D.” Although it would more appropriately be considered a secosteroid hormone, vitamin D was discovered during the period (the late teens and early 20s of the 20th century) when “vital amines” were being discovered in foods. The name implies “nutrient,” but it is exceptionally difficult to obtain adequate levels of vitamin D solely from the diet. The current Dietary Reference Intake (DRI) is 400 IU (or 10 mg) per day for adults over age 50, 600 IU (15 mg) for adults over age 70, and 200 IU (5 mg) for all other adults and children. Some researchers recommend an even higher dose of 800 IU (20 mg) per day for older adults (Table 1). Each day, one would need to eat approximately 25 to 100 g of fatty, oily fish to obtain the daily requirement of vitamin D. Eating 25 g of sardines provides between 7 and 10 mg of vitamin D; other fatty, oily fish generally provide much lower amounts. Besides these fish, most other foods naturally provide little or no vitamin D (Table 2). Because of the limited natural sources of vitamin D, the United States government chose to fortify milk to prevent rickets. Other dairy products, such as cheese, yogurt, ice cream, and cottage cheese, are typically not fortified. As long as whole milk is drunk daily in quantity (each cup should provide at least 100 IU), this method works relatively well. However, much of the world’s populations cannot process lactose after weaning from the breast. High rates of lactose malabsorption occur among populations in Asia and Africa, among Native Americans in North and South America, and among southern Europeans. Therefore, while a goal of achieving adequate levels of vitamin D through fortification of milk may be attainable for those of northern European ancestry, even in this TABLE 1. Dietary Reference Intake (DRI) for Various Age Groups*