Vitamin A

Vitamin A is a fat-soluble vitamin that plays a vital role in the body. It occurs naturally in food and can also be taken in the form of supplements. Although vitamin A is often thought of as a single nutrient, the name actually stands for a group of fat-soluble compounds that include retinol, retinal and retinyl esters. Two forms of vitamin A are found in the diet. "Finished" vitamin A - retinol and retinyl esters - are found exclusively in animal products such as dairy products, liver and fish, while provitamin A catotenoids are found in plant foods such as fruits, vegetables and oils (1). Only different carotenoids are found in plants. They have a provitamin character and can be partially converted to vitamin A by humans. How well the carotenoids can be converted into vitamin A depends on their structure. Beta-carotene has the most favorable structure. It can be converted into two vitamin A molecules under certain conditions.

To use these forms of vitamin A, the body must first convert them into retinal and retinoic acid - the active forms of the vitamin. Because vitamin A is a fat-soluble vitamin, it can be stored in body tissues for later use. Most vitamin A in this case is stored in the liver in the form of retinyl esters (2). These esters are broken down into trans-retinol, which binds to retinol binding protein (RBP). In this bound form, vitamin A subsequently enters the bloodstream for use by the body (3). The structural nature of vitamin A and carotenoids makes these substances very sensitive to light, oxygen and acids. As a result, they lose their biological effectiveness. If food is stored and prepared incorrectly, this can lead to the bioavailability of vitamin A and carotenoids being reduced by up to half. Long cooking times and unrefrigerated, light storage should be avoided if possible.

Using carrots as an example, it can be shown that the bioavailability of these substances can certainly be improved using simple kitchen techniques. In fresh carrot juice, the cell walls of the root (which contain carotene), which are otherwise difficult to digest, are broken down. Thus, the carotenoids are "exposed" and are more available to the digestive system. Raw, uncrushed cells are for the most part excreted again and with them the carotenes and other biochemicals of the cell. The same applies to blanching. The cell walls are made more easily digestible by the brief application of heat, thus increasing bioavailability. Tip: Fat ingested with food further increases the absorption capacity of carotenoids. This is due to their fat-soluble property.

Metabolism

Carotenoids and vitamin A are present in the diet mainly in a specific form of fats (esters). As a result, their absorption is closely linked to fat metabolism. They are broken down by bile acid and enzymes (esterases). Thus "broken down", they can be absorbed by the epithelial cells (skin cells) of the small intestine. In the skin cells they are converted into retinol and from there transported to the liver. The liver serves as a storage organ for vitamin A. When needed, the liver releases retinol into the blood and "sends" it to the target cells.

However, the transport of retinol is only possible if it is bound to a specific protein synthesized (formed) in the liver. The protein is called RBP (retinol-forming protein). Binding to RBP is necessary for retinol uptake in cells for two reasons. First, cells recognize the vitamin-RBP complex. This "recognition" enables the uptake of retinol into the cells. Second, this complex prevents uncontrolled uptake of the vitamin and thus toxic or undesirable effects in the cells. Retinol is excreted via the urine, bile and intestines.

Functions of vitamin A

Vitamin A is essential for health, aiding cell growth, immune function, fetal development and vision. Perhaps the best known functions of vitamin A include its role in vision and eye health. Retinal, the active form of vitamin A, combines with the protein opsin to form rhodopsin-a molecule necessary for color vision and low-light vision (13). It also helps maintain the cornea of the eye - the outermost layer of the eye - and the conjunctiva - the thin membrane that covers the surface of the eye and the inside of the eyelids (14). In addition to this, vitamin A helps maintain surface tissues such as the skin, intestinal mucosa, lungs, bladder, and inner ear. It also supports immune function by assisting in the growth and distribution of T cells - a type of white blood cell that protects the body from infection (15). Furthermore, vitamin A supports healthy skin cells, the male and female reproductive systems, and fetal development (16).

It has not yet been fully clarified for which tasks retinol is responsible in detail. In addition to the functions already mentioned, it is also involved in the following processes:

• Growth and formation of skin cells
• Spermatogenesis
• Placenta development
• Testosterone production
• Promotes the growth of bones and teeth
• healthy skin and hair

Potential health benefits of vitamin A

Vitamin A is important for eye health and good vision

Vitamin A protects the eyes from night blindness and age-related vision loss Vitamin A is essential for maintaining vision. This vitamin is needed to convert light that enters the eye into electrical signals that can be sent to the brain. In fact, a condition called night blindness may be one of the first symptoms of vitamin A deficiency (18). Night blindness occurs in people with vitamin A deficiency because this vitamin is a primary component of the pigment rhodopsin. Rhodopsin is found in the retina of the eye and is extremely sensitive to light. People who suffer from night blindness can continue to see normally during the day, but exhibit reduced vision in the dark as the eyes struggle to perceive light at low brightness. In addition to preventing night blindness, beta-carotene, when consumed in sufficient amounts, may help reduce the decline in vision that occurs in some people as they age (18).

Age-related macular degeneration is the leading cause of blindness in the Western world. Although the exact cause is unknown, this disease is believed to be the result of cellular damage in the retinal area that can be attributed to oxidative stress (19). A study looking at age-related eye disease found that an antioxidant supplement (including beta-carotene) may help reduce the risk of developing advanced macular degeneration by 25% in people over 50 with vision degeneration (20). However, a recent study review concluded that beta-carotene supplements alone cannot prevent or slow vision decline caused by age-related macular degeneration (21).

Summary: Consumption of adequate amounts of vitamin A may prevent the development of night blindness and help slow age-related decline in vision.

Vitamin A is a powerful antioxidant

Provitamin A Carotenoids such as beta-carotene, alpha-carotene and beta-cryptoxanthin are precursors of vitamin A and possess antioxidant properties. Carotenoids fight free radicals - highly reactive molecules that can damage the body by generating oxidative stress (22). Oxidative stress has been linked to numerous chronic diseases such as diabetes, cancer, heart disease, and cognitive decline (23). A diet rich in carotenoids is associated with a reduced risk of many of these diseases including heart disease, lung cancer, and diabetes (24, 25, 26).

Abstract: Carotenoids can protect the body from oxidative stress and the development of chronic diseases.

Vitamin A supports a healthy immune system

Vitamin A plays a crucial role in maintaining the body's natural defenses. This includes the mucosal barriers of the eyes, lungs, digestive tract and genitals, which help keep bacteria and other infectious elements out. It is also involved in the production and function of white blood cells, which help remove bacteria and other pathogens from the bloodstream. Conversely, this means that vitamin A deficiency may increase susceptibility to infection and, in the case of illness, may also slow down the healing process (27, 28). Indeed, it has been shown that correcting vitamin A deficiency in children in countries where infectious diseases such as measles and malaria are prevalent can reduce the risk of dying from these diseases (29).

Summary: Adequate dietary vitamin A intake helps keep the immune system healthy and fully functional.

Vitamin A can reduce the risk of acne

Acne is a chronic, inflammatory skin disease. People who suffer from acne develop painful pimples on the face, back and chest. These pimples develop when the sebaceous glands of the skin become clogged with dead skin particles and oil. The sebaceous glands are located in the hair follicles of the skin and produce sebum - an oily, waxy substance that keeps the skin moist and waterproof. Even though these pimples are harmless, acne can possess serious mental health implications, leading to decreased self-confidence, anxiety, and depression (30). The exact role that vitamin A plays in the treatment of acne remains unclear (31).

However, vitamin A deficiency is thought to increase the risk of developing acne by causing overproduction of the protein keratin in hair follicles (32, 33). This would increase the risk for acne by making it harder to remove dead skin cells from hair follicles, which can lead to blockage of the sebaceous glands. Some vitamin A-based medications are available by prescription. Isotretinoin is an example of an oral retinoid that has been shown to be effective in treating severe acne. However, this drug may have serious side effects and therefore should be used only under medical supervision (34, 35)

Abstract: The exact role that vitamin A plays in the prevention and treatment of acne is unclear. Nevertheless, vitamin A-based products are widely used to treat acne.

Vitamin A supports bone health

The key nutrients for maintaining healthy bones as we age are protein, calcium, and vitamin D. However, consumption of adequate vitamin A is also necessary for bone growth and development, and deficiency of this vitamin has been associated with poor bone health (36). In fact, people with low blood levels of vitamin A have a higher risk of bone fractures than those with healthy vitamin A levels (36). In addition, a recent meta-analysis of observational studies found that people with the highest amounts of vitamin A in their diet had a 6% lower risk of fractures (36). However, low vitamin A levels may not be the only problem when it comes to bone health.

Some studies have found that people with high vitamin A intakes also have a higher risk of bone fractures (37). However, these results are based on observational studies that cannot determine cause and effect. This means that the link between vitamin A and bone health is not fully understood at this time and that further controlled studies are needed to confirm what has been observed in observational studies. It should be kept in mind that vitamin A status does not by itself determine the risk of fractures and that the availability of other key nutrients such as vitamin D also plays a role (38).

Summary: Consuming adequate amounts of vitamin A may help protect bones and reduce the risk of fractures, although the link between vitamin A and bone health is not fully understood.

Vitamin A promotes healthy growth and reproductive system

Vitamin A is important for maintaining a healthy reproductive system in both males and females, and for ensuring normal embryonic growth and development during pregnancy. Rat studies that have examined the importance of vitamin A in male reproduction have shown that vitamin A deficiency blocks sperm cell developments and leads to infertility (39, 40). Other studies conducted with animals suggest that vitamin A deficiency in female animals can negatively affect reproduction by reducing the quality of oocytes and interfering with oocyte implantation in the uterus (39).

In pregnant women, vitamin A is also involved in the growth and development of the major organs and structures of the unborn child, including the skeleton, nervous system, heart, kidneys, eyes, lungs, and pancreas. However, too much vitamin A during pregnancy can also be harmful to the growing baby and lead to birth defects (41, 42). For this reason, many health experts recommend that women avoid foods such as liver, which contain concentrated amounts of vitamin A, and supplements containing vitamin A during pregnancy.

Summary: Adequate amounts of vitamin A are essential for reproductive health and the health of the unborn baby during pregnancy.

Vitamin A could reduce the risk of certain types of cancer

Cancer occurs when abnormal cells begin to grow and divide in an uncontrolled manner. Because vitamin A plays an important role in cell growth and development, its influence on cancer risk and its role in preventing cancer are of interest to scientists (43, 44). In observational studies, consumption of higher amounts of vitamin A in the form of beta-carotene has been associated with a reduced risk of certain types of cancer including Hodgkin's disease, cervical cancer, lung cancer, and bladder cancer (45, 46, 47, 48). However, although high intakes of vitamin A in the form of plant foods have been associated with reduced cancer risk, a similar association has not been established with animal foods containing active forms of vitamin A (49, 50).

Even with vitamin A supplements, such a link has not yet been shown (51). In some studies, an increased risk of lung cancer was even observed in smokers taking beta-carotene supplements (52, 53, 54). At the present time, the link between vitamin A levels in the body and cancer risk is not fully understood. However, available data suggest that adequate vitamin A intake-particularly in the form of plant sources-is important for healthy cell division and may reduce the risk of certain cancers (55).

Abstract: Adequate intake of vitamin A from plant sources may reduce the risk of certain cancers including Hodgkin's disease, cervical cancer, lung cancer, and bladder cancer, although the exact relationship between vitamin A and cancer is not yet known.

Deficiency symptoms

Although vitamin A deficiency is rare in the Western world, such deficiency is common in developing countries because people in these countries have limited access to foods containing vitamin A or provitamin A carotenoids. Vitamin A deficiency can lead to a number of health problems. For example, according to WHO, vitamin A deficiency is the leading cause of preventable blindness in children worldwide. Vitamin A deficiency also increases the severity of infections and the risk of dying from infections such as measles or diarrhea (8, 9). In addition to this, vitamin A deficiency in pregnant women increases the risk of anemia and death and impairs fetal development by slowing growth and development (10). Less serious symptoms of vitamin A deficiency include problems such as hyperkeratosis and acne (11, 12).

As an early stage in retinol deficiency, changes in vision occur. This includes, for example, night blindness and sensitivity to light. Further symptoms are disturbances in the metabolism of the skin and mucous membranes. In the eye, this initially leads to drying of the conjunctiva and can later lead to cornification of the corneal cells and ultimately to blindness. In children and adolescents, retinol deficiency causes impaired growth and bone formation. Vitamin A deficiency can occur despite an adequate supply of retinol if the organism does not have enough RBP available to bind the vitamin to itself and transport it to the cells. However, this is only to be feared in vegans who do not consume sufficient proteins. Certain groups of people such as premature infants, people suffering from cystic fibrosis, pregnant or lactating women, and people in developing countries have an increased risk of vitamin A deficiency (7).

Hypervitaminosis

Poisoning (hypervitaminosis) can be caused by too high a vitamin A intake. The reason for this is the high storage capacity of the vitamin. However, this can only occur in individual cases with excessive consumption of animal products (liver). Retinol poisoning is not to be expected with a balanced diet.

In case of short-term and high overdose (one hundred times the daily requirement and more) during retinol deficiency treatment, the following symptoms may occur:

• Nausea
• Vomiting
• Loss of appetite
• Muscle coordination disorders
• Skin damage
• Drowsiness

Symptoms of chronic hypervitaminosis A appear after months and years of excessive retinol intake exceeding ten times the daily requirement. Symptoms of poisoning may additionally include the following:

• Visual disturbances
• Pain in bones and joints
• Hair loss
• Light sensitivity
• Headache
• Dry or itchy skin
• Liver damage and jaundice
• Delayed growth
• Confusion

The symptoms may disappear after the vitamin is discontinued. Hypervitaminosis A can occur only with the intake of preformed vitamin A. Carotenoids are regulated and adjusted to the body's needs when they are converted to retinol. Prolonged cooking, oxygen and light damage vitamin A. Therefore, foods containing vitamin A should always be stored unpeeled or wrapped and in the dark - preferably in the refrigerator. Cooking losses range from 10 to 30 percent. The consumption of liver during pregnancy is not recommended. (Retinol is probably teratogenic, i.e. damaging to the amniotic fluid) Consequence: miscarriages, malformations. Hypervitaminosis occurs with intake of more than 200 milligrams per day.

Food sources

There are a variety of food sources for vitamin A and provitamin A carotenoids. In plants, it is contained as the precursor beta-carotene, which is then further processed in the organism to form vitamin A. Beta-carotene is therefore also called provitamin A. Since beta-carotene is converted into retinol only when needed and, unlike retinol, is not toxic even in larger quantities, it should be preferred in dietary supplements over retinol itself. Retinol is found in dairy products, egg yolk and cod liver oil.Vitamin A is important for the growth, function and structure of skin and mucous membranes, as well as for the visual process, as it is a building block of the visual pigment. The utilization of this vitamin in the body can be disturbed by low-fat diets, liver damage and the use of estrogen preparations. Good sources of vitamin A are fish, fish liver oils (cod liver oil), liver, butter, egg yolks, milk and milk products. Beta-carotene is found in vegetables, for example carrots, spinach, broccoli and kale, as well as in some orange to red fruits such as oranges.

The aldehyde of vitamin A is retinal, which together with opsin forms the visual pigment rhodopsin. Deficiency of vitamin A leads to night blindness and keratinization of the photoreceptors of the eye. The body's ability to convert carotenoids such as beta-carotene into finished vitamin A depends on many factors, which include genetic predisposition, diet, general health, and any medication (4). For this reason, people who follow a plant-based diet - and especially vegans - should be careful to consume sufficient carotene-rich foods.

The foods with the highest content of finished vitamin A include:

• Egg yolk
• Beef liver
• Liver sausage
• Butter
• Fish oil
• Chicken liver
• Salmon
• Cheddar
• Mackerel
• Trout

Foods rich in provitamin A carotenoids such as beta-carotene include (5, 6):

• Sweet potatoes
• Edible pumpkin
• Carrots
• Cabbage
• Spinach
• Dandelion leaves
• Chard
• Red bell bell pepper
• Parsley

Demand

With preformed retinol, adult women need 0.8 and men 1 mg of retinol. The organism can directly incorporate vitamin A, which is ingested from animal food, into the metabolism. The structural nature of carotenoids suggests that two vitamins A are formed from ß-carotene and one from all other carotenoids. The conversion rate does not behave so optimally in practice. To meet the body's requirement of 1 mg of vitamin A, for example, 6 mg of ß-carotene or 12 mg of other carotenoids are needed.

Demand in sport

As retinol, one should take 5,000-10,000I.U. per day with a meal. As beta-carotene: 15-60mg per day.

Conclusion

Vitamin A is a fat-soluble nutrient that is critical for immune system function, eye health, the reproductive system and fetal development. Both deficiency and excessive intake can bring serious side effects, so it is important to stay within the recommended range of 700 to 900 mcg and not exceed the daily upper limit of 3,000 mcg.

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