Vitamin A: necessary for sight and not only

Vitamin A is important for the development and integrity of numerous tissues and organs. That’s why a deficiency can cause a wide range of symptoms.

Vitamin A is important for the development and integrity of numerous tissues and organs. That’s why a deficiency can cause a wide range of symptoms.

Vitamin A is a fat-soluble vitamin (which means that it dissolves well in fats), present in nature in various versions: as retinol or other similar compounds, called retinoids (all of animal origin), or in the form of carotenoids (of plant origin), which represent the precursors of active vitamin A and which, for this reason, are considered a sort of “provitamin”.

There are about 600 known carotenoids, but only about fifty of these can be transformed into vitamin A by the human body (in the intestine and liverand be used as such. Other carotenoids contained in foods are not transformed into vitamin A, but they are still very useful for the body thanks to their strong antioxidant power: the main ones are lycopene, zeaxanthin and lutein.

What vitamin A is used for

Vitamin A is essential for the mechanism of vision, but also for the growth and differentiation of cells and for the maintenance of good immune defenses.

Involvement in these processes makes vitamin A crucial for the development, regeneration and integrity of most tissues and organs in the body.

A strong deficiency of this element is associated, therefore, with a wide range of symptoms and disorders, stunted growth and an increased risk of infections.

In terms of visual function, vitamin A is an indispensable and irreplaceable primary element.

The photoreceptors (rods) present in the retina, in fact, can capture light only thanks to the presence of a photosensitive pigment, rhodopsin, consisting of a protein, opsin, and vitamin A. This central role also explains why one of the first symptoms of vitamin A deficiency is represented by the appearance of “night blindness”, i.e. a considerable difficulty in distinguishing images in poorly lit environments or at dusk.

If after birth and in the first years of life the intake of vitamin A is very low, the eyes can be permanently damaged, up to develop blindness.

In fact, vitamin A deficiency in neonatal and pediatric age is the main cause of acquired blindness, unfortunately still widespread in developing countries characterized by poor nutritional standards.

To avoid this severe biological damage, prevention programs have been launched in these countries for several years now, based on the administration of therapeutic doses of vitamin A, two or three times a year, to all children, from birth to seven years.

Food Sources of Vitamin A and Beta-Carotene

The only natural source of vitamin A (retinol) immediately usable by the human body is represented by foods of animal origin with a fair content of lipids, such as milkcheese, butter, eggs, liver (by far the food that is richest in it), fatty fish from the North Seas (salmon, mackerel, herring, etc.), eels and fish oils. , such as the famous cod liver oil, historically used as a natural supplement of vitamin A and D for fragile and inappetent children at risk of deficiency.

Foods of plant origin do not contain appreciable amounts of vitamin A, but they can provide plenty of precursor carotenoids. Overall, about fifty different carotenoids are known to be transformed into vitamin A, but it is above all beta-carotene that is present in significant quantities in food and can be used efficiently by the human body.

Like vitamin A, beta-carotene is absorbed in the small intestine, with the help of bile salts, but only provided that a certain amount of fat is also introduced simultaneously with food.

Within the intestinal mucosa, each beta-carotene molecule is broken down into two retinol molecules, which, as such or further converted and inserted into lipid vesicles called chylichromes, enter the blood and are transported to the liver, where they can undergo other transformations before being used or stored. From the liver, when required, retinol and beta-carotene are re-introduced into the blood bound to proteins (retinol binding protein, RBP) or lipoproteins respectively and transported to the tissue in which they are to be used.

The intestinal cleavage of beta-carotene and the process that follows does not occur in a systematic way, but only if it is really necessary.

The human body is, in fact, able to self-regulate intestinal absorption in relation to its needs and, as an adult, can build up reserves of vitamin A so substantial as to be enough to respond to physiological demands for one or two years, even without further intake through food or targeted supplementation.

If there is no immediate biological need for vitamin A, beta-carotene is stored in adipose tissue, giving it its typical yellowish color. On a practical level, this means that eating large quantities of foods rich in beta-carotene in order to protect vision or stimulate tanning is basically useless, because the body will always use only the quota provided by genetically and physiologically determined needs. This does not mean, on the other hand, that you can eat without problems plant foods rich in beta-carotene, healthy and beneficial for general health thanks to the presence of other vitamins, minerals and antioxidant compounds.

In this regard, it should be noted that, although the name can be misleading, beta-carotene is present in abundance not only in carrots, but also in many other foods of plant origin. To contain significant quantities are especially those of yellow-orange color, such as apricots, peaches, melons, papaya, mangoes, mandarins, oranges, peppers, tomatoes and pumpkins, and vegetables with leaves or other dark green parts, such as spinach, broccoli, chard, dandelion, parsley, basil, arugula and other salads.

Although it is always preferable to consume these foods raw, it should be considered that beta-carotene is one of the most stable and resistant natural compounds to cooking, just do not boil vegetables for too long. Therefore, steaming, grilling, pan or baked for vegetables, fish, eggs and liver is welcome, preferably with the addition of a small amount of a fatty but healthy seasoning, such as extra virgin olive oil, to facilitate the absorption of the active compound.

In contrast, beta-carotene is easily oxidized by oxygen in the air and light; moreover, it deteriorates rapidly over time. To be sure to take appreciable quantities with the diet, therefore, it is better to avoid blending, centrifuging, extracting, squeezing, chopping or storing fruit and vegetables for too long. If it is not really possible to consume fresh plant foods rich in beta-carotene and other vitamins, the advice is to freeze them, raw or cooked, in medium-sized pieces, depending on the type, practicality of use and individual preferences.

Recommended daily doses of vitamin A

By convention, vitamin A intake levels are expressed as RE (Retinol Equivalent), or, in English, RAE (Retinol Activity Equivalent): 1 RE corresponds to 1 μg of retinol, 2 μg of beta-carotene dissolved in oil, 12 μg of beta-carotene present in common foods and 24 μg of other carotenoids transformed into vitamin A present in food (α-carotene, γ-carotene, or β-cryptoxanthine).

Less frequently, the daily requirement and the vitamin A or beta-carotene content of foods or food supplements can also be expressed using International Units (IU) as a unit of measurement, where 1 IU corresponds to 0.3 μg retinol (vitamin A), 0.6 μg beta-carotene and 1.2 μg other carotenoids.

According to the indications of the Italian Society of Human Nutrition (SINU), the recommended intake levels of vitamin A vary according to age and sex (in adults).

Infants 6-12 months 450 μg
Children-adolescents 1-3 years 300 μg
4-6 years 350 μg
7-10 years 500 μg
11-14 years 600 μg
Males 15-17 years 700 μg
> 18 years 700 μg
Females 15-17 years 600 μg
> 18 years 600 μg

For women, the recommended intake increases slightly during pregnancy (by about 100 μg, reaching a total of 700 μg) due to the supply of vitamin A to be ensured to the fetus and about 400 μg (reaching a total of 1,000 μg) for the duration of breastfeeding.

It should be emphasized that if you follow a varied and balanced diet for the quality and quantity of food consumed it is almost impossible to experience significant deficiencies of vitamin A or beta-carotene.

In all cases where it is still necessary to reasonably increase the intake of these elements, in addition to including in the diet greater quantities of foods that are naturally rich in them, you can take multivitamin supplements that can be purchased in pharmacies, which contain adequate quantities and do not expose you to risks of overdose.

According to the most recent provisions of the Ministry of Health, in accordance with European regulations, the quantities per daily dose allowed in food supplements correspond to 1,200 μg (corresponding to 1,200 RE) of vitamin A and 7.5 mg (corresponding to 1,250 RE) of beta-carotene, as the only source.

Overdose and toxicity

Symptoms of acute toxicity resulting from excessive intake of vitamin A (overdose) occur for intakes equal to or greater than 300 mg of vitamin A that may cause nausea, vomiting, migraine, loss of coordination and visual disturbances.

More frequent are the cases of chronic toxicity, resulting from the prolonged intake of doses of vitamin A even not too high, but higher than the ability of the liver to store and eliminate it. In these cases, the typical symptoms include liver damage, loss of appetite, muscle pain, anemia, fatigue, hair loss and neurological disorders of various types (blurred vision, irritability, hyperactivity, headache, etc.).

Situations of chronic toxicity can arise either due to excessive consumption of foods rich in vitamin A (such as liver) or through the inappropriate use of vitamin supplements or, more easily, following prolonged pharmacological treatment with retinoids, used for example for the treatment of dermatological diseases such as acne or psoriasis.

Therapeutic doses of retinoids, if taken by pregnant women, are also very harmful to the fetus, so much so that in women of childbearing age their use must be associated with a reliable contraceptive method.

Joycelyn Elders is the author and creator of EmpowerEssence, a health and wellness blog. Elders is a respected public health advocate and pediatrician dedicated to promoting general health and well-being.

The blog covers a wide range of topics related to health and wellness, with articles organized into several categories.

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