Fat – soluble vitamins Lec. 16

Fat-soluble vit. (A, D, E and K). Unlike water-soluble vitamins. They need regular replacement in the body. Fat-soluble vitamins are stored in the liver and fatty tissues. They are eliminated much more slowly than water-soluble vitamins.

They are absorbed from the small intestines, along with dietary fat, which is why fat malabsorption resulting from various diseases (e.g., cystic fibrosis, ulcerative colitis, Crohn's disease) is associated with poor absorption of these vitamins.

Vitamin A Vitamin A comprises the preformed retinoids. Retinoids is a collective term for retinol, retinal, and retinoic acid, all of which are biologically active. This vitamin is a constituent of the retinal pigment rhodopsin. It is essential for normal synthesis of mucopolysaccharides and growth of epithelial tissue. Vitamin A have three basic functions: vision, growth and development of tissues, and immunity.

Vision: Vitamin A combines with a protein called opsin to form rhodopsin in the rod cells of the retina. When vitamin A is inadequate, the lack of rhodopsin causes night blindness and severe cases may lead to complete loss of vision. Vitamin A is present in the diet and can be synthesized from dietary carotenes.

Vitamin A can be measured in plasma in which it is bound to prealbumin and retinol binding globulin. Low binding protein conc. can cause the plasma conc. of vitamin A to be low and impair its delevary to tissue even when hepatic stores of the vitamin is sufficient. Sources of vitamin A liver, fortified milk, fish liver oils, orange, dark green, and yellow vegetables, orange fruits.

Vitamin D Vitamin D plays a critical role in the body’s use of calcium and phosphorous. It increases the amount of calcium absorbed from the small intestine and helps form and maintain bones. Endogenous synthesis: - By the action of ultraviolet light on 7- dehydrocholesterol in the skin to produce Vit. D-3 cholecalciferol. - From the diet.

It is made in the liver and intestine. Dietary vit D is vitamin D-2 (ergocalciferol). Food sources of vitamin D are milk and other dairy products prepared with vitamin D. It is also found in oily fish (e.g.salmon and sardines) as well as in cod liver oil and margarines. Vit D3 and D2 undergo the same metabolic changes and physiological actions.

Vitamin D deficiency Vitamin D deficiency can result from inadequate intake coupled with inadequate sunlight exposure, disorders that limit its absorption, conditions that impair conversion of vitamin D into active metabolites and decreased endogenous synthesis.

Binding of calcium in the gut by dietary phytates contribute to Osteomalacia. Infant at high risk of vitamin D deficiency due to the little amount of vit D in breast milk. Cholecalciferol has little physiological activity Within the liver, cholecalciferal is hydroxylated to 25-hydroxycholecalciferol by the enzyme 25-hydroxylase. Within the kidney, 25-hydroxycholecalciferol serves as a substrate for 1-alpha-hydroxylase, yielding 1,25-dihydroxycholecalciferol, the biologically active form (Calcitirol)

These metabolites are transported in the circulation by a specific binding protein. Calcitirol, hormone play important role in calcium homoeostasis. Vit.D can be measured of plasma concentration of calcidiol.

The classical manifestations of vitamin D deficiency is rickets, which is seen in children and results in bony deformaties including curved long bones. Deficiency in adults leads to the disease osteomalacia. Vitamin D toxicity: Excessive exposure to sunlight does not lead to overproduction of vitamin D. Vitamin D toxicity is inevitably the result of overdosing on vitamin D supplements.

Vitamin K Vitamin K is involved in the carboxylation of certain glutamate residues in proteins to form gamma-carboxyglutamate residues. It play key roles in the regulation of three physiological processes: blood coagulation bone metabolism (binding Calciun to protein) vascular biology.

Sources of Vitamin K Vitamin K is found in leafy green vegetables such as spinach and lettuce; cabbage, broccoli, and wheat bran; cereals; some fruits, such as avocado, kiwifruit and bananas; meats; cow milk, other dairy products, eggs and soybeans

Vitamin K-deficiency results in an increase in prothrombin time which use as a test of liver function. Its most frequent use is on monitoring of patients on anticoagulant treatment e.g. (warfarin).

Vitamin E Tocopherol, or vitamin E, is a fat-soluble vitamin is an important antioxidant. In cell membranes mainly. Protecting unsaturated fatty acid residues against free radical attack. Sources of Vitamin E vegetable oils such as palm oil, sunflower, corn, soybean, olive oil, Nuts, sunflower seeds, kiwi, fish, peanut butter, and green leafy vegetable.

Deficiency of vit E may occure in severe malabsorption (in infant) Haemolytic anaemia and neurological dysfunction.

Vitamins as drugs Nutraceuticals Folic acid to reduce the risk of nural tube defect. Vit E antioxidant and reduce the risk of coronary heart disease Vit. C, β-carotene Vit A and Vit D are toxic in excess and even water soluble vitamins Excessive amount of Vit C may causes diarriah.