ROLE OF VITAMIN A IN HUMAN METABOLIC PROCESS
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.
