Vitamin D

Introduction of vitamin D

The generic term vitamin D designates a group of chemically related compounds that possess antirachitic activity. The two most prominent members of this group are vitamin D2 (ergo￾calciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is derived from a common plant steroid, ergosterol, and is the form that was employed for nutritional vitamin D fortification of foods from the 1940s to 1960s. Vitamin D3 is the form of vitamin D obtained when radiant energy from the sun strikes the skin and converts the precursor 7-dehydrocholesterol. Since ,The body is capable of producing vitamin D3, vitamin D does not meet the classical definition of a vitamin. A more accurate description of vitamin D is that it is a prohormone; thus,vitamin D is metabolized to a biologically active form that functions as a steroid hormone 1,2]. However, since vitamin D was first recognized as an essential nutrient, it has historically been classified among the lipid-soluble vitamins. Even today it is thought of by many as a vitamin for public health reasons [3], although it is now known that there exists a vitamin D endocrine system that generates the steroid hormone 1a,25-dihydroxyvitamin D3 [1a,25(OH)2D3] [4].Vitamin D functions to maintain calcium homeostasis together with two peptidehormones, calcitonin and parathyroid hormone (PTH). Vitamin D is also important for phosphorus homeostasis [5–7]. Calcium and phosphorus are required for a wide variety of biological processes (see Table 2.1). Calcium is necessary for muscle contraction, nerve pulse transmission, blood clotting, and membrane structure. It also serves as a cofactor for such enzymes as lipases and ATPases and is needed for eggshell formation in birds. It is an important intracellular signaling molecule for signal transduction pathways such as those involving calmodulin and protein kinase C (PKC). Phosphorus is an important component of DNA,RNA, membrane lipids, and the intracellular energy-transferring ATP system. The phosphory￾lation of proteins is important for the regulation of many metabolic pathways. The mainten￾ance of serum calcium and phosphorus levels within narrow limits is important for normal bone mineralization. Any perturbation in these levels results in bone calcium accretion or resorption. Disease states, such as rickets, can develop if the serum ion product is not main￾tained at a level consistent with that required for normal bone mineralization. Maintaining a homeostatic state for these two elements is of considerable importance to a living organism.The active form of vitamin D3, 1a,25(OH)2D3, has been shown to act on novel target tissues not related to calcium homeostasis. There have been reports characterizing receptors for the hormonal form of vitamin D and activities in such diverse tissues as brain, pancreas,pituitary, hair follicle, skin, muscle, immune cells, and parathyroid (Table 2.2). These studies suggest that vitamin D status is important for insulin and prolactin secretion, hair growth,muscle function, immune and stress response, and melanin synthesis and cellular differentiation of skin and blood cells. A number of recent and comprehensive reviews [1,8–22] cover many aspects of vitamin D and its endocrinology.

Sources of vitamin D

 Daylight – Exposure to daylight causes the skin to produce vitamin D.

 Oily fish - Amazing sources of vitamin D include salmon, mackerel, and fish.

 Energised food options - Vitamin D has been added to several food sources, including milk, cereal, and squeezed oranges.

 Egg yolks: One large egg yolk supplies around 6% of the recommended daily intake of vitamin D.

 Mushrooms - Some mushrooms, such as shiitake, provide trace amounts of vitamin D.

6. A teaspoon of cod liver oil contains about 450% of the recommended daily intake of vitamin D.

Supplements - Vitamin D supplements are often available as tablets, liquids, and containers.

Function of vitamin D

The major role of vitamin D is to promote appropriate blood calcium levels by aiding the body's absorption of calcium. Strong bones and teeth require calcium, and vitamin D is essential for calcium metabolism. Additionally, vitamin D is crucial for:

immunological system performance - Vitamin D helps control immunological responses and may help lower the risk of developing some autoimmune illnesses.

muscular function - Vitamin D is necessary for healthy muscular function and may help older persons avoid fractures and falls.

Brain function: Some research points to a possible role for vitamin D in the regulation of mood and cognition.

Cardiovascular health - Vitamin D's role in regulating blood pressure may help lower the risk of heart disease.