Vitamin D is well-known for its role in bone growth and remodeling by promoting calcium absorption from the GI tract. Based on the discovery of vitamin D receptors throughout the body, vitamin D is also thought to be involved with cell growth and immune function.1 Although it can be obtained through sunlight exposure or through diet, vitamin D is classified as a fat-soluble vitamin or nutrient.
BENEFITS OF VITAMIN D
Low levels of vitamin D cause serum calcium levels to drop, potentially causing rickets in children and osteomalacia in adults. Vitamin D also plays a role in cardiovascular health and BP regulation, insulin production, immune system function, and cell growth regulation—all of which may have protective effects against cardiovascular disease, certain types of cancers, and immune system diseases.1 However, insufficient evidence exists to prove these claims.2 Risk factors for vitamin D deficiency include decreased intake (inadequate oral intake or malabsorption), limited sun exposure, severe liver disease, renal dysfunction, and advanced age.3
RECOMMENDED INTAKE
Based on limited sun exposure, the Institute of Medicine's (IOM) 2010 dietary guidelines for daily vitamin D intake are as follows: 0 to 6 months, 400 international units (IU); 6 to 12 months, 400 IU; 1 to 3 years, 600 IU; 4 to 8 years, 600 IU; 9 to 70 years, 600 IU; and older than 70 years, 800 IU. Because too much vitamin D may be harmful, the IOM's upper level intake guidelines are as follows: 0 to 6 months, 1,000 IU; 6 to 12 months, 1,500 IU; 1 to 3 years, 2,500 IU; 4 to 8 years, 3,000 IU; 9 to 70 years, 4,000 IU; and older than 70 years, 4,000 IU.2
SOURCES OF VITAMIN D
Food sources that are naturally rich in vitamin D include fatty fish such as salmon and tuna (400-800 and 150-200 IU per serving, respectively). Vitamin D-fortified milk and orange juice (100 IU) and cereal (40-100 IU) are also good sources. Vitamin D supplements, multivitamins, and some calcium supplements also contain vitamin D. Although exposure to ultraviolet light is a source of vitamin D, the amount of vitamin D synthesized in response to sun exposure is dependent on season, time of day, cloud cover, skin melanin content, and sunscreen use. Due to these variables and the risk of skin cancer, it is difficult to recommend a safe amount of sun exposure as a source for vitamin D.4
VITAMIN D TESTING
Vitamin D level is assessed by measuring serum levels of the vitamin D metabolites 25(OH)D and 1,25(OH)2D. Vitamin D is rapidly metabolized to 25(OH)D. A fraction of 25(OH)D is then metabolized to the more active metabolite 1,25(OH)2D and is returned to the serum. It is generally recommended that levels of both 25(OH)D and 1,25(OH)2D be measured for screening and detailed testing and assessment of the patient's response to vitamin D therapy, respectively. Mayo Medical Laboratories use the following values to interpret 25(OH)D levels: 24 ng/mL or lower as vitamin D deficiency, 25 to 80 ng/mL as optimal, and greater than 80 ng/mL as possible toxicity.3 The IOM's Dietary Reference Intakes for calcium and vitamin D recommend maintaining a 25(OH)D level above 20 ng/mL to preserve bone health. Different labs may define deficiency differently.
Although routine screening for serum vitamin D levels is not universally supported, vitamin D testing may be beneficial for patients at high risk for deficiency or those with laboratory or radiographic findings that indicate vitamin D deficiency—including hypocalcemia, hypophosphatemia, elevated parathyroid hormone level, and decreased bone mineral density.3
TREATMENT
Patients can take vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol) supplements. Both appear to be effective as long as sufficient levels of serum 25(OH)D are reached. Vitamin D3 has a longer half-life, allowing for more infrequent dosing. For severe deficiency, a loading dose is often recommended for a short period of time followed by maintenance dosing. Loading doses of 50,000 IU once a week for 2 months followed by 800 to 1,000 IU daily has proven to be effective.3 JAAPA
Angela Dunaway practices in family medicine and urgent care and as a pharmacist in Georgia. The author has indicated no relationships to disclose relating to the content of this article.
Mary L. Hewett, PA-C, MS, department editor
REFERENCES
1. Caballero B, Allen L, Prentice A, eds. Encyclopedia of Human Nutrition. Vol 4. 2nd ed. Oxford, England: Elsevier Academic Press; 2005:368-378.
2. Ross AC, Taylor CL, Yaktine AL, et al, eds; Committee to Review Dietary Reference Intakes for Vitamin D and Calcium Food and Nutrition Board. Institute of Medicine Dietary Reference Intakes, Calcium and Vitamin D. Washington, DC: The National Academies Press; 2010. http://books.nap.edu/openbook.php?record_id=13050. Accessed June 7, 2011.
3. Kennel KA, Drake MT, Hurley DL. Vitamin D deficiency in adults: when to test and how to treat. Mayo Clin Proc. 2010;85(8):752-757; quiz 757-758.
4. U.S. Department of Agriculture, Agricultural Research Service. USDA national nutrient database for standard reference, release 23. http://www.ars.usda.gov/ba/bhnrc/ndl. Updated December 2, 2010. Accessed June 7, 2011.
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