Vitamin A is a collective name of retinoids, a number of related compounds with effects that are partly different. Presently, four retinoids are known: retinol, retinal, retinoic acid, and retinyl esters. Animal food is their only source. Under ideal conditions, however, it is possible for the body to convert certain carotenoids1 – whose source is vegetarian – to retinoids. This means that it is not necessary to eat animal food in order to get vitamin A. These carotenoids are sometimes called pro-vitamin A. Far from all carotenoids have pro-vitamin A capacity though, only four of them have shown to possess that quality: alfa- beta- and gamma-carotene, plus the xantophyll beta-cryptoxanthin.
For this conversion to take place, you must possess an almost perfect intestinal microflora, plus have an enzyme called 15-15'-dioxygenase. (Many mammals have this enzyme, but a few don't. Cats, for instance, are lacking it and cannot convert carotenoids to vitamin A. This is perfectly logical, cats are made for eating only animal food, they cannot digest anything else and should never eat anything else. They have no use for an ability to extract nutrients from the plant kingdom.) Some presence of a little dietary fat is also necessary, since both carotenoids and vitamin A are fat-soluble. Without some fat (as a solvent) in the intestines, neither conversion nor absorption will take place.
The use of drugs and certain medicines, the presence of toxins, a bad diet, as well as everything else that corrupts the intestinal microflora, might seriously disturb, or even block conversion.
Vitamin A in animal foodstuffs comes as a retinyl ester, mainly retinyl palmitate, which is converted to retinol in the intestinal area. Retinol functions as the basic form of vitamin A, and it is the form in which it is stored in the body. That's why vitamin A is sometimes described as retinol. Chemically, it is a fat-soluble aliphatic alcohol.
Retinal, the retinoid which is the result of conversion of carotenoids, is intraconvertible with retinol. This means that retinol can be converted to retinal, and retinal can be converted to retinol. Retinoic acid, however, is created from retinal in an oxidation process that is irreversible.
Based on the fact that retinol and retinal are intraconvertible, we are able to conclude that vitamin A from animal and vegetarian sources are of equal value for human needs, and that we don't need both. We can get all the vitamin A we need from either one.
If you rely on carotenoids as your source of vitamin A, it might be useful to know that beta-carotene per weight unit results in twice as much retinol equivalents as alfa-carotene, gamma-carotene, or beta-cryptoxanthin. (Retinol equivalents, or retinol activity equivalents, are units of measurement of the degree to which carotenoid-based pro-vitamin A can be converted to retinoid vitamin A.)
There is a widespread fear of vitamin A overdose. Indeed it can be dangerous, but the risk is small, the dose required for that is very large. Deficiency is a much more realistic risk.
Risk of overdose is eliminated by the use of carotenoids, since no more vitamin A than you need is produced. Carotenoid-based pro-vitamin A is not converted to retinal unless it is needed. But this source is unreliable, unless your intestinal environment is perfect. Some intake of genuine retinol is certainly desirable anyway.
Carotenoids are fat-soluble red-orange-yellow pigments from plants. They are important nutrients in their own right and have many qualities. Here I mention them only for their pro-vitamin A quality. Otherwise they are treated in "Not Only Beta-Carotene: Carotenoids (Carotenes & Xanthophylls) ".
Note that contrary to how they are sometimes described, carotenoids are not vegetable forms of vitamin A. Carotenoids are not to be classified as vitamin A at all! Four out of more than 500 known carotenoids have the capacity of being converted to retinoid vitamin A under certain circumstances, but that is just a little of what this family of substances offers in beneficial qualities.
It is a well-known fact that vitamin A activates thymus, thus improving immunity and detoxification.
Thymus is a gland in the chest, just above the heart. Until quite recently its function was unknown, or misunderstood. Today it is seen as a central part of the immune system, and the levels of thymosin (a hormone produced in thymus) is related to the speed of ageing.
This gland grows to its full size (about 30 g) during puberty, after that it tends to atrophy (shrink) rapidly.
Medical literature often describes the early atrophy as "normal", and in a statistical sense of the word it is; it happens to most people. But we know that certain nutrients can cause an increase of the gland's weight. There is absolutely no reason to believe that an early atrophy is "natural" or unavoidable. It can as well be some form of deficiency and degenerative disease.
Results of unpublished studies available to me indicate that very high dose vitamin A can possibly retard, stop, or even reverse thymus atrophy. This would be of importance for anti-ageing purposes, but more research is needed before anything can be said for certain.
Vitamin A strengthens the cells that cover the body's inner and outer surface. It is important for skin health; and for healthy mucous membranes, for whose continuous re-generation it is crucial. A deficiency leaves the membranes thin and weak.
The eyes need vitamin A, not at least for their ability to see in bad light. All sorts of activities which strain the eyes increase the need for vitamin A. That includes watching television or a computer screen.
As retinal, vitamin A is at the heart of vision, and it is not a coincidence that the retinoids share their word-stem with the retina of the eye. In Purple Light? It Doesn't Exist! And what is Visual Purple? I briefly outlined its role in connection with rhodopsin, visual purple.
As retinoic acid, vitamin A is important for gene transcription and bone metabolism.
Most of the effects of vitamin A are caused by retinoic acid, which is an oxidation product of retinal. There are exceptions, however; for instance eyes and vision, which need pure retinal; and fertility and reproduction, which need retinol, locally present in the body, converted to retinoic acid.
Finally, vitamin A is an anti-oxidant, albeit a relatively weak one. The carotenoids, however, are strong antioxidants. But that has nothing to do with their pro-vitamin A property.
The RDA is 4500-5000 I.U., double for pregnant or breast-feeding women.
As usual, this is the dose required to avoid symptoms of acute avitaminosis, but optimal health requires more. 10000 I.U. is a better level, and 20000-30000 I.U. is normally without danger - but do not double that in pregnancy or breast-feeding. If you are not sure, stay at 10000 I.U. That is for retinol, which can be toxic if the dose is too high (although normally that requires in excess of 100000 I.U. per day for a long time). If you use beta carotene, this risk is eliminated.
Early symptoms of overdose can be nausea, vomiting, headache, loss of appetite, hair loss, or nosebleed. If you stop taking vitamin A, the symptoms disappear.
A possible danger with high dose beta-carotene, in itself a strong anti-oxidant, has been indicated for heavy smokers and drinkers. The reasons are still insufficiently studied, but if you happen to be among those who abuse alcohol or tobacco, some caution is wise.
Avitaminosis is any illness or symptom caused by a deficiency of a vitamin. While chronic deficiency of vitamin A might give symptoms from a large number of systems and organs and contribute to the development of various diseases, the typical result of vitamin A deficiency is related to the eyes. There are two symptoms to be aware of:
One is nyctalopia, or night blindness, which means the ability to see in low light is reduced or lost.
The other is Xerophthalmia, which, if untreated, might progress to keratomalacia.
Xerophtalmia is when no tears are produced and the eye becomes very dry. This can lead to keratomalacia, which is a dry cornea. This condition eventually destroys the cornea and causes blindness.
Xerophtalmia is common in certain parts of the world, especially amongst children; it is a common cause of blindness.
As a prophylaxis, children should always have a vitamin A supplement. Breastfed children can start after 6 months, but those who are not breastfed need it sooner.
For retinol, the official daily maximum for children of 0-12 months of age is 600 micrograms per day, and it might be wise not to exceed that. Carrot juice, however, can be introduced in the diet early and become a risk-free vitamin A supplement, since the provitamin A of carrots will not convert to retinal unless the child needs and can use more vitamin A. But don't sweeten the juice, refined sugar would corrupt the intestinal microflora and harm the conversion.
Retinoid vitamin A occurs only in animal food.
Fish, especially the liver, is a particularly rich source. Sadly, fish liver is often so full of environmental toxins that it is dangerous to eat nowadays. Otherwise the liver of animals that live on fish is also very rich in vitamin A. I refer mainly to seals, and the richest source of all, polar bear liver, which is actually the only natural source that has ever caused a deadly overdose of vitamin A.
The liver of these animals is not within reach for most consumers, and they, too, are as full of environmental toxins as fish liver.
Fish, excluding the liver, still remains a major dietary source of vitamin A.
Another good source within reach in most parts of the world is egg yolk. How much vitamin A it consists depends on the diet of the birds producing the eggs.
Dairy products are a fairly good source, and in some parts of the world, synthetic vitamin A is added to milk. Some studies suggest that this synthetic vitamin A added to milk might actually cause a vitamin deficiency, but this is not sufficiently studied to make any results conclusive. Synthetic vitamin A otherwise seems to mimic natural vitamin A fairly well. I think, however, that it is wise to avoid the synthetic one unless you use therapeutic (very high) doses or live under conditions where natural sources cannot possibly cover your need.
Plenty of vegetarian pro-vitamin A occurs in, for instance, sweet potato (a very rich source of beta carotene), carrot, spinach, tomato, papaya, passion fruit, bell pepper, cayenne pepper, cantaloupe, apricot, celery, oranges, mango, plum, watermelon, Brussels sprouts, green peas and beans, squash, asparagus, leeks, lettuce, turnip greens, Swiss chard, broccoli, parsley, peppermint, and basil.
Generally, pro-vitamin A is present in most dark green fruits and vegetables, and in most yellow-orange and some red fruits.
Remember that in order to get an intestinal conversion of pro-vitamin A to retinal, and also to facilitate absorption, the presence of some amount of dietary fat is required.
(The article is based on material previously published in Meriondho Leo, and in my e-book “Nutrients & Dietary Supplements”, 2019.)
The Importance of Dietary Solvents
Not Only Beta-Carotene: Carotenoids (Carotenes & Xanthophylls)
The Body's Inner & Outer Surface (and its inhabitants)
Purple Light? It Doesn't Exist! And what is Visual Purple?
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