Vitamin D3, also known as cholecalciferol, is an essential vitamin that enables the body to absorb calcium. The human body naturally produces vitamin D3 when exposed to sunlight, and the vitamin may also be obtained from several food products as well as vitamin supplements. Both the recommended intake and upper limits for vitamin D3 have been increased in recent years.
Functions of Vitamin D
Vitamin D is the single most important vitamin enabling the human body to absorb calcium and regulate calcium levels in the bloodstream. Vitamin D's action begins in the digestive tract, where it enables calcium compounds to move from the intestines into the bloodstream. In the bloodstream, vitamin D is a key regulating agent of calcium and phosphorus concentrations, facilitating the two minerals' incorporation into bones and teeth both during crucial formative years and throughout life. Inadequate vitamin D at any point in life can lead to severe and life-threatening diseases of bone and muscle weakening and skeletal deformation, including rickets and osteoporosis.
Vitamin D Subtypes
Vitamin D exists in two basic forms: D2 and D3. D2, also known as ergocalciferol, is produced by some plants and can be obtained through certain supplements. Vitamin D3 is produced by the body and available in certain foods and supplements. While research on the question continues, D3 is generally considered the more potent and nutritionally desirable of the two vitamin D forms. An adequate intake of vitamin D3 obviates the need for D2.
Sources of Vitamin D3
The simplest way to obtain vitamin D is to get it naturally. Natural ultraviolet light activates vitamin D3 production in the body. Just 15 to 20 minutes in sunlight, several times a week, is generally sufficient to provide the body with the vitamin D3 it requires; however, many people don't regularly spend this amount of time in direct sunlight, especially if they live in climates, or have jobs, that are not conducive to extended outdoor activity. The most common food source of vitamin D3 is pasteurized milk. Certain fish, such as sockeye salmon, tuna and mackerel, also contain substantial vitamin D3. Larger intakes of vitamin D3 can be obtained by taking vitamin supplements.
Recommended Daily Vitamin D3 Intake
Standards established by the Institutes of Medicine recommend that infants, from birth to 1 year old, receive 400 International Units, or IU, of vitamin D daily. From age 1 to 70 years, the recommended daily vitamin D intake increases to 600 IU and increases again to 800 IU from age 70 years onward. Pasteurized milk is the most important single source of vitamin D3 in the United States, particularly for children, with each 8-oz. serving supplying about 30 percent of the vitamin's daily recommended intake. According to the National Institutes of Health, vitamin D fortification of pasteurized milk has eliminated childhood rickets.
Upper Limits of Vitamin D3
According to the Institutes of Medicine's standards, for newborns through age 6 months, the tolerable upper intake for vitamin D is 1,000 IU daily. This rises to 1,500 IU at age 6 months and goes to 2,500 IU from ages 1 to 3 years. The maximum daily intake is 3,000 IU from ages 4 to 8 years, and then 4,000 IU for the rest of life. The National Institutes of Health notes that immediate vitamin D toxicity is unlikely at dosages under 10,000 IU; however, persistent lower vitamin D overdoses may exert negative health impacts over time. Vitamin D supplements may provide anywhere from several hundred IU to many thousands of IU in a single dose, sometimes exceeding the tolerable upper intake guidelines.
Treating Vitamin D Deficiency
The above limits on acceptable daily intake are not universally endorsed, and some studies, including research published in the "Journal of Steroid Biochemistry and Molecular Biology," have advocated raising both the recommended daily intake and tolerable upper limit. Treatment of diseases resulting from vitamin D deficiency may temporarily use substantially larger doses than the tolerable upper limit, or even "megadoses" on a single day, or several days, consisting of several hundred thousand IU. Such doses should never be administered without medical guidance, and you should consult a physician or nutritional expert before starting any vitamin D supplementation regimen.
References
- National Institutes of Health: Dietary Supplement Fact Sheet; Vitamin D
- "Journal of Steroid Biochemistry and Molecular Biology"; Why The Optimal Requirement For Vitamin D3 Is Probably Much Higher Than What Is Officially Recommended For Adults; Vieth R.; May 2004
- U.S. FDA: Vitamin Fortification of Fluid Milk Products
- Mayo Clinic: Vitamin D Dosing
- "Evidence Report - Technology Assessment"; Effectiveness and Safety of Vitamin D In Relation To Bone Health; A. Cranney et al.; August 2007
- Harvard School of Public Health; Vitamin D and Chronic Disease; Dr. Edward Giovannucci
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