Vitamin A
Main Characteristics:
• It was named Retinol due to its
action on the retina of the eyes.
• It is fat-soluble, that is, it
is soluble in fatty media.
• It is considered an essential
growth factor.
• It can be found in nature in
two forms: preformed (retinol, retinal, retinoic acid) or as a provitamin
(carotenes).
• Trans retinol is the most
active form.
• It is absorbed in the
intestine.
• It is stored in the liver and
adipose tissue, so when there is a deficit it takes a long time to show
symptoms of deficiency and because they are deposited in tissue, they may
present hypervitaminosis.
• Of the absorbed retinol, 80 to
90% is deposited in the liver in the form of retinol ester. It is released
according to needs, undergoes hydrolysis and passes into the circulation as
free retinol and is then transported by a protein. Retinol is mobilized from
the liver by an enzyme dependent on zinc and proteins.
Main Functions and Mechanisms of
Action
• Acts on vision and the
integrity of photoreception: It is one of the components that form RHODOPSIN
and IODOPSIN – pigments essential to vision.
• Acts on growth, cell
differentiation, structural integrity of membranes and functional integrity of
epithelia: In the presence of Vitamin A, basal cells differentiate, forming a
thick epithelium; in the absence of Vitamin A, the cells do not differentiate
and quickly keratinize and begin to desquamate.
• Acts on reproduction and
participates in the differentiation of reproductive cells/spermatogenesis:
Controls protein synthesis and cell mitosis.
• Acts on the growth and
development of the skeleton and soft tissues: Vitamin A stimulates osteoclasts,
stimulating bone renewal, so that when it is deficient, fibroblasts
predominate, which can lead to bone deformation.
• Acts on teeth: contributes to
the formation of enamel.
• Acts as a barrier against
infection: retinoic acid is necessary for the differentiation of basal cells
into mucus-secreting epithelial cells and also participates in the synthesis of
T lymphocytes.
• Acts in the gastrointestinal
system: it is important for the production of mucus in the gastrointestinal
wall.
• Prevents the development of
bladder, breast, stomach and skin tumors (animal study) – anticancer role.
• Has antioxidant action: it acts
by promoting the removal of free radicals.
Bioavailability
• Under normal conditions,
approximately 70 to 90% of dietary retinol is absorbed and, even in high doses,
this absorption remains high. However, preformed vitamin A, as well as
carotenoids, are fat-soluble substances and, therefore, depend on the concomitant
ingestion of lipids to be adequately absorbed.
• Free retinol, in physiological
concentrations, is absorbed via facilitated diffusion by a carrier that has not
yet been identified, but in pharmacological concentrations, it is absorbed by
simple diffusion.
• Carotenoid absorption is
similar to that of lipids. Therefore, the second step in the absorption process
and, consequently, the bioavailability of carotenoids, involves the
incorporation and release of these together with micelles. In turn, the formation
of these micelles is dependent on the presence of lipids in the intestine.
• Margarines enriched with
sterols and supplementation with dietary pectin also reduce the absorption of
beta carotene. Dietary fiber present in vegetables is another factor that
interferes with the bioavailability of carotenoids and vitamin A, due to its
interaction with bile salts and lipids, resulting in greater fecal excretion of
bile acids, thus reducing the absorption of lipids and fat-soluble substances
such as carotenoids and cholesterol.
• Ethanol consumption results in
the depletion of hepatic vitamin A in animals and humans. Considering that both
retinol and ethanol are alcohols, there is a potential for overlapping
metabolic pathways of these two compounds and competition for similar enzymes.
Possible interactions
• Drugs such as cholestyramine,
colestipol, neomycin and mineral oil that sequester bile acid, may reduce the
absorption of vitamin A.
• High doses of vitamin A
(greater than 10,000 IU) have been shown to increase the effectiveness of
anticoagulants.
• There is evidence from some
scientific studies that medications that reduce fat absorption also reduce the
absorption of the vitamin, since it depends on lipids to be absorbed.
Interaction between Nutrients
• Iron: There is a suggestion
that vitamin A deficiency impairs the mobilization of iron from stores and that
vitamin A supplementation increases the concentration of hemoglobin.
• Zinc: Zinc is essential in the
synthesis of RBP, an enzyme that mobilizes vitamin A from the liver into the
circulation. Zinc can also influence the conversion of beta carotene into
vitamin A through retinal reductase, another enzyme dependent on this mineral.
Another important issue observed is that zinc participates in the synthesis of
rhodopsin, a pigment essential to vision.
Scientific evidence regarding the
effectiveness of supplementation
• Some studies have demonstrated
advantages in relation to vitamin A supplementation, especially in children
with diarrhea, measles, and respiratory infections. Other studies have also
pointed to a protective role of vitamin A supplementation in relation to
mortality in children with AIDS.
• Vitamin A supplementation in
children with goiter, due to vitamin A and iodine deficiency, has been observed
to increase serum retinol levels and reduce excess TSH, reducing the risk of
goiter and its consequences.
• During pregnancy, vitamin A
supplementation is indicated to increase vitamin levels in breast milk and in
newborns, and thus reduce mortality by 22%.
• Supplementation of vitamin A
and other antioxidants in individuals with cancer has been shown to reduce the
risk of mortality, especially from colorectal cancer, and tumor response, and
reduce toxicity during chemotherapy.
Risks of overdose
• It has been shown that there is
a catabolic mechanism that allows the excretion of excess retinol. However,
with very high intakes, this system becomes saturated, which may be one of the
aspects of retinol toxicity, since there is no longer capacity for its
catabolism and excretion. Retinyl esters stored in the stellate cells of the
liver are slowly released into the parenchyma cells for catabolism, and retinol
has a considerable chronic toxic effect.
Genetic factors
Beta-carotene is found in orange,
red, green or purple vegetables. It is a provitamin, which means that it needs
to be activated by the BCMO1 gene, whose function is to convert beta-carotene
into retinal, and in turn, into retinoic acid. Some people have genetic
variants that can reduce this conversion by more than 50%. A tan where your
skin turns orange could indicate a poor conversion, which prevents your ability
to convert beta carotene into vitamin A. Vegetarians and vegans with these
genetic variants in BCMO1 should talk to their doctor or dietitian to make sure
their vitamin A levels are not too low.
Food sources
Foods (100g) Amount of vitamin A
(retinol)
Grilled beef liver 10,700 mcg
Brie cheese 329 mcg
Butter 565 mcg
Ricotta cheese 120 mcg
Boiled egg 149 mcg
Whole milk 56 mcg
Cod liver oil 30,000 mcg
Foods rich in beta-carotene (100
g) Beta-carotene (mcg)
Acerola 2,600
Tommy mango 1,400
Melon 2,200
Watermelon 470
Formosa papaya 610
Peach 330
Guava 420
Passion fruit 610
Broccoli 1,600
Pumpkin 2,200
Carrot 2,900
Sweet cabbage 3800
Tomato juice 540
Tomato paste 1100
Spinach 2400
Sweet potato 11508
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