Preformed retinol versus a provitamin precursor
Retinol is preformed vitamin A, found in animal foods such as liver, eggs, and dairy, and the body can use it directly. Beta-carotene is a provitamin from plants (carrots, sweet potato, leafy greens) that the body must cleave into retinol before use. That conversion is inefficient and variable, so the vitamin A activity of beta-carotene is much lower per microgram than retinol, which is why intake is expressed in retinol activity equivalents.
Conversion efficiency differs between people, partly due to genetics and partly to factors like dietary fat and overall vitamin A status.
Different forms, different toxicity profiles
The two forms differ in safety as well as potency. Preformed retinol can accumulate and reach toxic levels at high sustained intakes, with particular caution in pregnancy because excess can cause birth defects. Beta-carotene does not cause vitamin A toxicity because the body limits conversion; very high intakes mainly turn the skin orange, which is harmless and reversible. High-dose beta-carotene supplements, however, raised lung cancer risk in smokers in major trials.
This is general educational information, not medical advice. Vitamin A supplementation, especially in pregnancy or for smokers, should be discussed with a clinician.