Two different forms
'Vitamin A' actually refers to two kinds of compounds the body handles differently [1]:
- Preformed vitamin A (retinol, retinyl esters) comes from animal foods — liver, fish, eggs, dairy — and from many supplements. It's ready to use and stored in the liver.
- Provitamin A carotenoids (mainly beta-carotene) come from colorful plants — carrots, sweet potato, leafy greens. The body converts only as much as it needs, which makes beta-carotene self-limiting as a vitamin A source.
Why the units (RAE) matter
Because the forms differ in potency, intake is measured in retinol activity equivalents (RAE). NIH notes 1 mcg RAE equals 1 mcg retinol, 2 mcg of supplemental beta-carotene, or 12 mcg of dietary beta-carotene [1]. The adult RDA is 900 mcg RAE for men and 700 mcg RAE for women [1].
Only preformed vitamin A has an upper limit
This is the key safety point: the Tolerable Upper Intake Level (3,000 mcg/day for adults) applies only to preformed vitamin A, not to beta-carotene [1]. That's why high-dose retinol supplements and liver can cause toxicity, while eating lots of carrots won't (it may temporarily tint skin orange, which is harmless).
The beta-carotene smoker warning
There's an important exception to beta-carotene's safety: high-dose beta-carotene supplements raised the risk of lung cancer in smokers. NIH cites the CARET trial, in which supplements increased lung-cancer risk by about 28% in current and former smokers [1]. See antioxidant supplements.
Practical guidance
- For most people, getting vitamin A from a varied diet (including plant carotenoids) is the safest route.
- Avoid high-dose preformed vitamin A unless directed; it's the form that accumulates (see [vitamin A intake and toxicity](/learn/vitamin-a-intake-and-toxicity)).
- Smokers should avoid high-dose beta-carotene supplements.