ABSTRACT
Introduction .......................................................................................................................... 2
Nutritional Aspects of Vitamin A and Carotenoids.............................................................. 2
Historical ........................................................................................................................... 2
Definitions of Vitamin A, Retinoids, and Carotenoids ..................................................... 3
Properties, Nutritional Equivalency, and Recommended Intakes ..................................... 3
Properties of Nutritionally Important Retinoids ........................................................... 3
Properties of Nutritionally Important Carotenoids ....................................................... 7
Nutritional Equivalency................................................................................................. 8
Transport and Metabolism.................................................................................................. 10
Transport and Binding Proteins ...................................................................................... 10
Retinol-Binding Protein ............................................................................................... 12
Albumin ....................................................................................................................... 13
Lipoproteins................................................................................................................. 13
Intracellular Retinoid-Binding Proteins ....................................................................... 13
Nuclear Retinoid Receptors......................................................................................... 14
Intestinal Metabolism ...................................................................................................... 15
Conversion of Provitamin A Carotenoids to Retinoids............................................... 15
Intestinal Absorption of Vitamin A............................................................................. 17
Reesterification, Incorporation into Chylomicrons, and Lymphatic Secretion ........... 18
Hepatic Uptake, Storage, and Release of Vitamin A ...................................................... 18
Hepatic Uptake............................................................................................................ 18
Extrahepatic Uptake .................................................................................................... 19
Storage ......................................................................................................................... 19
Release ......................................................................................................................... 20
Plasma Transport ............................................................................................................ 20
Plasma Retinol ............................................................................................................. 20
Conditions in Which Plasma Retinol May Be Low..................................................... 22
Other Retinoids in Plasma ........................................................................................... 23
Plasma Carotenoids ..................................................................................................... 23
Plasma Retinol Kinetics and Recycling ....................................................................... 23
Intracellular Retinoid Metabolism .................................................................................. 23
Hydrolysis .................................................................................................................... 23
Oxidation-Reduction and Irreversible Oxidation Reactions ....................................... 24
Formation of More Polar Retinoids............................................................................ 24
Conjugation ................................................................................................................. 25
Isomerization ............................................................................................................... 25
Vitamin A and Public Health.............................................................................................. 25
Prevention of Xerophthalmia .......................................................................................... 25
Actions of Vitamin A in the Eye.................................................................................. 26
Morbidity and Mortality ................................................................................................. 27
Subclinical Deficiency .................................................................................................. 27
Immune System Changes ............................................................................................. 27
Medical Uses of Retinoids............................................................................................... 28
Dermatology ................................................................................................................ 28
Acute Promyelocytic Leukemia ................................................................................... 29
Prevention of Hypervitaminosis A of Nutritional Origin................................................ 29
Excessive Consumption of b-Carotene ........................................................................ 29 References ........................................................................................................................... 30
Vitamin A (retinol) is an essential micronutrient for all vertebrates. It is required for normal
vision, reproduction, embryonic development, cell and tissue differentiation, and immune
function. Many aspects of the transport and metabolism of vitamin A, as well as its
functions, are well conserved among species. Dietary vitamin A is ingested in two main
forms-preformed vitamin A (retinyl esters and retinol) and provitamin A carotenoids
(b-carotene, a-carotene, and b-cryptoxanthin)—although the proportion of vitamin A obtained from each of these form varies considerably among animal species and among
individual human diets. These precursors serve as substrates for the biosynthesis of two
essential metabolites of vitamin A: 11-cis-retinal, required for vision, and all-trans-retinoic acid,
required for cell differentiation and the regulation of gene transcription in nearly all tissues.
