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Vitamin A & Carotenoids
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In one pass Walk into a dark cinema and it takes a while before you can see the seats. Not this — β-carotene supplements prevent cancer (even for smokers) — CARET 1996 was halted early — β-carotene supplementation raised lung cancer in smokers. Antioxidant ≠ safe.
Educational content, not medical advice — consult a clinician.
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Chapter 1
Animal and plant sources
Walk into a dark cinema and it takes a while before you can see the seats. What gets you there is a switch that vitamin A flips in your retina.
Vitamin A is not a single molecule but a group of compounds that can be turned into retinol, and it enters your body through two doors.
Animal sources supply preformed A: liver (Organ Meat), egg yolk, and dairy contain mainly retinol and retinyl esters, which are almost ready to use once absorbed.
Plant sources supply carotenoids: the beta-carotene in carrots (see Carrot), pumpkin (see Pumpkin / Winter Squash), sweet potato (Sweet Potato), and dark leafy greens first has to be cut in two by an enzyme inside the gut cells before it becomes retinol. The orange of mango flesh takes the same road (see Mango). How efficient that cut is varies widely from person to person: whether there is fat in the same meal, how healthy the gut is, and which gene variants you carry all make a difference.
Too much preformed A is poisonous: severe headache and repeated vomiting after eating a large amount of animal liver or taking a high-dose vitamin A supplement call for medical care immediately.
Vitamin A is not a single molecule but a group of compounds that can be turned into retinol, and it enters your body through two doors.
Animal sources supply preformed A: liver (Organ Meat), egg yolk, and dairy contain mainly retinol and retinyl esters, which are almost ready to use once absorbed.
Plant sources supply carotenoids: the beta-carotene in carrots (see Carrot), pumpkin (see Pumpkin / Winter Squash), sweet potato (Sweet Potato), and dark leafy greens first has to be cut in two by an enzyme inside the gut cells before it becomes retinol. The orange of mango flesh takes the same road (see Mango). How efficient that cut is varies widely from person to person: whether there is fat in the same meal, how healthy the gut is, and which gene variants you carry all make a difference.
Too much preformed A is poisonous: severe headache and repeated vomiting after eating a large amount of animal liver or taking a high-dose vitamin A supplement call for medical care immediately.
Mechanism · Why some people convert less
That pair of scissors sits in the gut cells and is called BCO1 (formerly BCMO1), an enzyme that cuts beta-carotene down the middle. The beta-carotene in carrots, pumpkin, sweet potato, and dark leafy greens has to be cut by it before it can become retinol.BCO1 gene variants are an underrated detail. In one small study, about 45% of people carried variants such as R267S or A379V, and they converted carotenoids into retinol 30–70% less efficiently. That figure comes from a single group of volunteers, and the share may differ in other populations.
What this means in practice: from the same bowl of carrot soup, some people make plenty of retinol and others find it hard. This may explain why some strict vegans who eat plenty of carrots and pumpkin still have low serum retinol.
In practice: building on plant foods is fine, but if you rely entirely on plant sources for vitamin A, it is better to ask a doctor to check serum retinol or retinol-binding protein than to count how much beta-carotene you ate.
Another often-overlooked factor: carotenoid absorption depends on fat. In the salad trial of Brown 2004, people absorbed clearly more carotenoids when the salad came with full-fat dressing than with reduced-fat or fat-free dressing. A vegetable salad with no oil at all yields very little vitamin A.
Chapter 2
Absorbed together with fat
Vitamin A is fat-soluble. Like vitamins D, E, and K, it needs bile acids to emulsify dietary fat into micelles (tiny oil droplets) before it can enter the gut cells, where it is packed into chylomicrons (fat-carrying particles) and carried away through the lymph.
That explains three things:
A very low-fat diet reduces carotenoid absorptionToo little bile, fatty diarrhea, or poor gut absorption raises the risk of vitamin A deficiencyCarrots and pumpkin are better not eaten completely oil-free; a little fat clearly improves absorption
Most absorbed vitamin A is sent to the liver for storage. The liver works like a vitamin A warehouse: when needed, it binds retinol to retinol-binding protein 4 (RBP4) and ships it to tissues throughout the body.
That explains three things:
A very low-fat diet reduces carotenoid absorptionToo little bile, fatty diarrhea, or poor gut absorption raises the risk of vitamin A deficiencyCarrots and pumpkin are better not eaten completely oil-free; a little fat clearly improves absorption
Most absorbed vitamin A is sent to the liver for storage. The liver works like a vitamin A warehouse: when needed, it binds retinol to retinol-binding protein 4 (RBP4) and ships it to tissues throughout the body.
Myth · Can eating carrots turn you orange?
Eating lots of carrots, sweet potato, and pumpkin for a long time can turn your skin orange. This really happens and is called carotenodermia, but it is not vitamin A poisoning:Excess beta-carotene settles in the outer skin layer and the fat under the skin, turning it yellow-orangeIt shows most on the palms, soles, and the sides of the nose (places with many oil glands and thick outer skin)The whites of the eyes stay white, which is the key difference from jaundice (jaundice yellows the whites of the eyes first)It is fully reversible, usually fading on its own 2-6 weeks after you cut backIt does not cause true vitamin A poisoning: the body regulates how much beta-carotene it turns into retinol, so no matter how much you eat, the amount actually converted has a ceiling
This is the key safety difference between retinol (vitamin A from animal sources) and beta-carotene: the first can poison you, while the second essentially cannot cause vitamin A poisoning. But that statement covers poisoning only. It does not mean high-dose beta-carotene supplements carry no other risk. In two large randomized trials, smokers who took high-dose beta-carotene supplements got more lung cancer, not less: ATBC (Finnish male smokers) and CARET (smokers and asbestos-exposed workers). In the AREDS2 trial (people aged 50–85 at risk of advanced macular degeneration), lung cancer occurred in 2.0% of those taking a beta-carotene formula versus 0.9% of those taking one without it, mostly in former smokers. So current and former smokers should not take high-dose beta-carotene supplements; the amounts you get from vegetables are not part of this.
Chapter 3
How eyes see in dim light
The most classic vitamin A mechanism is in the retina. Vitamin A is turned into 11-cis-retinal, which sits inside rhodopsin (the light-sensing protein in rod cells) like a switch waiting to flip.
When a photon hits it, it flips into all-trans-retinal. Rhodopsin changes shape in response, the rod cell sends a nerve signal, and the brain perceives light.
Afterward, the spent retinal is reduced, carried away, and turned back into the 11-cis shape, completing one round of the visual cycle.
When vitamin A runs short, vision in dim light fails first: walking from a bright place into the dark, the eyes are slow to adjust. That is the mechanism of night blindness (nyctalopia), and it is the earliest clinical sign.
When a photon hits it, it flips into all-trans-retinal. Rhodopsin changes shape in response, the rod cell sends a nerve signal, and the brain perceives light.
Afterward, the spent retinal is reduced, carried away, and turned back into the 11-cis shape, completing one round of the visual cycle.
When vitamin A runs short, vision in dim light fails first: walking from a bright place into the dark, the eyes are slow to adjust. That is the mechanism of night blindness (nyctalopia), and it is the earliest clinical sign.
Clinical · How vitamin A deficiency harms the eye
Xerophthalmia is the full range of eye disease caused by vitamin A deficiency, progressing step by step with severity (WHO grading codes in brackets):1. Night blindness (XN): the earliest sign, from too little rhodopsin being regenerated
Difficulty finding the way at dusk, known in Chinese folk speech as sparrow eyes
2. Conjunctival xerosis (X1A): the conjunctiva stops secreting mucus and loses its sheen
3. Bitot spots (X1B): silvery-white, foamy patches appear on the conjunctiva
They are made of shed keratinized cells plus commensal bacteria (Corynebacterium xerosis)Pathologically, they are keratin build-up on the dry conjunctivaFound in both eyes, mostly on the side of the eyeball toward the earThey are one of the signs WHO uses to diagnose vitamin A deficiency
4. Corneal xerosis (X2): the cornea loses its shine and looks sandy
5. Keratomalacia (X3): irreversible
The cornea softens, dies, and perforatesVision is lost permanentlyDeath usually follows within < 6 months (with infection and malnutrition)
The three-dose rescue regimen (WHO-recommended for severely deficient children):
200,000 of vitamin A on each of Day 1, Day 2, and Day 15; infants under 1 year get lower doses, set by a doctor according to age in monthsThe early stages, X1-X2, can be fully reversed; by X3, keratomalacia, it is too late
Why it is still seen in developed regions:
Cystic fibrosis and chronic fat malabsorptionExtremely restrictive diets (infants wrongly fed only rice water; teenagers on strict vegan diets who do not supplement)Alcohol-related cirrhosis: the liver's ability to store vitamin A is badly disrupted
Chapter 4
Barrier & immunity
Vitamin A's active form, retinoic acid, is a signal that controls genes. It enters the cell nucleus, binds a complex made of two kinds of retinoic acid receptors (RAR/RXR), and switches on the set of genes that makes a cell develop into a particular type.
It matters most for two kinds of tissue:
Epithelial barriers: the skin and the linings of the airways and gut all rely on vitamin A to develop normally and to secrete mucus. Without vitamin A, epithelial cells turn flat and keratinized (squamous metaplasia) and lose their protective mucusThe immune system: the development of T cells and B cells, and the production of the antibody IgA on mucous membranes, are all regulated by retinoic acid. Children who lack vitamin A die of infections such as measles at a clearly higher rate
It matters most for two kinds of tissue:
Epithelial barriers: the skin and the linings of the airways and gut all rely on vitamin A to develop normally and to secrete mucus. Without vitamin A, epithelial cells turn flat and keratinized (squamous metaplasia) and lose their protective mucusThe immune system: the development of T cells and B cells, and the production of the antibody IgA on mucous membranes, are all regulated by retinoic acid. Children who lack vitamin A die of infections such as measles at a clearly higher rate
Clinical · Prescription drugs derived from vitamin A
Retinoic acid is one of the few molecules that has given rise to several families of prescription drugs from a nutrient. These drugs are used at pharmacologic doses, which is a different matter from taking a vitamin.Topical retinoids (acne and anti-aging):
Retinol: an over-the-counter anti-aging ingredient, the mildestTretinoin (all-trans retinoic acid): a prescription drug, the classic treatment for acne and sun damageAdapalene (brand name Differin): third generation, switched to over-the-counter in the US in 2016Tazarotene: used for psoriasis and acne, more irritating
How they work: they bind the retinoic acid receptors (RAR/RXR) in skin cells directly and speed up the turnover of keratin-making cells, which treats acne, softens fine lines, and evens out pigment.
Oral retinoids (severe acne):
Isotretinoin: the most effective drug for severe nodular and cystic acne; many people stay clear for a long time after one courseTypical dose 0.5-1 mg/kg/day for 4-6 monthsStrongly teratogenic (it causes birth defects): two forms of contraception at once and regular pregnancy tests are usually required (the US has the iPLEDGE registry), because the risk persists for a while after stoppingSide effects: dry lips, dry skin, raised liver enzymes, and possible effects on mood
Treating acute promyelocytic leukemia (APL):
All-trans retinoic acid (ATRA) plus arsenic trioxide (ATO) is a regimen that can cure APLUsing ATRA for APL was first reported in 1988 by Wang Zhenyi's team in Shanghai. The ATRA here is a drug used at pharmacologic doses; this is not a vitamin curing cancerComplete remission rate > 95%
Important: pregnancy contraindications:
Oral isotretinoin, acitretin, and etretinate: strict contraception, because they cause birth defectsTopical tretinoin and adapalene: very little is absorbed, but they are still avoided in pregnancy (as a precaution)
So vitamin A is more than a molecule in the retina: of all the fat-soluble vitamins, it has the longest history in medicine and has been turned into prescription drugs most thoroughly.
Chapter 5
Too much
The risk of vitamin A poisoning comes from preformed retinol, not from the beta-carotene in ordinary vegetables. Long-term high doses of retinol build up in the liver and cause headache, peeling skin, liver damage, and falling .
Pregnancy is especially critical: too much preformed vitamin A clearly causes birth defects. Retinoic acid is the signal that tells cells where they are during embryonic development, and an excess disrupts the development of the limbs, heart, head, and face.
The adult upper limit (): 3000 µg a day (counting only preformed vitamin A, that is, retinol; beta-carotene from vegetables does not count). The limit in pregnancy is the same as outside pregnancy, 2800–3000 µg/day depending on age, not higher; pregnant women should not take high-dose vitamin A supplements on their own.
How to arrange it: build on dark-colored fruit and vegetables, eat liver now and then (not every day), and use supplements only for a confirmed deficiency or on a doctor's advice.
Pregnancy is especially critical: too much preformed vitamin A clearly causes birth defects. Retinoic acid is the signal that tells cells where they are during embryonic development, and an excess disrupts the development of the limbs, heart, head, and face.
The adult upper limit (): 3000 µg a day (counting only preformed vitamin A, that is, retinol; beta-carotene from vegetables does not count). The limit in pregnancy is the same as outside pregnancy, 2800–3000 µg/day depending on age, not higher; pregnant women should not take high-dose vitamin A supplements on their own.
How to arrange it: build on dark-colored fruit and vegetables, eat liver now and then (not every day), and use supplements only for a confirmed deficiency or on a doctor's advice.
Safety · Why too much liver can poison you
The most dramatic historical cases of acute vitamin A poisoning involve polar explorers who fell ill after eating the liver of polar bears, seals, or sled dogs:The story:
In 1596, Willem Barentsz's Arctic expedition recorded crew members being poisoned after eating bear liverIn 1913, on Mawson's Antarctic expedition, a team member died after eating large amounts of sled-dog liver along the way, with headache, blurred vision, and widespread peeling skin before death; later researchers suggested vitamin A poisoning as one cause, which remains debatedThe modern explanation: polar bear liver has been measured at about 24,000 /g of vitamin A, so a 100 g piece holds 2.4 million IU, far more than an adult can handle at once
Why polar animals store so much vitamin A:
The fish, seal, and polar bear food chain concentrates retinol at each stepPolar bears themselves tolerate very high retinol (a metabolic adaptation), but a person who eats bear liver is poisoned
Symptoms of acute vitamin A poisoning:
Headache (from raised pressure inside the skull, called pseudotumor cerebri)Dizziness, nausea, vomitingBlurred visionWidespread peeling skin (often starting around the mouth)Cracked, bleeding lips and mucous membranes
If severe headache, repeated vomiting, or blurred vision follows eating a large amount of animal liver or taking a high-dose vitamin A supplement, seek medical care immediately; the same goes for a child who has swallowed an adult's vitamin A supplements.
Chronic vitamin A poisoning (more common today):
The adult upper limit is equivalent to 10,000 IU (~3000 µg) a day; chronic poisoning is mostly seen with supplements taken for a long time at doses far above that, over months to yearsHeadache, hair loss, bone pain, liver fibrosis, lower Abnormal blood lipids, and it may also interfere with vitamin D (one hypothesis is that the two compete for the same receptor partner, RXR)Most people recover after stopping, but it takes months
In practice:
An ordinary diet almost cannot cause poisoning; eating liver now and then is fine, and the risk comes only from eating a lot of it every dayDuring pregnancy, avoid large amounts of animal liver and high-dose retinol supplementsDo not take a multivitamin, cod liver oil, and a separate vitamin A supplement at the same time (the doses add up easily)Check which form of vitamin A a combination supplement uses: beta-carotene does not cause vitamin A poisoning (though smokers taking high-dose beta-carotene supplements got more lung cancer in the ATBC and CARET trials); for retinol (retinyl palmitate or acetate), check the dose
Chapter 6
Where deficiency is still common
Vitamin A deficiency is one of the world's most serious preventable nutrient deficiencies, especially in sub-Saharan Africa and Southeast Asia.
Blindness: each year roughly 250,000 to 500,000 children worldwide go blind from vitamin A deficiency (a WHO estimate), and about half of them die within a year of losing their sightChild deaths: children lacking vitamin A die of infections such as measles at a clearly higher rate. A Cochrane review by Imdad 2017 pooled randomized trials in children aged 6 months to 5 years (mostly in places where deficiency is common) and found that vitamin A supplements lowered deaths from all causes by about 12–24% (two statistical models gave 12% and 24%)Global action: many countries fortify flour and cooking oil and hand out vitamin A capsules to children
In Chinese cities, vitamin A deficiency is uncommon overall; groups at risk include poor rural children, people with fat malabsorption, and people who eat a very narrow range of foods.
Blindness: each year roughly 250,000 to 500,000 children worldwide go blind from vitamin A deficiency (a WHO estimate), and about half of them die within a year of losing their sightChild deaths: children lacking vitamin A die of infections such as measles at a clearly higher rate. A Cochrane review by Imdad 2017 pooled randomized trials in children aged 6 months to 5 years (mostly in places where deficiency is common) and found that vitamin A supplements lowered deaths from all causes by about 12–24% (two statistical models gave 12% and 24%)Global action: many countries fortify flour and cooking oil and hand out vitamin A capsules to children
In Chinese cities, vitamin A deficiency is uncommon overall; groups at risk include poor rural children, people with fat malabsorption, and people who eat a very narrow range of foods.
Background · Can Golden Rice supply vitamin A?
Golden Rice is a biofortification project. The grain of ordinary rice does not accumulate beta-carotene; genetic modification lets it do so, so that people who live on rice and lack vitamin A (in Southeast Asia and sub-Saharan Africa) can get beta-carotene from their staple food. A small human study (Tang 2009, Am J Clin Nutr) found that roughly every 3.8 parts of the beta-carotene in Golden Rice are converted into 1 part of retinol (by weight), showing that it can indeed supply vitamin A.It is also one of the most contested projects in the politics of genetically modified (GMO) crops: more than twenty years of regulatory and public wrangling passed between the first report and the start of commercial planting in the Philippines. But that is a story about agriculture and policy. Looking at nutrition alone: fighting vitamin A deficiency does not depend on any single technology. WHO recommends working on several fronts at once: a varied diet, fortified foods, and micronutrient supplementation (more orange and yellow fruit and vegetables, plus vitamin A capsules). The steep fall in vitamin A deficiency in China happened alongside economic growth, urbanization, and easier access to fruit and vegetables, and had nothing to do with Golden Rice.
tang-2009-golden-rice
References · 9
- National Institutes of Health, Office of Dietary Supplements. (2025). Vitamin A and Carotenoids — Fact Sheet for Health Professionals. ods.od.nih.gov/factsheets/VitaminA-HealthProfessional
- Brown, M. J., Ferruzzi, M. G., Nguyen, M. L., Cooper, D. A., Eldridge, A. L., Schwartz, S. J., & White, W. S. (2004). Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. The American Journal of Clinical Nutrition, 80(2), 396-403. Dietary fat markedly raises absorption of fat-soluble carotenoids. 10.1093/ajcn/80.2.396
- The ATBC Cancer Prevention Study Group. (1994). The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. NEJM, 330(15), 1029–1035. 29,133 male smokers aged 50-69 in south-western Finland; alpha-tocopherol 50 mg/day, beta-carotene 20 mg/day, both, or placebo; follow-up 5-8 years. Lung cancer: alpha-tocopherol -2% (-14 to 12%), beta-carotene +18% (3 to 36%). Fewer prostate cancers with alpha-tocopherol, more deaths from haemorrhagic stroke; total mortality 8% higher with beta-carotene (abstract, PMID 8127329). 10.1056/NEJM199404143301501
- Omenn, G. S., et al. (1996). Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease (CARET). NEJM, 334(18), 1150–1155. 10.1056/NEJM199605023341802
- Age-Related Eye Disease Study 2 Research Group. (2013). Lutein + zeaxanthin and omega-3 fatty acids for age-related macular degeneration: the Age-Related Eye Disease Study 2 (AREDS2) randomized clinical trial. JAMA, 309(19), 2005-2015. 4,203 people aged 50-85 at risk of advanced AMD (bilateral large drusen, or large drusen in one eye and advanced AMD in the other); 2 x 2 factorial of lutein 10 mg + zeaxanthin 2 mg and/or DHA 350 mg + EPA 650 mg vs placebo, on top of the AREDS formulation, with a secondary randomization removing beta-carotene and/or lowering zinc. Median follow-up 5 years. In the primary analyses, adding lutein + zeaxanthin (HR 0.90, 98.7% CI 0.76-1.07), DHA + EPA (0.97) or both (0.89) did not further reduce progression to advanced AMD. More lung cancers with beta-carotene than without (23 [2.0%] vs 11 [0.9%], nominal P = .04), mostly in former smokers, so the authors call lutein + zeaxanthin an appropriate carotenoid substitute for beta-carotene (abstract, PMID 23644932). 10.1001/jama.2013.4997
- Sommer, A. (2008). Vitamin A deficiency and clinical disease: an historical overview. The Journal of Nutrition, 138(10), 1835–1839. 10.1093/jn/138.10.1835
- Institute of Medicine. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press. www.ncbi.nlm.nih.gov/books/NBK222310
- World Health Organization. (2009). Global prevalence of vitamin A deficiency in populations at risk 1995–2005: WHO Global Database on Vitamin A Deficiency. iris.who.int/handle/10665/44110
- Imdad, A., Mayo-Wilson, E., Herzer, K., & Bhutta, Z. A. (2017). Vitamin A supplementation for preventing morbidity and mortality in children from six months to five years of age. Cochrane Database of Systematic Reviews, (3), CD008524. 10.1002/14651858.CD008524.pub3