Vitamins PDF

# VITAMINS ## Dr. P. N. ANSIL ## What are vitamins? - Essential organic compounds that are required in small amounts for normal growth, maintenance of good health and for the proper utilization of other nutrients. - Nutrients that our body does not make on its own. Thus we must obtain them from t...

# VITAMINS ## Dr. P. N. ANSIL ## What are vitamins? - Essential organic compounds that are required in small amounts for normal growth, maintenance of good health and for the proper utilization of other nutrients. - Nutrients that our body does not make on its own. Thus we must obtain them from the foods we eat, or via vitamin supplements. ## Classification of Vitamins | | | | | :------------------------------------------------------------------------------------------ | :-------------- | :--------------------- | | **VITAMINS** | **Fat Soluble** | **Water Soluble** | | | Vitamin A | B-Complex | | | Vitamin D | Non B-Complex | | | Vitamin E | Vitamin C | | | Vitamin K | | | **Energy releasing** | | | | Thiamine | | | | Riboflavin | | | | Niacin | | | | Pyridoxine | | | | Biotin | | | | Pantothenic acid | | | | Folic acid | | | | Hematopoietic | | | | Vitamin B₁₂ | | | ## Difference between fat soluble and water soluble vitamins | Fat soluble vitamins | Water soluble vitamins | | :-------------------------------------------------- | :-------------------------------------------------------- | | Soluble in fat | Soluble in water | | Absorbed along with other lipids | Absorption is simple | | Requires carrier proteins | No requirement of carrier protein | | Stored in liver | Excreted in urine | | Deficiency manifests only when stores are depleted | Deficiency manifests rapidly as there is no storage | | Toxicity - Hypervitaminosis may result | Unlikely, since excess is excreted | | Single large does may prevent deficiency | Regular dietary supply is required | | E.g. A,D,E & K | E.g. B complex & C | ## Lipid soluble vitamins - common features - Hydrophobic - Isoprene derivatives - They cannot be synthesized by the body - Supplied by the diet - Absorption along with fat - Transport: By lipoprotein & Specific binding protein - Surplus amount stored - liver & adipose tissue - Excess consumption leads to accumulation & toxic effects. ## Vitamin A - Vitamin A is an essential nutrient needed in small amounts for the normal functioning of the visual system, and maintenance of cell function for growth, epithelial integrity, red blood cell production, immunity and reproduction. - Vitamin A deficiency (VAD) is a major nutritional concern in poor societies, especially in lower income countries like INDIA. ## Where does it come from? ### Animal Sources - Eggs - Meat - Cheese - Milk - Liver - Kidney - Fish liver oils ### Plant Sources - Carrots - Sweet Potatoes - Apricots - Broccoli - Spinach - Pumpkin - Papaya - Mango ## Vitamin A (Preformed and Provitamin) - Retinoids: Natural & synthetic forms of Vitamin A - Preformed: Three preformed compounds that are metabolically active and found in animal products - Retinol - alcohol form - Present in animal tissues as retinyl ester with long chain fatty acid - Retinal or retinaldehyde - aldehyde form - Obtained by oxidation of retinol - Retinal & Retinol are interconvertible - Retinoic acid - acid form - Produced by oxidation of Retinal - Provitamin: Carotenoids (β-carotene) can yield 2 retinols when metabolized in the body - β-carotene - found in plants - Has 2 β-ionone rings connected by a polyprenoid chain ## Structures of vitamin A and related compounds. - α-Carotene (a carotenoid) - Retinol (vitamin A) - Retinol phosphate - Retinal (All-trans-retinal) - Retinoic acid ## Retinal **(All-trans-retinal)** - 11 - 12 - (A11-cis-retinal) - C - H - H - C ## Absorption - Retinoids - Retinyl esters broken down to free retinol & FA in small intestine requires bile, digestive enzymes, integration into micelles - Once absorbed, retinyl esters reformed in intestinal cells - 90% of retinoids can be absorbed - Carotenoids - Absorbed intact, absorption rate much lower - Intestinal cells can convert carotenoids to retinoids ## Absorption - Approximately 80% is absorbed. - It is passed along with fat through the lymphatic system into blood stream. - Absorption is poor in case of diarrhea, jaundice and abdominal disorder. - Absorption increases if taken with fat. - Vitamin A which is not absorbed is excreted within 1 or 2 days in feces. ## Transport - Transported via chylomicrons from intestinal cells to the liver - Transported from the liver to target tissue as retinol via retinol-binding protein (RBP; MW. 21,000). | | | | :----------- | :------------------------------------ | | From intestine | Liver | | From liver | tissue | - Chylomicron - Plasma retinol binding protein (RBP)- to cell membrane- - Cellular retinoic-acid binding protein (CRBP) -retinol- - cytoplasm-HRE-GENE EXPRESSION ## Summary of Vitamin A absorption - Intestinal cell - Liver - Target cell ## Summary of the functions of Vitamin A compounds - β-Carotene (antioxidant) - Retinol (steroid hormone) - Retinal (visual cycle) - Retinyl phosphate (glycoprotein synthesis) - Retinoic acid (steroid hormone) ## Storage - The liver has enormous capacity to store in the form of retinol palmitate. - Under normal conditions a well-fed person has sufficient Vitamin A reserves to meet his need for 6 to 9months or more ## Excretion of Vitamin A - Not readily excreted - Kidney disease and aging increase risk of toxicity because excretion is impaired. ## Functions of Vitamin A - Vision: Vitamin A is a component of the visual pigment rhodopsin. Retinal is bound to the protein opsin. - Growth: Vitamin A deficiency causes loss of appetite. Slow bone growth. Affects CNS. - Reproduction: Retinol and retinal are essential for normal reproduction - Maintenance of epithelial cells: Essential for normal differentiation of epithelial tissues and mucus secretion ## Role of Vitamin A in Vision ### Visual Cycle (Wald's Visual Cycle) - A process by which light impacting on the retina of the eye is converted to an electrical signal. - The optic nerve carries the electrical signal to the brain (nerve impulse). - The brain processes the signal into an image. - Retina is a light-sensitive layer of cells at the back of the eye where an image is formed. - Retina consists of: Rod and cone cells (photosensitive cells) - Rod cells process black & white image - Cone cells process color image - Iris - Lens - Retina - Front of eye - Pupil - Optic nerve - Light focused on retina ## Microscopic Anatomy of the Retina - Horizontal Cells - Amacrine Cells - Pigmented Cells - Cone Cells - Rod Cells - Bipolar Cells - Ganglion Cells - Light from the Lens - Figure 5 - Rod Cell - Cone Cell ## Role of Vitamin A in Vision - Normal vision depends on the retina and on adequate vitamin A - In the retina, vitamin A in the form of retinal binds to a protein called opsin to make rhodopsin [11-cis - retinal-opsin) in rod cells. - Rhodopsin is a light-sensitive pigments. ## Wald's Visual Cycle - Rhodopsin (Visual pigment) - Photo-receptor - Opsin - 11-cis-retinal - All-trans-retinal - Retinal epithelium - cis-retinal - trans-retinal - Blood - Liver - cis-retinal - NADH - ADH in liver - NAD+ - cis-retinol - trans-retinal - George Wald - NP 1967 - 1906-1997 - Retinol isomerase in liver - NADH - ADH in liver - NAD+ - trans-retinol ## The Visual Cycle - In the dark - Opsin and Il-cis retinal enzymatically combined to regenerate rhodopsin. - All-Trans retinal enzymatically converted back to Il-cis retinal - Opsin triggers reaction cascade message sent to brain. - Cessation of dark current. - In the light - Rhodopsin absorbs photan of light - Il-cis refinal isomerizes to all-trans retinal in rhodopsin - All-Trans retinal separates from opsin ## Role of Vitamin A in Vision - When stimulated by light, vitamin A isomerizes from its bent 'cis' form to a straighter 'trans' form and detaches from opsin - The opsin molecule changes shape, which sends a signal to the brain via optic nerve and an image is formed - Most retinal released in this process is quickly converted to trans-retinol and then to cis-retinal, to begin another cycle ### Dark Adaptation time - Bright light depletes rhodopsin (photobleaching) - Sudden shift from bright light to darkness causes difficulty in seeing - Rhodopsin is synthesized in a few minutes and vision is improved in the dark - The time required to synthesize rhodopsin in the dark is called dark adaptation time. - It is increased in vitamin A deficiency. ## Bleaching of Rhodopsin - Bleaching - When exposed to light, the colour of rhodopsin changes from red to yellow - Occurs in a few milliseconds - Unstable intermediates are formed: - Rhodopsin - Prelumirhodopsin - Lumirhodopsin - All-trans-retinal + Opsin - Metarhodopsin II - Metarhodopsin 1 ## Visual cascade and cGMP - Light strikes the retina - biochemical changes - membrane hyperpolarization - generate nerve impulse. - Hyperpolarization of the membrane is brought about by a visual cascade involving cGMP ## Visual cascade involving cGMP - Rhodopsin - Photon - Metarhodopsin II - GTP - GDP - Transducin (inactive) - Transducin (active) - Phosphodiesterase (inactive) - Phosphodiesterase (active) - 3',5'-cGMP - 5'-GMP - Decrease in cGMP closes the Na+ channels in the membranes of the rod cells. - Result in hyperpolariztion - an excitatory response transmitted through the neuron network to the visual cortex of the brain ## Colour vision - Cones are specialized in bright and colour vision - Governed by colour sensitive pigments: - iodopsin-green - cyanopsin - blue - porphyropsin - red - These pigments are complexes of protein & Vitamin A - Bright light strikes retina - depending on the particular colour of the light - one or more pigments are bleached - passes nerve impulse to brain as specific colour ## Functions of Vitamin A: Growth and Differentiation of Cells - Retinoic acid is necessary for cellular differentiation - Important for embryo development, gene expression - Retinoic acid influences production, structure, and function of epithelial cells that line the outside (skin) and external passages (mucus forming cells) within the body ## Epithelial cell 'integrity' - Many epithelial cell require Vitamin A for proper differentiation and maintenance - Lack of Vitamin A leads to dysfunction of epithelia - The skin becomes keratinized and scaly, and mucus secretion is suppressed ## Functions of Vitamin A: Immunity - Deficiency leads to decreased resistance to infections - Supplementation may decrease severity of infections in deficient person ## Functions of Vitamin A - Role in Prevention of cardiovascular disease - Antioxidant capabilities - Role in Cancer prevention: - Antioxidant capabilities - Lung, oral, and prostate cancers - Studies indicate that vitamin A-containing foods are more protective than supplements ## What does RDA mean? - Recommended Dietary Allowances - These are suggested levels of essential nutrients considered adequate to meet nutritional needs of healthy individuals. | | | | :-------------- | :------------- | | Infants and children | 400-600µg/day | | Men | -750-1000µg/day | | Women | -750µg/day | | Pregnancy | -1000µg/day | | Lactation | -1200µg/day | ## Vitamin A Deficiency and Diseases - Nyctalopia (night blindness) - Xerophthalmia - Bitot's spots - Keratomalacia - Complete blindness (in severe deficiency) ## 1. Night Blindness - The visual acuity is diminished in dim light. - Occurs as a result of inadequate pigment in the retina. - The dark adaptation time is increased. - Found in pregnant women in some instances, especially during the last trimester of pregnancy when the vitamin A needs are increased. ## 2. Xerophthalmia - It is the most specific VAD, and is the leading preventable cause of blindness. - Conjunctiva becomes dry, thick and wrinkled. - Conjunctiva gets keratinized and loses its normal transparency. - Cornea becomes glazy & lusterless. ## 3. Bitot's Spot - Tissue spots that develop around the eye ball, causing severe dryness in the eyes. - Seen as grayish-white triangular plaques firmly adherent to the conjunctiva. ## 4.Keratomalacia - One of the major cause for blindness in India. - Cornea becomes soft and may burst. - Prolonged xerophthalmia leads to keratomalacia (softening of the cornea). - If the eye collapses, vision is lost. ## Other Symptoms of VAD - Alteration of skin and mucous membrane - Hepatic dysfunction - Headache - Drowsiness - Peeling of skin