Vitamin D is a fat-soluble vitamin that exists in two physiologically relevant forms, ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Ergocalciferol is synthesized by plants and mushrooms, while cholecalciferol is synthesized by humans in the skin when it is exposed to ultraviolet-B (UVB) rays from sunlight. Some foods may also be fortified with vitamin D, such as milk and breakfast cereals. The current average daily intakes of vitamin D for Americans are well below suggested adequate intakes1, and much of the world’s population is deficient in this important vitamin 2.
The main function of vitamin D is to regulate serum calcium and phosphorus concentrations within the normal range by enhancing the efficiency of the small intestine to absorb these minerals. By influencing the absorption of calcium, vitamin D helps to form and maintain strong bones and teeth 3,4.
Vitamin D supplementation helps maintain physical performance in the elderly4,5. Adequate vitamin D intake may also be important for maintaining immune health6,7, nervous system health8, may help improve mood during the winter months9,10 and improve overall quality of life11.
Vitamin D is generally well tolerated at recommended doses. The U.S. Food and Drug Administration (FDA) has established a Reference Daily Intake (RDI) of 400 international units (IUs) vitamin D for adults and children 4 or more years of age. RDIs are a set of dietary references for essential vitamins and minerals that are considered amounts sufficient to meet the daily requirements of healthy individuals. RDIs serve as the basis for calculating the percent daily value (% DV) amounts found on dietary supplement and food labels.
According to the Endocrine Society’s Vitamin D Clinical Practice Guidelines, individuals who are at risk for vitamin D deficiencies should ask their physician to have their blood tested for the vitamin D metabolite [25(OH)D]. For individuals with blood 25(OH)D levels <75 nmol/L, higher amounts of vitamin D intake are suitable: children ages 1–18 may need 600–1,000 IU daily, adults >18 age may need 1,500–2,000 IU vitamin D daily12.