TABLE OF CONTENTS

TITLE PAGE        –         –         –         –         –         –         –         –         i

CERTIFICATION           –         –         –         –         –         –         –         ii

DEDICATION      –         –         –         –         –         –         –         –         iii

ACKNOWLEDGEMENT         –         –         –         –         –         –         iv

TABLE OF CONTENTS          –         –         –         –         –         –         v-vii

CHAPTER ONE: INTRODUCTION                 

1.1     Vitamin A    –         –         –         –         –         –         –         –         1-3

CHAPTER TWO: VITAMIN A METABOLISM

2.1     Deficiency of Vitamin A           –         –         –         –         –         8-11

2.2     Side Effect of Vitamin A           –         –         –         –         –         12-14

2.3     Recommended Daily Allowance          –         –         –         –         15-18

2.4     Source of Vitamin A        –         –         –         –         –         –         18-20

2.5.    Population at Risk and Consequences

of Vitamin A deficiency  –         –         –         –         –         –         20-22

2.5.1  Age and Gender     –         –         –         –         –         –         –         22-24

2.5.2  Risk Factors           –         –         –         –         –         –         –         24-26

2.5.3  Morbidity    –         –         –         –         –         –         –         –         26-28

2.6     World and Regional Supply of Vitamin A     –         –         –         29-31

2.7     Indicator of Vitamin A deficiency       –         –         –         –         31-33

CHAPTER THREE: BIOCHEMICAL MECHANISM

FOR VITAMIN A FUNCTION       

3.2     Roles of Vitamin A in Human Metabolism Process           –         36-37

3.2.1  Vision          –         –         –         –         –         –         –         –         37-38

3.2.2  Gene Transcription          –         –         –         –         –         –         38-40

3.2.3  Immune Function  –         –         –         –         –         –         –         41-42

3.1.3  Dermatology          –         –         –         –         –         –         –         43-44

CHAPTER FOUR: SUMMARY AND CONCLUSION        

4.1     Summary     –         –         –         –         –         –         –         –         45

4.2     Conclusion  –         –         –         –         –         –         –         –         46

References

CHAPTER ONE

INTRODUCTION

1.1     Vitamin A

Vitamin A (retinol) is an essential nutrient needed in small amounts by humans for the normal functioning of the visual system; growth and development; and maintenance of epithelial cellular integrity, immune function, and reproduction. These dietary needs for vitamin A are normally provided for as preformed retinol (mainly as retinyl ester) and pro-vitamin A carotenoids

Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta-carotene). Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system, and for good vision.Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells. Yet the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol) in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.

All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are essential for vitamin activity. The orange pigment of carrots (beta-carotene) can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-caroteneand gamma-carotene also have a single retinyl group, which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.

Vitamin A can be found in two principal forms in foods: (i) Retinol: This is the form of vitamin A absorbed when eating animal food sources, it is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate. (ii) The carotenes, alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin (all of which contain beta-ionone rings), but no other carotenoids, function as provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene 15,15′-dioxygenase which cleaves beta-carotene in the intestinal mucosa and converts it to retinol.