Vitamin A PDF

HNUT10010 HNUT10020 Human Nutrition 1: Understanding Nutrients Aifric O’Sullivan BSc, MSc, PhD UCD Institute of Food and Health School of Agriculture and Food Science Vitamin A Harvest Plus Project Increase vitamin A intake Vitamin A deficiency is a major public health concern More than 600,000...

HNUT10010 HNUT10020 Human Nutrition 1: Understanding Nutrients Aifric O’Sullivan BSc, MSc, PhD UCD Institute of Food and Health School of Agriculture and Food Science Vitamin A Harvest Plus Project Increase vitamin A intake Vitamin A deficiency is a major public health concern More than 600,000 deaths a year among children under five Vitamin A Alcohol form Aldehyde form Preformed Vitamin A Acid form ProVitamin A Cleaved to yield retinaldehyde The Nutrition Society Textbook Series, Introduction to Human Nutrition, Second Edition © 2009, 2002 Vitamin A Metabolism 50+ carotenoids: , β, -carotene and cryptoxanthin Retinol activity equivalent RAE = 1µg retinol Biological activity of carotenes is lower β –carotene: ~ 12µg = 1µg retinol/RAE Other carotenes: ~24µg = 1µg retinol The Nutrition Society Textbook Series, Introduction to Human Nutrition, Second Edition © 2009, 2002 Retinol Activity Equivalent • Poor absorption • Poor intestinal activity of carotene dioxygenase • Other carotenoids may inhibit the action of carotene dioxygenase • Sometimes cleavage occurs at different parts resulting in 8, 10, 12 apo-carotenes which are oxidized to retinoic acid not retinol Vitamin A Foods RDA men = 700 µg women = 600 µg RAE μg/d Sweet potato, baked in skin, 1 whole 1,403 Beef liver, pan fried, 3 ounces 6,582 Spinach, frozen, boiled, ½ cup 573 Carrots, raw, ½ cup 459 Peppers, sweet, red, raw, ½ cup 117 Mangos, raw, 1 whole 112 Breakfast cereal, regular bowl 140 Milk, glass 112 Egg, hard boiled, 1 large 75 Vitamin A Requirements RAE μg/d Children 1-3 4-6 7-10 Males 11-14 15-17 18-64 65+ Pregnancy Lactation¶ 400 400 500 Males 600 700 Males 700 700 700 950 RAE μg/d Females 600 600 Females 600 600 FSAI, 1999 Vitamin A Metabolism Retinol Binding Protein and transthyretin Stored mainly in liver and adipose +A Chylomicron +A Retinol 70-90% efficiency Carotene 5-60% efficiency Vitamin A Functions • • • • Vision Growth Reproduction Immune function Vitamin A Metabolic Function Participates in the conversion of light into nerve impulses Vision Opsin and trans-Retinal split sending a signal to the brain Vitamin A helps to maintain the cornea Trans-retinal converted to cis-retinal and Rhodopsin is reformed or converted to Retinoic acid Vision https://lpi.oregonstate.edu/mic/vitamins/vitamin-A Linus Pauling Micronutrient Information Centre Vitamin A Deficiency Vision In vitamin A deficiency the time taken to adapt to darkness and the 1 ability to see in poor light are impaired.2 In deficiency extra retinal is not available to restore cis-retinal Vitamin A and Gene Expression RARE bound to specific regions on DNA Gene expression Nucleus 1 Retinoic acid response element (RARE) DNA 2 1. 2. 3. 4. ,β, retinoic acid receptor (RAR) ,β, retinoic X receptor (RXR) All-trans retinoic acid 9-cis retinoic acid 3 Activates transcription Retinoic acid moves into the nucleus Binds to receptors (RAR/RXR) part of the RARE in the promoter region of certain genes Binding to receptors on DNA enhances the transcription of selected genes Depending on the tissue, vitamin A regulation will have slightly different function e.g. Epithelial cells = cell differentiation Vitamin A Metabolic Function • Retinoic acid, regulates the expression of various genes that encode for: – structural proteins (e.g., skin keratins) – enzymes (e.g., alcohol dehydrogenase) – cell differentiation (e.g. epithelial cells) – immune cells • Embryonic development • Immune function • Carotenoids function as antioxidants Vitamin A Deficiency • Night blindness – Impaired production of rhodopsin • Xerophthalmia – Dryness of the eye due to inadequate mucus – Progresses to irreversible blindness – Bitot’s spots (small white cell accumulation) • Hyperkeratosis (too much keratin) – Immature skin cells fail to differentiate properly Vitamin A Deficiency Vitamin A Status • Direct measure is liver biopsy (invasive) • Ability to adapt to dim light is sometimes used to assess deficiency (dark adaptation time) • Bitot’s spots: dead cells on surface of eye • Relative Dose Response test for status (after vitamin A ingestion) – measure % change in serum concentration Vitamin A Toxicity • Limited capacity to metabolize • Excessive intake leads to accumulation in liver and other tissues • Unbound vitamin A leads to liver and bone damage • Recommended upper limits of dietary intake*** • Teratogenic effect: associated with birth defects (UL Pregnant women = 3000-3300 µg) Reading • Gibney M, Lanham-New S, Cassidy A, Vorster H, eds. Introduction to Human Nutrition. 2nd ed: Wiley-Blackwell, 2009. Chapter 8 • Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism, International Edition 6th ed: Wadsworth Cenage Learning, 2013. Chapter 9/10 • Whitney E, Rolfes SR. Understanding Nutrition, 13th Edition: Wadsworth, Cengage Learning, 2013. Chapter 10/11