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
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.
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.