Key Takeaways (expand)
- Rather than being a single compound, vitamin A is a group of fat-soluble retinoids (namely retinal, retinol, and retinyl esters) that have vitamin A functions in the body.
- The term “vitamin A” is often used interchangeably with beta-carotene, but the latter is actually a vitamin A precursor; it must undergo a (very inefficient) conversion into retinoids before exhibiting vitamin A activity.
- Vitamin A helps regulate the expression of over 500 genes—including those needed for hormones, growth factors, cytokines, lipid metabolism, and cell growth and differentiation (knowing what type of cell to become as it matures).
- Vitamin A is particularly important for immune function, including by helping immune cells proliferate, migrate, and differentiate.
- Low levels of vitamin A in the body are associated with higher risk of infectious disease, respiratory complications, measles-related mortality in children, flu, and autoimmune disease.
- Vitamin A is an essential player in vision and eye health: it’s a precursor for and component of rhodopsin, a protein in specialized photoreceptor cells in the eyes (called “rods”) that’s needed for seeing in low-light conditions.
- Vitamin A plays a similar role for another photoreceptor cell in the eye (“cones”), which are needed for color vision.
- Vitamin A is also needed for the eye to be structurally healthy, due to supporting the proper functioning of the eye’s conjunctival membranes and cornea.
- Vitamin A has been shown to help protect against age-related macular degeneration and retinitis pigmentosa (an inherited genetic disorder that causes the progressive loss of photoreceptor cells in the retina).
- Vitamin A is needed for maintaining normal thyroid function, and its deficiency can cause the thyroid gland to increase in size, raise circulating levels of thyroid hormones, reduce the thyroid’s iodine uptake, and increase the secretion and synthesis of TSH.
- Vitamin A is needed for healthy reproductive function, prenatal development, and postnatal development.
- Female reproductive cycles require adequate amounts of vitamin A in the body; likewise, sperm production and male genital tract health depend on vitamin A.
- During fetal development, vitamin A is vital for organ development (including the heart, lungs, ears, and eyes).
- During childhood, vitamin A is needed for growth and survival, and helps protect against childhood mortality.
- The skin also needs vitamin A to be healthy: this nutrient helps epithelial cells maintain their integrity and can regulate the production of sebum (skin oil).
- As a fat-soluble vitamin, excess vitamin A poses toxicity risks—in particular, a condition called hypervitaminosis A, which can cause nausea, dizziness, skin problems, appetite loss, cerebral edema, and eventually liver damage or coma.
- Vitamin A toxicity is typically only seen in cases of extreme supplementation or chronically high intake of vitamin-A rich foods like liver.
- High levels of vitamin A can interfere with vitamin D’s ability to maintain calcium homeostasis, and can subsequently increase the risk of bone fracture.
- During pregnancy, excess intake of preformed vitamin A (especially as supplements) can raise the risk of birth defects.
- Vitamin A deficiency can occur due to low dietary intake and/or decreased absorption by the body, and includes symptoms like loss of night vision, increased susceptibility to infections, a progressive eye disease called xeropthalmia, and eventually changes in the eye’s structure that produces blindness.
- The only sources of active vitamin A are animal products (particularly liver, cod liver oil, egg yolks, grass-fed high-fat dairy products, and seafood); meanwhile, vitamin A precursors are found in yellow, orange, and green vegetables and fruits.
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Vitamin A isn’t a single compound, but a group of fat-soluble retinoids that have vitamin A functions in the body—namely retinal, retinol, and retinyl esters. And while the term “vitamin A” is often used interchangeably with its precursor, beta-carotene, these are not the same thing! Carotenoids like beta-carotene are phytonutrients with antioxidant and anticancer properties, and they must be converted into retinoids by our bodies before exhibiting vitamin A activity—a process that’s extremely inefficient. (Plus, only about 10% of the many hundreds of carotenoids produced by plants are able to be converted to retinol; these particular phytonutrients are called provitamin A carotenoids.)
This nutrient has a long, complex scientific history, with no clear discovery date. The first evidence of its effects came in 1816, when a physiologist named François Magendie found that malnourished dogs developed corneal ulcers and early death. From the late 1800s to early 1900s, additional researchers determined that contrary to the prevailing belief that all fats were nutritionally similar, a special “unknown substance” or “accessory factor” existed in butter and egg yolks that supported growth and survival in a way that olive oil and lard did not. In 1918, this “accessory factor” was termed “fat-soluble vitamin A,” and then renamed simply “vitamin A” in 1920. It wasn’t until 1937 that vitamin A was officially isolated, and research continued well through the 1990s in attempt to understand its effects on immunity and childhood survival!
Vitamin A is absolutely essential for several physiological functions: vision, reproduction, immunity, and cellular communication. In fact, virtually every cell in our bodies requires it for growth!
The only food sources of retinoids are animal products, and the richest sources are liver, cod liver oil, egg yolks, grass-fed high-fat dairy products, and seafood (especially shrimp, salmon, sardines, and tuna). Vitamin A precursors are found in yellow, orange, and green vegetables and fruits, such as carrots, sweet potatoes, spinach, broccoli, cantaloupe, mangoes, winter squash, lettuce, tomatoes, apricots, and bell peppers. Importantly, while vitamin A precursors found in plant foods are often touted as great sources of vitamin A, the actual amount of vitamin A we can convert from plant-based precursors is highly variable and generally low.
The Biological Roles of Vitamin A
Vitamin A compounds are mainly stored in the liver as retinyl esters, which can then be hydrolyzed and bound with retinol binding protein (RBP) to enter the bloodstream as an all-trans-retinol/RBP complex. This complex, in turn, binds with a protein called transthyretin, which transports vitamin A (in the form of all-trans-retinol) to peripheral tissues throughout the body. Similarly, chylomicrons containing vitamin A (in the form of retinyl esters) can help deliver vitamin A to various tissues outside of the liver. These mechanisms allow vitamin A to go where it needs to in order to support the many functions it’s involved in!
Although dietary carotenoids can contribute to our available vitamin A pool, they must be converted to vitamin A in our bodies—which is highly variable and depends upon many factors such as food preparation, individual differences in digestion and absorption, and genetics. Typically, 3% or less of ingested carotenoids are actually absorbed, and the conversion rate can be very low in general: a conversion factor of 28:1 for beta-carotene to retinal has been found for some plant foods, including leafy greens.
One of vitamin A’s most important functions is supporting cell growth and differentiation (knowing what type of cell to become as it matures), which it does by helping regulate gene transcription. Multiple forms of vitamin A play a role here! For example, enzymes called retinol dehydrogenases can convert retinol into retinal, which can then be oxidized into retinoic acid, which acts as a ligand for certain nuclear receptors (known as retinoic acid receptors) that are bound to DNA. Via this process, vitamin A (in the form of retinoic acid) helps regulate the expression of over 500 genes! And, vitamin A in the form of retinol has a regulatory function for some genes: the membrane receptor/transporter STRA6 accepts retinol, which subsequently activates the JAK/STAT signaling pathway—which in turn regulates the expression of a number hormones, cytokines, and growth factors. Vitamin A as retinal, too, is specifically involved in regulating genes needed for lipid metabolism, and has also been found to inhibit the expression of gluconeogenic genes in rodent livers (more research is needed in humans).
Likewise, vitamin A (in the form of retinoic acid) is needed for regulating the differentiation, migration, and antigen-presenting capacity of dendritic cells (which are integral for the adaptive immune system), as well as for the differentiation of naïve CD4 T-lymphocytes into regulatory T-lymphocytes (important immune components that prevent autoimmunity, while also regulating immune response strength in order to protect the host from damage).
Vitamin A has some unique functions in maintaining vision and eye health: it’s a component of a protein called rhodopsin, which exists in specialized photoreceptor cells in the eyes (called “rods”) and absorbs light as it hits the retina (hence the chemical name retinal/retinol). Vitamin A in the form of all-trans-retinol accumulates in the pigmented cell layer of the retina (called the retinal pigment epithelium), where it becomes esterified and stored as retinyl esters. These retinyl esters then go through a series of conversions (hydrolysis, isomerization, and oxidation) to ultimately become 11-cis-retinal, which is the form of vitamin A needed to create rhodopsin. Rhodopsin converts light into an electrical signal and is essential for being able to see in low-light conditions; this is why vitamin A deficiency is famous for causing night blindness. Vitamin A plays a similar role for another type of photoreceptor cell in the eye (called “cones”), which are needed to absorb photons from visible light and allow us to see color. And, vitamin A supports the proper functioning of certain parts of the eye—conjunctival membranes and cornea, specifically. As a result, vitamin A is needed for the retina to function, for seeing in dim light conditions, for color vision, and for the eye to be structurally healthy!
Interactions with Other Nutrients
Vitamin A has some known interactions with other nutrients.
Research has shown that vitamin A deficiency may exacerbate iron deficiency anemia, due to causing alterations in iron metabolism. Likewise, animal studies suggest that iron deficiency could influence the levels of vitamin A present in the blood and liver.
Zinc deficiency, too, is believed to interfere with the metabolism of vitamin A, due to decreasing the synthesis of retinol-binding protein (which is needed for transporting retinol to peripheral tissues), decreasing the enzymatic release of retinol from retinyl palmitate (its storage form) in the liver, and reducing the activity of the enzyme that converts retinol into retinal.
Additionally, high levels of vitamin A can interfere with vitamin D’s ability to maintain calcium balance; some research has found that vitamin A increases the risk of bone fracture among women with the lowest intakes of vitamin D. There is also some evidence that high doses of vitamin A could decrease vitamin K absorption.
Vitamin A in Health and Disease
Through its various functions in the body, vitamin A helps reduce the risk of infection, protects against night blindness and age-related macular degeneration, reduces the risk of autoimmune disease, helps maintain normal thyroid function, allows us to see in low-light situations, supports healthy reproductive function, maintains skin health, and allows for normal growth and development from the fetal years through childhood.
Vitamin A and Immunity
Due to vitamin A’s necessity for helping immune cells proliferate and differentiate, this nutrient is essential for maintaining immune function. In fact, it was once termed “the anti-infective vitamin” due to having such a significant role in preventing infection! Studies have shown that low serum retinol, excess secretion of vitamin A in the urine, and depleted liver reserves of vitamin A are all associated with greater risk of infectious disease.
In children, vitamin A is particularly necessary for protecting against respiratory complications, diarrhea, and measles-related mortality; research in developing countries has shown that vitamin A supplementation can decrease the incidence and severity of measles complications. (The World Health Organization even recommends that measles-infected children who live in areas with prevalent vitamin A deficiency receive 200,000 IU of vitamin A for two days in a row as part of their treatment!)
Observational research has likewise linked plasma retinol levels to indicators of flu infection, and limited data suggests that vitamin A-deficient women with HIV are more likely to pass HIV onto their infants (though much more research is needed here, since controlled trials generally haven’t found a benefit of vitamin A supplementation during pregnancy or breastfeeding on HIV transmission). Not surprisingly, given its role regulating immunity, vitamin A (particularly as retinoic acid) has also been strongly linked to lower risk of autoimmune disease development.
Vitamin A and Eye Health
Because of its importance in maintaining vision health, vitamin A can benefit certain eye-related disorders.
For example, vitamin A can help protect against age-related macular degeneration—a condition where the eye’s macula becomes damaged, leading to blurred central vision.
And, one trial found that retinitis pigmentosa—an inherited genetic disorder that causes the progressive loss of photoreceptor cells in the retina, ultimately leading to blindness—responds positively to vitamin A supplementation, with 15,000 IU daily of retinyl palmitate significantly slowing the loss of retinal function over the course of four to six years. However, the subjects of the study all had the most common form of retinitis pigmentosa and were all adults; more research is needed to determine the effects of vitamin A on less common forms of the disease, as well as in children and teens.
Vitamin A and Thyroid Health
Vitamin A also plays a role in maintaining thyroid health. Animal studies have shown that vitamin A deficiency interferes with the pituitary-thyroid axis in a number of ways: increasing the size of the thyroid gland, increasing the secretion and synthesis of thyroid-stimulating hormone, raising circulating concentrations of thyroid hormones, reducing the thyroid gland’s iodine uptake, and impairing the synthesis and iodination of thyroglobulin (a protein produced by the thyroid gland).
Likewise, in iodine-deficient populations, vitamin A status influences how well people respond to preventative iodine supplementation. And, a study of children with both iodine deficiency and vitamin A deficiency found that the more severe the vitamin A deficiency, the greater the likelihood of developing goiters and abnormal thyroid hormone levels; what’s more, when these children were given salt enriched with either iodine or iodine plus vitamin A, the addition of vitamin A led to a significant decrease in thyroid volume and TSH concentration. Intriguingly, another study of iodine-deficient children found that vitamin A supplementation alone could reduce thyroid gland volume, TSH, and thyroglobulin concentrations—indicating that vitamin A has its own thyroid-protective effects independent of its interactions with iodine.
Vitamin A and Reproductive Health and Development
Sufficient intake of vitamin A are also required for healthy reproductive function, prenatal development, and postnatal development. Female reproductive cycles depend on adequate amounts of serum vitamin A, and male sperm production, as well as the maintenance of the male genital tract, require vitamin A too.
During fetal development, vitamin A (in the form of retinoic acid) is critical for organ development—including development of the eyes, ears, heart, lungs, limbs, and other organs. This is due in part to the involvement of retinoid signaling in the expression of many extracellular matrix proteins, including collagen, proteoglycans, and laminin—all of which contribute to proper organ morphology and function.
Adequate vitamin A is also needed to help protect the health of preterm infants, who are at high risk of eye disease, respiratory problems, and gastrointestinal diseases: several randomized controlled studies found that supplementing premature newborns with vitamin A significantly reduced their risk of developing bronchopulmonary dysplasia (a form of chronic long disease where the lungs and airways become damaged), and also reduced mortality.
Even beyond the immediate postnatal period, vitamin A is extremely beneficial for childhood growth and survival. In regions where vitamin A deficiency is common, prophylactic (preventative) use of vitamin A has been shown to significantly reduce the risk of childhood mortality: for children aged 6 months to five years, a 25% reduction in all-cause mortality and a 30% reduction in diarrhea-specific mortality by 30%.
Vitamin A and Skin Health
Additionally, the epithelial cells lining the inside and outside of the body (such as skin cells and gut cells) require enough vitamin A for proper differentiation. When there isn’t enough vitamin A, epithelial cells lose their integrity; for example, skin will become scaly and hard, and mucus secretion will be suppressed throughout the gastrointestinal tract. Vitamin A also has anti-inflammatory properties and an ability to regulate the production of sebum (an oily substance secreted by the skin’s sebaceous glands); not surprisingly, some skin disorders have responded well to treatment with retinoids, such as a synthetic retinoid called acitretin (used for psoriasis) and oral isotretinoin (used for treating acne).
Health Effects of Vitamin A Deficiency
Vitamin A deficiency can occur either due to low dietary intake levels (primary deficiency) or due to factors that decrease its absorption or increase the body’s vitamin A requirement (secondary deficiency). Typically, primary vitamin A deficiency happens from eating a diet low in vitamin A from animal foods and vitamin A precursors from plant foods—a problem that’s more common in developing countries. Meanwhile, secondary vitamin A deficiency can occur in people with poor lipid absorption, such as due to impaired pancreatic or bile secretion (such as from cystic fibrosis) or from inflammatory bowel diseases (like celiac disease or Crohn’s disease). Some genetic variations, too, can influence the conversion of vitamin A precursors into active vitamin A—particularly variations in the beta-carotene monooxygenase 1 (BCMO1) gene, which governs the conversion of beta-carotene into retinol, and can reduce the amount of vitamin A obtained from carotenoids. And, vitamin A requirements increase during pregnancy and breastfeeding, giving a higher risk of developing deficiency during these times; in fact, the prevalence of vitamin A deficiency rises significantly during the third trimester, when fetal growth is most rapid.
Symptoms of vitamin A deficiency generally begin with a loss of night vision, called night blindness or nyctalopia. The deficiency can also lead to a progressive eye disease called xerophthalmia, which includes dryness and wrinkling in the eye’s outer membrane (called the conjunctiva) and foamy, subtle lesions called Bitot’s spots. Prolonged or severe vitamin A deficiency ultimately results in changes in the eye’s cornea cells (the clear covering on the eye) that can lead to ulcers, scarring, and eventually blindness. About half of children who develop vitamin A-deficiency induced blindness subsequently die within a year.
Vitamin A deficiency can also increase the risk of infections; in fact, vitamin A deficiency could be viewed as an acquired immunodeficiency disease due to its extreme impact on the immune system.
Problems From Too Much Vitamin A
Unlike many vitamins (especially water-soluble ones), it’s possible to ingest enough vitamin A to induce toxicity—although this can only occur from eating animal-based sources of vitamin A, is rarely seen in the context of whole foods (one exception being polar bear liver, if you happen to get your hands on some: just 1/10th of a gram of their liver supplies the entire RDA for vitamin A!). Acute vitamin A toxicity can occur in children consuming extremely high doses (greater than 300,000 IU at one time), and chronic vitamin A toxicity can occur from consuming greater than 30,000 IU each day for months. That being said, there’s some evidence that the elderly, people genetically predisposed to high cholesterol, and chronic alcohol users may be more likely to experience vitamin A toxicity at somewhat lower (but still very high!) levels.
When it does occur, vitamin A toxicity causes a condition called hypervitaminosis A. When acute (from short-term extreme intake), it includes symptoms such as nausea, fatigue, dizziness, headache, dry skin, loss of appetite, and cerebral edema; when chronic (from longer-term high intake), it includes symptoms such as itchy skin, weight loss, loss of appetite, enlarged liver, enlarged spleen, anemia, and bone and joint pain. In severe, untreated cases, vitamin A toxicity can result in liver damage, hemorrhaging, and coma.
Importantly, high-dose supplements of preformed vitamin A—especially during early pregnancy—can also be harmful for reproductive health and fetal development, including increasing the risk of birth defects. For that reason, pregnant or potentially pregnant women are advised to avoid supplements like isotretinoin, and to monitor their intake of retinoids from fortified foods, supplements, and animal products like liver.
It’s also worth noting that beta-carotene supplements have been linked with a higher risk of lung cancer in people already susceptible to the disease. Specifically, cigarette smokers and asbestos-exposed individuals who supplement with beta-carotene have an increased lung cancer risk over time—possibly due to changes in lung cell oxidative status that cause beta-carotene to produce harmful oxidation products, rather than its normal beneficial antioxidant activity. Fortunately, this increased risk appears limited to dietary supplements, with no evidence that foods naturally high in carotenoids (like fruits and vegetables) would increase cancer risk!
How Much Do We Need?
Although vitamin A is more often listed in the form of international units (IU) for food and supplements, the recommended dietary allowance for vitamin A is presented as retinol activity equivalents (RAE)—a calculation that accounts for variations in the bioavailability of different vitamin A sources. For example, 1 IU of retinol and 1 IU of supplemental beta-carotene equals 0.3 micrograms of RAE; 1 IU of dietary beta-carotene equals 0.05 micrograms of RAE; and 1 IU of alpha-carotene or beta-cryptoxanthin equals 0.025 RAE. The RDA is listed as 700 micrograms of RAE daily for women, and 900 for men.
Best Food Sources of Vitamin A
The following foods have high concentrations of vitamin A, containing at least 50% of the recommended dietary allowance per serving, making them our best food sources of this valuable vitamin!
Good Food Sources of Vitamin A
The following foods are also excellent or good sources of vitamin A, containing at least 10% (and up to 50%) of the daily value per serving.
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