Sources of vitamin D and its metabolism

The major natural source of vitamin D is directly produced from a cholesterol-like precursor (7-dehydrocholesterol) in the skin by exposure to sunlight. The ultraviolet B (UVB) rays/energy (wavelength of 290–315 nanometers) in the sunlight will convert 7-dehydrocholesterol to vitamin D3, which is carried to liver and then to kidneys. In the liver, vitamin D incorporate extra oxygen and hydrogen molecules to become 25-hydroxyvitamin D3 [25(OH) D3]. This form of vitamin D can be measured in the blood to diagnose low vitamin D levels. Finally, the kidneys will further convert it to active vitamin D3 [1,25-dihydroxyvitamin D3; 1,25(OH)2D3] in the mitochondria of the proximal convoluted tubules (Figure 1). The active vitamin D3 and vitamin D-binding protein are then transported to different organs for further metabolism and action (Figure 1 and 2).

• Endocrine action: the hormone is distributed in blood and to distant target cells.

• Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.

• Autocrine action: the hormone acts on the same cell that produced it.

Reference: http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/basics/hormones.html

The minor source of natural vitamin D is derived from the diet. Dietary sources of vitamin D are limited (Table 1). There are two froms of vitamin D in the diet. Vitamin D2 (ergocalciferol) is from plants. Vitamin D3 (cholecalciferol) is from animals. After absorption in the small intestine, it will transport to the liver and then to the kidneys as previously mentioned (Figure 1).

The form of vitamin D used in supplement products can be either vitamin D2 or D3 but neither will function until the kidneys change them into active form. Vitamin D2 is manufactured by exposing a plant sterol (ergosterol in yeast or mushrooms) to ultraviolet energy to produce vitamin D2. Vitamin D3 supplement is industrially produced in the same way but using natural 7-dehydrocholesterol from animal origin [e.g. fish, wool grease (lanolin) from sheep]. Their function is almost identical, so they are grouped together under the name “vitamin D”. In addition, many foods in some countries are artificially fortified with vitamin D (e.g. dairy products, breakfast cereals, and fruit juice). Both the United States and Canada mandate the fortification of infant formula with vitamin D: 40–100 international units (IU)/100 kcal in the United States and 40–80 IU/100 kcal in Canada.