Vitamin A: Essential, Useful, And Potentially Harmful

Vitamin A is most often associated with vision, but in the body, it performs a much broader range of functions. It is necessary for maintaining mucous membranes, skin, and the normal functioning of the immune system. How effectively the body recovers from damage and copes with infectious load depends on it.

The role of vitamin A becomes especially apparent during infections. During illness, its level in the body may decrease because it is actively used for the restoration of mucous membranes and the support of immune defense. According to the World Health Organization, vitamin A deficiency is associated with a more severe course of measles and an increased risk of complications. For this reason, WHO includes vitamin A in measles treatment protocols for patients with vitamin A deficiency.

At the same time, vitamin A cannot be considered unconditionally safe. It belongs to fat-soluble vitamins and is able to accumulate in the body. The range between a beneficial and an unsafe dose is relatively narrow. At high doses, vitamin A is associated with an increased risk of lung cancer in smokers and with teratogenic effects when used in early pregnancy. Therefore, vitamin A is not suitable for “just in case” use and requires a cautious and well-grounded approach.

Retinol and beta-carotene: a key difference within dietary forms

Although retinol and beta-carotene are often grouped under the general name “vitamin A,” for the body they are fundamentally different substances.

• Retinol is ready-made vitamin A. The body can use it directly, without additional conversions.
• Beta-carotene is a precursor from which vitamin A must still be synthesized.

In foods and supplements, retinol is usually present in the form of esters, most commonly retinyl palmitate and retinyl acetate. From the body’s perspective, retinol and its esters belong to the same functional category: ready-made vitamin A that does not require additional activation steps.

It is precisely retinol and its esters that are used in clinical protocols and international guidelines, including those of the World Health Organization, because they provide a predictable supply of active vitamin A.

Beta-carotene, by contrast, comes exclusively from plant sources and is not vitamin A by itself. Its conversion into retinol requires several enzymatic steps. The efficiency of this process varies significantly between individuals and may be reduced due to genetic factors as well as disorders of intestinal or liver function.

Under these conditions, beta-carotene may accumulate in tissues as a pigment while providing little active vitamin A to the body. As a result, a person may regularly consume foods rich in carotenoids and still show signs of vitamin A deficiency-especially during periods of increased demand, such as during infections, chronic inflammation of mucous membranes, or recovery after illness.

Thus, retinol (including its esters) and beta-carotene are not interchangeable forms. Retinol provides a direct and reliable supply of vitamin A, whereas beta-carotene remains only a potential source, the effectiveness of which depends on individual characteristics of the body.

Food sources of vitamin A

Vitamin A enters the body in different forms, and this fundamentally affects its use and clinical significance. Under the general term “vitamin A,” there are both ready-made forms that the body can use directly and precursors that require conversion.

Part of vitamin A is consumed in the form of retinol and its esters, which are found in animal-derived foods. These forms do not require metabolic conversion and can be used directly by tissues or stored in the liver for later use as needed. The main dietary sources of ready-made vitamin A are:

• Animal liver (beef, veal, chicken);
• Fish oil;
• Fatty fish;
• Egg yolk;
• Whole dairy products with preserved fat content.

These sources provide the most predictable supply of active vitamin A; however, they require moderation, since retinol is able to accumulate in the body.

Another portion of vitamin A is obtained in the form of provitamin A-carotenoids-which are found in plant foods. These compounds are not vitamin A by themselves and must be converted into retinol within the body. The main sources of carotenoids are:

• Carrots;
• Pumpkin;
• Sweet potato;
• Red and orange sweet peppers;
• Apricots;
• Mango;
• Kale and other dark green leafy vegetables.

The efficiency of converting carotenoids into active vitamin A varies significantly among individuals and depends on intestinal health, liver function and bile secretion, overall nutrition, and individual metabolic characteristics. For this reason, a diet that is formally rich in vegetables and fruits does not always ensure adequate retinol intake, especially during periods of increased demand.

With excessive carotenoid intake, the skin may acquire a yellowish or orange tint due to their accumulation in subcutaneous fat tissue. This condition is not associated with liver damage and does not reflect sufficient active vitamin A status, since carotenoids can accumulate as pigments without being converted into retinol.

Thus, sources of vitamin A are not equivalent. Animal products provide a direct and reliable supply of active vitamin A but require dose control, whereas plant sources are safer in terms of overdose risk but do not always meet physiological needs.

Medicinal forms of vitamin A (retinoids)

Medicinal forms of vitamin A belong to the same family of compounds as endogenous vitamin A in the body, because they act on the same retinoid receptors. However, they are used not as part of nutrition, but as pharmaceutical agents intended for pharmacological effects.

• First-generation retinoids (tretinoin / all-trans retinoic acid, isotretinoin / 13-cis-retinoic acid) act on nuclear retinoid receptors, influencing processes of cell maturation and differentiation; they are used in acute promyelocytic leukemia, neuroblastoma, as well as in severe forms of acne with impaired control of epithelial turnover.
• Second-generation retinoids (acitretin, etretinate) are used in skin diseases with pronounced disorders of keratinization and cell proliferation, such as psoriasis.
• Third-generation retinoids (adapalene, tazarotene) have more selective effects, reducing pathological proliferation and inflammation; they are used mainly for acne, photodamaged skin, and localized disorders of epithelial renewal.
• Oral retinoids (isotretinoin, acitretin, tretinoin) exert systemic effects on the expression of genes that control cell growth, differentiation, and apoptosis; they are used in severe dermatologic diseases and certain oncologic and oncohematologic conditions.
• Topical retinoids (tretinoin, adapalene, tazarotene) act locally, normalizing the epithelial cell renewal cycle and reducing hyperkeratosis; they are used for acne, hyperkeratosis, and photoaging of the skin.

Main functions of vitamin A in the body

After distinguishing forms and mechanisms of action, it is useful to return to the basic question: why the body needs vitamin A at all and what functions it performs under normal physiological conditions. These effects underlie both its role in nutrition and its clinical use.

• Vitamin A is essential for normal functioning of the visual system. In the form of retinal, it participates in the formation of retinal visual pigments and ensures adaptation of vision to low light conditions.
• Vitamin A participates in the functioning of the immune system. It influences the maturation and function of immune cells and the balance between protective responses and excessive inflammation, which determines the body’s ability to cope with infectious load.
• Vitamin A is necessary for the renewal and maintenance of the integrity of epithelial tissues. It participates in the regulation of cells of the skin and the mucous membranes of the respiratory tract, intestine, and genitourinary system. Through this mechanism, it supports the barrier function of mucous membranes and reduces their vulnerability to damage and infection.
• In processes of growth and reproduction, vitamin A participates in the regulation of tissue development. It is necessary for normal organ formation, maintenance of reproductive function, and proper embryonic development, which is why its balance is especially sensitive during periods of growth and hormonal changes.
• Vitamin A influences the activity of genes that determine what cells become and how they perform their functions. This effect underlies both its physiological actions in the body and the medical use of vitamin A derivatives.

Vitamin A deficiency and possible causes

Vitamin A deficiency can manifest in different ways, ranging from pronounced symptoms to hidden forms that remain unnoticed for a long time. Even in the absence of obvious complaints, it can significantly affect the condition of mucous membranes, skin, and immune system function.

• In pronounced deficiency, one of the earliest signs is impaired adaptation of vision to darkness, including worsening night vision, followed by a decline in overall visual quality.
• Later, dryness of the skin and mucous membranes, increased susceptibility to infections, and delayed tissue recovery may appear.
• On the skin, a characteristic keratotic rash often develops: areas of dry, rough skin with disordered keratinization, most commonly on the arms, thighs, and buttocks.
• Subclinical deficiency may manifest as frequent infections, prolonged inflammation of the respiratory or intestinal mucous membranes, dry skin, a tendency toward keratosis, and impaired recovery after illness.

What can cause vitamin A deficiency

• Importantly, vitamin A deficiency can develop even with externally normal or formally sufficient nutrition, especially during viral infections and in the presence of chronic inflammation of mucous membranes.
• Another common cause is impaired digestion and absorption of fats, since vitamin A is a fat-soluble vitamin.
• Finally, deficiency may be related to insufficient intake of substances necessary for vitamin A metabolism in the context of increased physiological demand.

Metabolic interactions: not only vitamin A

The function of vitamin A in the body is a team process. Its effectiveness depends on the presence of other substances:

• Zinc: Without it, the transformation and transport of vitamin A are not possible. If a person has zinc deficiency, blood retinol levels will be low, even if liver stores are large.
• Vitamin E: In moderate doses, it protects vitamin A from degradation in the intestine and tissues. However, excess vitamin E may, on the contrary, interfere with vitamin A absorption.
• Iron: Vitamin A participates in the mobilization of iron from storage. Deficiencies of these nutrients often coexist, and correction of only one of them may be less effective.

When to check vitamin A levels

• Vision: Difficulty driving or walking at dusk (“night blindness”), persistent sensation of eye dryness.
• Skin: “Goosebumps” (small dry bumps) on the elbows, thighs, and buttocks; cracks and very slow wound healing.
• Immunity: Endless colds and prolonged bronchitis, when “one infection follows another.”
• Gastrointestinal tract: Confirmed gallbladder or pancreatic problems (risk of impaired fat absorption).

Vitamin A testing

Assessment of vitamin A status in clinical practice is performed by measuring serum retinol levels. This is a standard laboratory marker used in medicine and international guidelines.

It should be taken into account that blood retinol levels are regulated quite tightly by the body. With moderate or gradually developing vitamin A insufficiency, the value may remain within reference ranges. A more pronounced decrease in retinol levels usually reflects an already significant deficiency.

During acute infections and inflammatory conditions, serum retinol levels may temporarily decrease. In such situations, the test reflects the body’s state during illness and does not always correspond to baseline vitamin A status outside the infectious process.

Recent supplement use also affects test results. Taking vitamin A shortly before testing may temporarily increase blood retinol levels and mask potential abnormalities. For this reason, for a more accurate assessment, it is usually recommended to discontinue vitamin A supplements for some time before testing, so that the result reflects a stable level rather than short-term intake.

Thus, vitamin A testing is applied when there are clinical indications-symptoms, risk factors, or the need for safety monitoring-and is used as part of an overall health assessment rather than as a routine “just in case” screening.

Vitamin A and immunity

The relationship between vitamin A and immune protection is primarily determined by its effect on the condition of mucous membranes. The mucous membranes of the respiratory tract and the intestine are the first to come into contact with viruses, and the integrity of these barriers largely determines whether infection can penetrate. Vitamin A participates in epithelial renewal and supports the barrier function of mucous membranes, limiting the spread of the infectious process.

At the level of local immunity, vitamin A influences the maturation of immune cells in mucosal tissues and helps maintain a balance between protective immune responses and excessive inflammation. This is especially important in viral infections accompanied by epithelial damage and disrupted immune regulation.

There is also a reverse relationship: frequent infections themselves contribute to the depletion of vitamin A stores. During viral illnesses, vitamin A is actively used for immune responses and tissue regeneration. Recurrent or severe infections accelerate this process, creating a vicious cycle in which deficiency worsens the course of infection, and infection deepens the deficiency.

With vitamin A deficiency, infections generally run a more severe course: damage to mucous membranes becomes more pronounced, and tissue recovery is delayed. This effect is well described in the case of measles.

The World Health Organization emphasizes that vitamin A deficiency does not increase the risk of contracting measles, but is associated with more severe disease and a higher risk of complications. During measles, vitamin A levels can drop sharply even in people who had adequate nutrition before illness, because the demand for mucosal repair and immune support increases.

On this basis, WHO recommends vitamin A as part of measles treatment in patients with vitamin A deficiency, especially in regions where deficiency is more common and viral circulation is higher.

At the same time, WHO specifically states that vitamin A does not prevent measles infection and does not replace vaccination: its use is considered a measure to reduce disease severity, the risk of complications, and mortality, rather than a means of preventing infection.

Source: WHO. Measles - Fact sheet.

Vitamin A in multivitamins

Vitamin A is often included in multivitamin formulations, which creates the impression that it can be safely taken on a daily basis. Multivitamins can give a false sense of control, when it seems that one pill automatically covers all of the body’s needs. In the case of vitamin A, this approach often distracts from assessing actual necessity and individual context.

Vitamin A belongs to fat-soluble vitamins and is capable of accumulating in the body. With regular intake, especially when vitamin A is obtained from multiple sources at once, excessive load may develop without obvious early symptoms.

In people without deficiency, regular intake of vitamin A has not demonstrated consistent health benefits. In a number of large observational studies, long-term use of vitamin A supplements or antioxidant complexes was associated with less favorable health outcomes. This does not mean that vitamin A is harmful in itself, but indicates that it is not intended for continuous use without indications.

From the perspective of clinical logic, vitamin A is in many ways comparable to iron. Both substances may be necessary in the presence of deficiency, but in its absence, routine long-term intake is not an optimal strategy. In practice, vitamin A, like iron, is more often used in courses-based on indications and for a limited duration.

Vitamin A, smoking, and carcinogenicity

Smoking significantly alters the balance between benefit and risk when using vitamin A. This is not related to vitamin A itself, but to how its forms interact with tissues under conditions of constant exposure to tobacco smoke.

Large studies involving smokers have shown that supplementation with vitamin A, and especially beta-carotene, was associated with an increased risk of cancer, primarily lung cancer. These findings were obtained from observations of large cohorts of smokers and led to a reassessment of approaches to the use of vitamin A and carotenoids in this population.

The most unfavorable effects were associated specifically with beta-carotene. In the context of smoking, it may behave differently than under normal conditions: instead of the expected protective effect, it may enhance cellular damage. Tobacco smoke creates pronounced oxidative stress, under which beta-carotene may lose its antioxidant properties and form products with adverse biological effects.

It is assumed that smoking intensifies oxidative processes, alters carotenoid metabolism, and disrupts the regulation of cell growth. As a result, supplements that may be neutral under other conditions can act differently in smokers and increase the risk of unwanted effects.

It is important to emphasize that these data do not mean that vitamin A itself is carcinogenic. The increased risk is related specifically to smoking and, to the greatest extent, to the use of provitamin forms such as beta-carotene.

In smokers, as well as in those who have recently quit smoking, the approach to vitamin A should be particularly cautious and individualized. The form, dose, and appropriateness of use in these cases require separate evaluation.

Vitamin A, pregnancy, and childbirth

Pregnancy is a period during which the approach to vitamin A requires special attention. During this time, vitamin A performs important physiological functions; however, the boundary between benefit and risk becomes significantly narrower than under usual conditions.

Vitamin A is necessary for the normal formation of fetal tissues and mucous membranes, as well as for proper organ development. Low, physiological doses of vitamin A support these processes and belong to the basic needs of the body during pregnancy. This refers to covering the required minimum, not to therapeutic or preventive doses.

At the same time, excessive intake of vitamin A during pregnancy poses a risk. High doses of retinol and its esters are associated with an increased likelihood of fetal developmental abnormalities; therefore, their use during this period is considered dangerous.

Women of reproductive age also deserve special attention. Even in the absence of a confirmed pregnancy, the possibility of an early or unrecognized pregnancy remains; therefore, any decisions regarding the use of vitamin A in high doses in this group must be made with particular caution and after assessing potential risks.

During childbirth and in the postpartum period, vitamin A may participate in the recovery of mucous membranes and tissues. However, here as well its use should remain within physiological needs and individual assessment, without the use of increased doses “just in case.”

Thus, during pregnancy and throughout the reproductive age, vitamin A requires a balanced and careful approach: low physiological doses may be acceptable and even necessary, whereas high doses create a risk for the fetus.

Toxicity and overdose of vitamin A

Vitamin A is a fat-soluble vitamin and, when consumed in excess, is not rapidly eliminated from the body. When the capacity of its physiological regulation is exceeded, intoxication may develop, known as hypervitaminosis A.

Hypervitaminosis A is a condition in which the amount of vitamin A exceeds the body’s ability to use and control it safely. It may occur after a single intake of very high doses, due to dosing errors, or as a result of prolonged regular intake of vitamin A without taking into account the total load from food and supplements.

Acute toxicity develops when the dose is sharply exceeded. Its clinical manifestations include nausea, vomiting, severe headache, dizziness, irritability, and visual disturbances. A characteristic and well-described manifestation is increased intracranial pressure, known as pseudotumor cerebri or drug-induced intracranial hypertension. This condition is associated with impaired cerebrospinal fluid dynamics in the context of excess vitamin A and requires medical evaluation.

A typical sign of acute intoxication also includes pronounced skin manifestations-intense peeling and shedding of the skin (desquamation). These changes reflect the toxic effect of retinoids on the epithelium and are especially rapid and pronounced in children.

Chronic toxicity develops gradually with long-term excessive intake of vitamin A. It may be accompanied by dryness of the skin and mucous membranes, hair loss, brittle nails, bone and joint pain, changes in liver function, and a general deterioration in well-being. Such manifestations are often underestimated because they develop slowly and are not always immediately linked to supplement use.

A separate role in the development of toxicity is played by dosing errors, especially when using concentrated liquid forms. Confusion between drops and volume units, as well as between different types of droppers, can lead to multiple fold increasement of the intended dose and the development of acute intoxication even over a short period of time.

Thus, the risk of vitamin A toxicity is determined by dose, form, duration of use, and dosing accuracy. Vitamin A requires careful and conscious use and is not suitable for uncontrolled or technically inaccurate application, especially in pediatric practice.

Conclusion

Vitamin A is a biologically active compound involved in immune regulation, maintenance of mucosal integrity, and processes of growth and tissue repair. Its effects are not universal and always depend on the physiological and clinical context.

The relevance of vitamin A increases when the balance between the body’s needs and its actual supply is disrupted — in deficiency states, during infections, with mucosal damage, or in conditions of impaired fat absorption. Outside these situations, regular vitamin A supplementation has not shown consistent benefit and requires caution due to its ability to accumulate in the body.

Different forms of vitamin A are not equivalent. Retinol and its esters provide direct delivery of active vitamin A, whereas beta-carotene remains a precursor with variable conversion efficiency. Pharmaceutical retinoids belong to a separate pharmacological category and should not be considered a form of nutritional vitamin A.

Smoking, pregnancy, and the reproductive period significantly affect the balance between potential benefit and risk, making an individualized approach essential in these groups.

Thus, vitamin A is best viewed not as a background daily supplement, but as an agent whose use is determined by specific conditions, form, and duration of application.