VITAMIN D

 Vitamin d is a fat soluble vitamin (however, now considered as a hormone). It resembles sterol in structure and function like hormone. 

CHEMISTRY:

Ergocalciferol (vitamin D2) is formed from ergosterol and is present in plants🌱. 

Ergocalciferol and cholecalciferol are sources for vitamin d activity and are referred to as provitamins. 

Vitamin d is sun-shine vitamin. 

The synthesis of vitamin D3 in the skin is proportional to the exposure to sunlight. Dark skin pigment (melanin) adversly influences the synthesis of cholecalciferol. 

ABSORPTION:

Vitamin D is absorbed in the small intestine for which bile os essential. 

TRANSPORT:

Through lymph, vitamin D enters the circulation bound to plasma alpha2 -globulin and  is distributed through body. 

STORAGE:

Liver and other tissue store small amount of vitamin D. 

SYNTHESIS OF 1,25-DHCC:

Formation of ergocalciferol from ergosterol. 

REGULATION OF THE SYNTHESIS OF 1,25-DHCC:

Biosynthesis of active from of vitamin D-calcitriol(1, 25DHCC) 

BIOCHEMICAL FUNCTION:

Calcitriol (1,25-DHCC) is the biologically active form of vitamin D. It regulates the plasma levels of calcium and phosphate. Calcitriol acts at 3 different levels (intestine, kidney and bone) to maintain plasma calcium (normal 9-11 mg/dl).

1. Action of calcitriol on the intestine : Calcitriol increases the intestinal absorption of calcium and phosphate. In the intestinal cells, calcitriol binds with a cytosolic receptor to form a calcitriol-receptor complex. This complex then approaches the nucleus and interacts with a specific DNA leading to the synthesis of a specific calcium binding protein (calbindin). This protein increases the calcium uptake by the intestine. The mechanism of action of calcitriol on the target tissue (intestine) is similar to the action of a steroid hormone.

2. Action of calcitriol on the bone: In the osteoblasts of bone, calcitriol stimulates calcium uptake for deposition as calcium phosphate. Thus calcitriol is essential for bone formation. The bone is an important reservoir of calcium and phosphate. Calcitriol along with parathyroid hormone increases the mobilization of calcium and phosphate from the bone. This causes elevation in the plasma calcium and phosphate levels.

Metabolism and biochemical function of vitamin D(1, 25DHCC,25-Dihydroxycholecalciferol, also called as Calcitriol is the active from of vitamin D;PTH-Parathyroid hormone) 

RECOMMENDED DIETARY ALLOWANCE (RDA) 

 The daily requirement of vitamin D is 400 International Units or 10 mg of cholecalciferol. In countries with good sunlight (like India), the RDA for vitamin D is 200 IU (or 5 µg chole calciferol).

DIETARY SOURCES:

Good sources of vitamin D include fatty fish, fish liver oils, egg yolk etc. Milk is not a good source of vitamin D.

Vitamin D can be provided to the body in three ways

1. Exposure of skin to sunlight for synthesis of vitamin D;                                                               2.Consumption of natural foods;                        3. By irradiating foods (like yeast) that  contain precursors of vitamin D and fortification of foods (milk, butter etc.).

DEFICIENCY SYMPTOMS:

  Vitamin D deficiency is relatively less common, since this vitamin can be synthesized in the body. However insufficient exposure to sunlight and consumption of diet lacking vitamin D results in its deficiency. 

   Vitamin D deficiency occurs in strict vegetarians ,chronic alcoholocs, individual with liver and kidney diseases or fat malabsorption syndromes. In some people, who cover the entire body (purdah) for religious customs, vitamin D deficiency is also observed, Vitamin D if the requirement is not met through diet.

Diagnosis of vitamin D deficiency: Estimation of plasma levels of 25-hydroxycholecalciferol (Reference range 30-40 ng/ml) are normally employed to evaluate vitamin D deficiency.

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