Vitamin A is mostly present in foods of animal origin, especially in the liver and spleen, followed by milk and derivatives and eggs. However, given that meat, milk and eggs are rich in cholesterol, it is recommended to consume it mainly through fish and derivatives, and through vegetable sources.
Carotenoids, from which the body obtains vitamin A, are particularly present in plant tissues and photosynthetic microorganisms. Among plant tissues, those with a yellow-orange color (since these factors are directly responsible for this type of color) and leafy ones are particularly rich.
Vitamin A comes in three different forms: alcoholic, aldehydic and acidic. They are isoprene derivatives, made up of the union of 4 isoprene chains.
The source of the vitamin A: foods of animal origin
Foods of animal origin contain mainly retinol and its esters, while in vegetables mainly carotenoids are found.
Before being absorbed, retinol esters are hydrolyzed by various enzymes. Lipase, carboxylester lipase of pancreatic origin and a retinylester hydrolase found on enterocyte membranes. The absorption of retinol occurs through a process of facilitated diffusion but, if the concentration of retinol is high, passive diffusion mechanisms may also intervene.
The absorption of retinol depends on the presence of lipids and bile salts. Vitamin A has numerous biological functions in the body which mainly concern vision and cellular differentiation. Retinaldehyde, which can also be formed from retinol via an alcohol dehydrogenase, is part of the mechanism of vision.
Retinaldehyde in the 11-cis form, in fact, is joined to a retinal protein, opsin, via an imine covalent bond with lysine residue number 296, forming rhodopsin. When a photon hits the rhodopsin, the retinal isomerizes into the all-trans form and this determines a conformational change of the rhodopsin and activation of a molecular cascade mediated by a G protein which determines the generation of electrical impulses.
The all-trans retinal detaches from the opsin and is reduced, by a NADPH-dependent dehydrogenase, to all-trans retinol which is then reuptake by the retinal epithelium and stored, after being esterased. All-trans retinol is subsequently converted back to 11-cis retinaldehyde via oxidation and isomerization reactions.