Vitamin A in general
Please use the following site to get an overview:
Linus Pauling Institute, Vitamin A
Vitamin A is any compound possessing the biological activity of retinol. The term retinoid includes any naturally occurring form of vitamin A and any retinol analog with or without biological activity (Blomhoff 1994).
Sources of vitamin A in the diet:
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- Retinyl esters: liver-products, some in fish and meat, milk products.
- β-carotene: green, yellow and red vegetables
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No animal species can synthesize vitamin A. De novo vitamin A synthesis is restricted to plants, bacteria, fungi and algae. Hence, animals must obtain vitamin A from the diet. Most of the vitamin A obtained from the diet stems from provitamin A carotenoids in various food plants, where the various carotenoids give rise to yellow, orange, red or purple colors. Vitamin A is also obtained from animal products, where vitamin A in the form of retinyl esters are present, like the liver, but also other tissues. Some foods are fortified with vitamin A, like milk, margarine and in some countries, breakfast cereals.
Recommendations
- Children: 400 µg
- Women: 700 µg
- Men: 900 µg
- Lactating: 1200-1300 µg
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Carotenoids are taken up by the enterocytes in the small intestine by passive diffusion. The bioavailability, i.e. the fraction that becomes available to the various tissues for utilisation or storage, is determined by several factors.
Vitamin A metabolites: all-trans retinoic acid (tretinoin, vitamin A acid), 11-cis retinal, 13-cis retinoic acid (isotretinoin)
Retinoids: all natural and synthetic analogs of retinol, with or without biological activity.
Uptake, transport, storage, metabolism
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Physiological metabolites of vitamin A:
RE (Retinyl esters)
ROH (Retinol)
RAL (Retinal)
RA (Retinoic Acid)
Transport proteins for vitamin A in plasma:
RBP (Retinol Binding Protein)
TTR (Transthyretin)
Intracellular transport proteins for vitamin A:
CRBP (Cellular Retinol Binding Protein)
CRABP (Cellular Retinoic Acid Binding Protein)
Important enzymes in the metabolism of vitamin A:
REH (Retinyl Ester Hydrolase)
β-carotene-15,15'dioxygenase
RDH (Retinol Dehydrogenase)
RALDH (Retinal Dehydrogenase)
LRAT (Lecithin:Retinol Acyltransferase)
ARAT (Acyl-CoA:Retinol Acyltransferase)
CYP26
Nuclear receptors activated by vitamin A:
RAR (Retinoic Acid Receptor)
RXR (Retinoid X Receptor)