Fat Soluble Vitamins 1 Lecture Notes PDF

Lecture 41 Fat Soluble Vitamins 1 Importance of Vitamin D 1  Specific Objectives By the end of this lecture students can be able to: Understand the synthetic pathway of vitamin D inside the body. Explain activation process of vitamin D. Explain the main functio...

Lecture 41 Fat Soluble Vitamins 1 Importance of Vitamin D 1  Specific Objectives By the end of this lecture students can be able to: Understand the synthetic pathway of vitamin D inside the body. Explain activation process of vitamin D. Explain the main functions of Vitamin D. Discuss the effects of vitamin D deficiency. 2  VITAMIN D The D vitamins are a group of sterols that have a hormone-like function. vitamin D is called “sun-shine vitamin”. As sunshine less in winter months, vitamin deficiency is seen in winter. The active molecule, 1,25- dihydroxycholecalciferol (Calcitriol) (1,25-diOH- D3), binds to intracellular receptor proteins. 3 Distribution of vitamin D 1. Diet: Ergocalciferol (vitamin D2), found in plants, and cholecalciferol (vitamin D3), found in animal tissues, are sources of preformed vitamin D activity 2. Endogenous vitamin precursor: 7-Dehydrocholesterol, an intermediate in cholesterol synthesis, is converted to cholecalciferol in the dermis and epidermis of humans exposed to sunlight. 4 5 Function of vitamin D The overall function of Calcitriol is to maintain adequate (increase) plasma levels of calcium. It help the body absorb and retain calcium and phosphorus; both are critical for building bone. 6 Effect of vitamin D on the intestine: 1,25-diOH-D3 (Calcitriol) stimulates intestinal absorption of calcium and phosphate. As a result, calcium uptake is enhanced by an increased synthesis of a specific calcium-binding protein (Calbindin). Thus, the mechanism of action of 1,25-diOH-D3 is typical of steroid hormones. 7 Effect of vitamin D in Renal Tubules: Calcitriol increases the reabsorption of calcium and phosphorus by renal tubules, therefore both minerals are conserved. 8 Effect of vitamin D on bone: Calcitriol stimulates osteoblasts which secret alkaline phosphatase. Due to this enzyme, the local concentration of phosphate is increased. The ionic product of calcium and phosphorus increases, leading to mineralization. 9 Other functions of vitamin D: It strengthens the immune system. It might prevent certain types of cancer, it may reduce cancer cell growth. It helps control infections. It reduces inflammation. It boosts your mood. It can aid in weight loss. It can lower the risk of rheumatoid arthritis. It lowers the risk of type 2 diabetes. It can help lower blood pressure. It might reduce the risk of heart disease. 10 11 D. Distribution and requirement of vitamin D Vitamin D occurs naturally in fatty fish, liver, and egg yolk. Milk, unless it is artificially fortified, is moderate source of the vitamin. AI for vitamin D is 200 IU to age 50, and 400-600 IU after age 50. 12  Deficiency of Vitamin D Causes for Vitamin D Deficiency i. Nutritional deficiency of vitamin D is the most common cause. This can occur in people who are not exposed to sunlight properly or not take adequate level of vitamin D source. ii. Malabsorption of vitamin (obstructive jaundice and steatorrhea). iii. Abnormality of vitamin D activation. Liver and renal diseases may retard the hydroxylation reactions. 13  Clinical indications Vitamin D deficiency causes a net demineralization of bone, resulting in rickets in children and osteomalacia in adults. Rickets is characterized by the continued formation of the collagen matrix of bone, but incomplete mineralization, resulting in soft, pliable bones. 14  In osteomalacia, demineralization of pre-existing bones increases their susceptibility to fracture. 15 Toxicity of vitamin D Like all fat-soluble vitamins, vitamin D can be stored in the body and is only slowly metabolized. High doses (100,000 IU for weeks or months) can cause loss of appetite, nausea, thirst, and stupor. 16 Reference Book: Vasudevan, D. M., Sreekumari, S., and Kannan, V.., 2011. Textbook of biochemistry for medical students, 6th Edition. 17