Vitamin D, commonly referred to as the "sunshine vitamin," plays an important role in your health. Exposure to sunlight stimulates the body to manufacture vitamin D and convert it to vitamin D3 (cholecalciferol), the active form of this vitamin in the human body. Vitamin D3 acts as a hormone and performs a variety of functions.
How It Is Made
Vitamin D is unique because it is made by the body when the skin is exposed to ultraviolet rays from the sun. When this happens, a cholesterol-like compound is converted to a vitamin D precursor and then to vitamin D3 (or cholecalciferol). Vitamin D3 is activated by enzymes from the liver and the kidney. When activated, vitamin D functions as a hormone. Just 15 minutes under the sun (for most people, without sunscreen), three times a week makes enough vitamin D. It can be stored for several months in the body.
Recommended Intake and Deficiency
The recommended adequate intake (AI) level for the vitamin D3 is 5 micrograms (mcg) or 200 International Units (IU) for adults aged 31 to 50 years. After the age of 50, the AI increases to 400 IU or 10 mcg. Vitamin D deficiency, or hypovitaminosis D, is common in Americans, especially those who live in colder, northern climates, as well as the elderly population. This is because your ability to produce the vitamin declines with age.
Food Sources
While vitamin D is strongest and most potent in its natural form, as made by the body through exposure to sunlight, a few foods in nature contain vitamin D. Fish, such as salmon, tuna and mackerel, as well as cod liver oil, beef liver, cheese and egg yolks are among the best food sources. Vitamin D in these foods is primarily in the form of vitamin D3 (cholecalciferol). Some mushrooms provide another form of vitamin D, D2 or ergocalciferol, which can be converted to D3 in the body. Fortified foods provide most of the vitamin D in the American diet, such as fortified milk.
Functions
In its active form, cholecalciferol travels through the bloodstream, targeting certain organs, such as the brain, affecting what that organ does. Vitamin D3 is essential for regulating cell growth, increasing calcium and phosphorus absorption, and maintaining immune system integrity and cardiovascular health. It may play a role in cancer prevention. Vitamin D3 helps to maintain blood calcium levels for building bone and teeth, muscle contraction, and the transmission of nerve impulses. Vitamin D3 is important for regulating mood, alleviating depression and preserving cognitive function.
Reduction in Overall Mortality
Serum (blood) concentration of vitamin D (known as 25-OHD) is the best indicator of vitamin D status in humans. Serum vitamin D concentrations of greater than 15 ng/mL (greater than 37.5 nmol/L) are recommended. Higher levels are suggested by some experts as desirable for overall health and disease prevention (greater than 30 ng/ml or greater than 75 nmol/L). For example, an epidemiological study published in the Archives of Internal Medicine in 2008 showed that low serum vitamin D levels were correlated with increased mortality (from all causes). In addition, a meta-analysis research study published in the Archives of Internal Medicine in 2007 revealed that use of vitamin D supplements was associated with a statistically significant reduction (7 percent) in overall mortality.
References
- Dietary Reference Index Tables for Americans: Vitamins
- USDA National Nutrient Database for Standard Reference: Vitamin D Content of Foods
- "Journal of Clinical Endocrinology and Metabolism;" Update in Vitamin D; J.S. Adams and M. Hewison; Feb 2010
- Update on Vitamin D; Position Statement by the Scientific Advisory Committee on Nutrition; 2007
- "Archives of Internal Medicine"; Vitamin D supplementation and total mortality: a meta-analysis of randomized controlled trials; P. Autier and S. Gandini; Sep 2007
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