Preformed vitamin A, supplied as retinyl palmitate or retinyl acetate in softgels and as retinol in cod liver oil. Unlike carotenoids, it is absorbed and stored directly, with no conversion step to limit how much reaches the tissues.
Vitamin A deficiency is one of the most consequential nutritional problems in the world — the leading cause of childhood blindness in low-income countries, and a major contributor to child mortality there. Supplementation in those settings works. In a well-fed population the picture inverts completely: preformed vitamin A accumulates in the liver, has no efficient excretion route, and both acute and chronic excess are well documented. It is the clearest example in this catalogue of a nutrient where the deficiency evidence and the supplementation-on-top-of-adequacy evidence point in opposite directions.
Retinol, Retinyl palmitate, Retinyl acetate, Preformed vitamin A
Regulation: Sold in the US as a dietary supplement with no pre-market review of efficacy or safety. The Institute of Medicine set a Tolerable Upper Intake Level of 3,000 µg RAE/day for preformed vitamin A in adults; several European regulators additionally advise pregnant women and postmenopausal women to limit retinol supplements. Labels still often express amounts in IU, which makes overshooting easy.
What the research used
The amounts, forms and durations published trials of Vitamin A (retinol) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
700–900 µg RAE
Strong
700 µg RAE/day for women and 900 µg RAE/day for men. Retinol activity equivalents combine preformed retinol with a fraction of provitamin A carotenoids — the two are not interchangeable on a microgram basis.
Studied for Tolerable Upper Intake Level, adults
3000 µg RAE
Strong
The IOM upper limit applies to preformed vitamin A only, and is set on liver toxicity and teratogenicity. It equates to roughly 10,000 IU/day. Provitamin A carotenoids are excluded because conversion is regulated.
Studied for Childhood deficiency programmes in low-income settings
100000–200000 IU
Strong
High-dose oral capsule, every 4–6 months
The WHO periodic dosing schedule for children aged 6–59 months in populations with endemic deficiency. This is a public-health intervention for a deficient population, not a dose with any bearing on a well-nourished adult.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
🍽️With food
Moderate
Absorbed with dietary fat through the same route as other lipids, so a fat-containing meal materially increases uptake. This is established pharmacology rather than convention — absorption studies show poor uptake of fat-soluble vitamins taken fasted.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Moderate
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Periodic high-dose vitamin A in children in deficient populations is associated with lower all-cause and measles-related mortality across many large trials. The finding is specific to deficient settings; equivalent trials in well-nourished populations have not shown the same, and one large neonatal trial programme produced inconsistent results.
What may be going on: Retinoic acid regulates epithelial barrier integrity and the differentiation of T and B lymphocytes, which is the plausible route from deficiency to infection susceptibility.
Moderate evidence. Several human studies point the same way, with real caveats.
In populations with established vitamin A deficiency, supplementation restores night vision and reduces xerophthalmia — this is well documented. In people whose intake is already adequate, extra vitamin A has not been shown to improve any measure of vision.
What may be going on: Retinal, the aldehyde form of retinol, is the chromophore of rhodopsin. Without it the rod photoreceptors cannot regenerate visual pigment, which is why night blindness is the first symptom of deficiency.
Moderate evidence. Several human studies point the same way, with real caveats.
There is not enough evidence that oral vitamin A supplements change skin structure or acne in people who are not deficient. The dramatic dermatological results belong to prescription retinoids such as isotretinoin at pharmacological doses under medical supervision, which is a different intervention entirely.
Not enough evidence. Not enough human research to say anything useful yet.
Randomised controlled trial · The American journal of clinical nutrition · 1997 · PMID 8988926
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Avoid
Teratogenic in pregnancy
Preformed vitamin A is a known human teratogen. Intakes above roughly 10,000 IU/day in early pregnancy have been associated with cranial-neural-crest birth defects, and the entire retinoid drug class carries the same warning. Pregnant women and women who might become pregnant are advised to avoid retinol supplements and liver, and to check that any multivitamin supplies vitamin A as beta-carotene rather than retinyl esters.
Too much
Hypervitaminosis A: liver injury and raised intracranial pressure
Chronic intake above the 3,000 µg RAE upper limit causes hepatotoxicity that can progress to fibrosis and cirrhosis, together with headache, blurred vision and papilloedema from raised intracranial pressure (pseudotumour cerebri). Acute massive doses cause nausea, vomiting, vertigo and skin peeling. Because retinol is stored in the liver, the damage accumulates silently over months.
Too much
Reduced bone density and hip fracture at chronic high intake
Cohort studies have linked high habitual retinol intake — largely from supplements and liver — with lower bone mineral density and increased hip fracture risk, and retinoic acid stimulates osteoclast activity in laboratory work. The association is not settled, but it argues strongly against long-term retinol supplementation in older adults, who are the group most exposed to fracture in the first place.
Interaction
Additive toxicity with retinoid drugs and interaction with anticoagulants
Taken alongside isotretinoin, acitretin or bexarotene, supplemental vitamin A adds directly to retinoid toxicity. High doses also appear to interfere with vitamin K–dependent clotting and may increase bleeding risk on warfarin.
Worth knowing
Liver disease and heavy alcohol use
Alcohol and vitamin A share metabolic pathways, and hepatotoxicity from retinol occurs at lower intakes in people with existing liver disease or heavy alcohol intake. Cod liver oil is a common unrecognised source, supplying both vitamin A and vitamin D in fixed proportion.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.