Vitamin A
Vitamin A is a fat-soluble vitamin that plays a crucial role in maintaining various physiological functions in the human body. It is essential for vision, immune system performance, cellular communication, and reproduction. Vitamin A is also vital for the proper functioning of the heart, lungs, kidneys, and other organs.
There are two primary forms of vitamin A: preformed vitamin A and provitamin A. Preformed vitamin A, also known as retinol, is found in animal products such as meat, fish, poultry, and dairy foods. Retinol is the active form of vitamin A that the body can readily use. Provitamin A, on the other hand, refers to carotenoids, which are plant-based compounds that the body can convert into retinol. The most well-known carotenoid is beta-carotene, which is abundant in colorful fruits and vegetables like carrots, sweet potatoes, and spinach.
Vitamin A is integral to maintaining healthy vision. It is a component of rhodopsin, a protein in the eyes that absorbs light in the retinal receptors. This function is crucial for low-light and color vision. A deficiency in vitamin A can lead to night blindness and, in severe cases, complete blindness. It can also cause xerophthalmia, a condition characterized by dryness of the conjunctiva and cornea.
Beyond vision, vitamin A is essential for immune function. It helps maintain the integrity and function of skin and mucosal cells, which serve as the body’s first line of defense against pathogens. Vitamin A also supports the production and activity of white blood cells, which are critical for fighting infections.
In terms of cellular communication, vitamin A is involved in the regulation of gene expression. Retinoic acid, a metabolite of vitamin A, acts as a hormone-like growth factor for epithelial cells and other cell types. This regulation is crucial for normal cell growth and differentiation, particularly during embryonic development.
Vitamin A’s role in reproduction includes supporting sperm production in males and maintaining healthy fetal development in females. Adequate levels of vitamin A are necessary for the proper formation of the heart, eyes, and ears during fetal growth.
The recommended dietary allowance (RDA) for vitamin A varies by age, sex, and life stage. For example, adult men are advised to consume 900 micrograms (mcg) of retinol activity equivalents (RAE) per day, while adult women should aim for 700 mcg RAE. Pregnant and lactating women have higher requirements due to the increased demands of fetal and infant development.
While vitamin A is essential for health, excessive intake can lead to toxicity, known as hypervitaminosis A. Symptoms of toxicity include nausea, headache, dizziness, and even more severe effects like liver damage and central nervous system disorders. Therefore, it is important to balance vitamin A intake, ensuring it is neither deficient nor excessive.
In summary, vitamin A is a vital nutrient with diverse roles in maintaining vision, immune function, cellular communication, and reproduction. Its presence in both animal and plant foods allows for varied dietary sources, making it accessible to different dietary preferences. However, careful attention to intake levels is necessary to avoid deficiency or toxicity.
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 Retinol (depicted) and retinal play a biological role in vision, but most of the effects of vitamin A are exerted by retinoic acid, which binds to nuclear receptors and regulates gene transcription. | |
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| AHFS/Drugs.com | Monograph |
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| Routes of administration | By mouth, intramuscular |
| Drug class | Vitamin |
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| ECHA InfoCard | 100.031.195 |
| Chemical and physical data | |
| Formula | C20H30O |
| Molar mass | 286.459 g·mol−1 |
| 3D model (JSmol) | |
| Melting point | 62–64 °C (144–147 °F) |
| Boiling point | 137–138 °C (279–280 °F) (10−6 mm Hg) |
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Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most notably β-carotene (beta-carotene). Vitamin A has multiple functions: growth during embryo development, maintaining the immune system, and healthy vision. For aiding vision specifically, it combines with the protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision.
Vitamin A occurs as two principal forms in foods: 1) retinoids, found in animal-sourced foods, either as retinol or bound to a fatty acid to become a retinyl ester, and 2) the carotenoids α-carotene (alpha-carotene), β-carotene, γ-carotene (gamma-carotene), and the xanthophyll beta-cryptoxanthin (all of which contain β-ionone rings) that function as provitamin A in herbivore and omnivore animals which possess the enzymes that cleave and convert provitamin carotenoids to retinol. Some carnivore species lack this enzyme. The other carotenoids do not have retinoid activity.
Dietary retinol is absorbed from the digestive tract via passive diffusion. Unlike retinol, β-carotene is taken up by enterocytes by the membrane transporter protein scavenger receptor B1 (SCARB1), which is upregulated in times of vitamin A deficiency (VAD). Retinol is stored in lipid droplets in the liver. A high capacity for long-term storage of retinol means that well-nourished humans can go months on a vitamin A-deficient diet, while maintaining blood levels in the normal range. Only when the liver stores are nearly depleted will signs and symptoms of deficiency show. Retinol is reversibly converted to retinal, then irreversibly to retinoic acid, which activates hundreds of genes.
Vitamin A deficiency is common in developing countries, especially in Sub-Saharan Africa and Southeast Asia. Deficiency can occur at any age but is most common in pre-school age children and pregnant women, the latter due to a need to transfer retinol to the fetus. Vitamin A deficiency is estimated to affect approximately one-third of children under the age of five around the world, resulting in hundreds of thousands of cases of blindness and deaths from childhood diseases because of immune system failure. Reversible night blindness is an early indicator of low vitamin A status. Plasma retinol is used as a biomarker to confirm vitamin A deficiency. Breast milk retinol can indicate a deficiency in nursing mothers. Neither of these measures indicates the status of liver reserves.
The European Union and various countries have set recommendations for dietary intake, and upper limits for safe intake. Vitamin A toxicity also referred to as hypervitaminosis A, occurs when there is too much vitamin A accumulating in the body. Symptoms may include nervous system effects, liver abnormalities, fatigue, muscle weakness, bone and skin changes, and others. The adverse effects of both acute and chronic toxicity are reversed after consumption of high dose supplements is stopped.