Vitamin D Metabolism
Vitamin D belongs to a group of sterols, with the two most important sterols being vitamin D2 and vitamin D3. The major difference between the two vitamin D sterols is the source from which they are obtained. When skin is exposed to ultraviolet sunlight, it synthesizes vitamin D3, the most readily available form. However, the ability of skin to synthesize vitamin D3 from sunlight exposure is adversely affected by factors that diminish the intensity of the exposure, such as poor air quality, extreme latitudes, and the winter season. In addition, factors that limit the skin's absorption of sunlight, such as increased pigmentation, the use of sunscreen, and advanced age, can also limit vitamin D3 synthesis. Foods such as oily fish are an additional source of vitamin D3, whereas plant sources provide vitamin D2. However, the amount of vitamin D obtained through diet is often low and limited by malabsorption syndromes. Therefore, many people rely on fortified foods and dietary supplements to meet their vitamin D needs during times of insufficient sunlight. Vitamin D3 supplements may be more effective at increasing serum levels of 25-hydroxyvitamin D (25(OH)D) than vitamin D2 supplements. This may be because vitamin D3 has a stronger affinity for vitamin D–binding protein, a transporter that mediates its delivery to muscle or fat for storage, or liver and kidney for bioactivation.
Both forms of vitamin D are biologically inert and must undergo two metabolic steps before becoming physiologically active. The first step occurs in the liver, which converts vitamin D to 25(OH)D, the main circulating form and the usual measure of vitamin D status. This conversion to 25(OH)D is impeded in those with liver disease. According to a recent study, 92% of patients with chronic liver disease are vitamin D deficient. Vitamin D deficiency is defined by serum 25(OH)D concentrations below 10 ng/mL (25 nmol/L), because values below this cut-off are associated with rickets and myopathy.
The second metabolic step occurs in the kidney and many other tissues (i.e., heart, brain, skin, reproductive tissue, skeletal muscle, spinal cord, and the placenta) and forms the physiologically active hormone 1,25-dihydroxyvitamin D [1,25(OH)2D]. The 1,25(OH)2D is released into the circulatory system and can stimulate target cells in close proximity through its autocrine and paracrine functions.[7,8] Moreover, 1,25(OH)2D can bind to vitamin D receptor (VDR) and modulate many cellular and immunologic processes. The expression of VDR has been identified in most organs and several types of white blood cells, indicating that the vitamin D endocrine system is essential for the development and maintenance of tissue structure and function.
J Midwifery Womens Health. 2010;55(6):550-560. © 2010
Elsevier Science, Inc.
Cite this: Relationship Between Vitamin D During Perinatal Development and Health - Medscape - Nov 01, 2010.