Vitamin D

Vitamin D is a fat-soluble vitamin that is essential for maintaining the mineral balance in the body. Its most active form in humans, vitamin D3 (‘cholecalciferol’), can be synthesized in the skin upon exposure to ultraviolet-B (UVB) radiation from sunlight (1). When exposure to UVB radiation is insufficient, adequate intake of vitamin D from the diet is essential for health.

Plants can synthesize ergosterol, which is converted to vitamin D2 (ergocalciferol), the less active form of vitamin D (less than 30% of D3), by ultraviolet light (2).

After it is consumed in the diet or synthesized in the skin, the biologically inactive form of vitamin D enters the circulation and is transported to the liver, where 25-hydroxyvitamin D [25(OH)D] is formed, the major circulating form of vitamin D and the indicator of vitamin D status

Increased exposure to sunlight or increased dietary intake of vitamin D increases blood levels of 25(OH)D, making the blood 25(OH)D concentration a useful indicator of vitamin D nutritional status.

In the kidney, 25(OH)D is converted to 1alpha,25-dihydroxyvitamin D [1,25(OH)2D], the most potent form of vitamin D. Most of the physiological effects of vitamin D in the body are related to the activity of 1,25(OH)2D (3). Its effects are mediated through the vitamin D receptor (VDR) (4). More than 200 genes in tissues throughout the body are known to be regulated by 1,25(OH)2D (5).

Health functions

Vitamin D is essential for the efficient utilization of calcium by the body (1). Maintenance of blood calcium levels within a narrow range is vital for normal functioning of the nervous system, as well as for bone growth and maintenance of bone density.

Disease risk reduction

Without sufficient vitamin D from sun exposure or dietary intake, calcium absorption in the digestive tract cannot be maximized.

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Things to know about Vitamin D

  • Intake recommendations

    In 1993, the European Scientific Committee for Food set population reference intakes (PRI) for vitamin D in micrograms (mcg) and international units (IU) (60).

  • Supply situation

    Surveys in several European countries, such as Germany (95), Austria (66), Ireland (67), The Netherlands (68) and the U.K. (69), indicate that a substantial part of the population has a vitamin D intake below the recommended levels.

  • Deficiency

    Although adult bones are no longer growing, they are in a constant state of turnover (‘remodeling’), a process that includes bone break down (‘resorption’) and bone formation.

  • Sources

    According to many experts, 10–15 minutes of sun (ultraviolet-B radiation) exposure on arms and legs or face and arms three times weekly between 10:00 a.m. and 3:00 p.m. during the spring, summer, and fall at 40 degrees latitude should provide a light-skinned individual with adequate vitamin D3, produced in the skin (87).

  • Safety

    Vitamin D toxicity (‘hypervitaminosis D’) has only been associated with excessive supplemental intake of daily doses greater than 50,000 IU of vitamin D (76), which is far higher than those necessary to achieve the health benefits.

  • References

    Consult the full list of scientific references.