GLOSSARY

Retinol is the alcohol form of preformed A, convertible into related forms or stored as esters.

Vitamin A includes preformed retinol and its esters plus provitamin carotenoids. Retinal supports vision and retinoic acid regulates gene expression, but these forms are not fully interchangeable in the body. A retinoid medicine is not simply an ordinary dietary supplement: structure, conversion pathways and intended use limit what can be inferred from their shared family name.

Liver and some animal or fortified foods provide preformed vitamin A; dark-green and orange vegetables can provide precursors. Contributions vary with food and portion. For athletes, varied sources are more useful than frequent liver consumption or stacking retinol products. Accompanying energy and fat also belong in the actual meal plan rather than being ignored because a vitamin is present.

Retinol activity equivalents compare vitamin A activity rather than simply adding pigment masses. Under the common convention, 1 microgram retinol corresponds to 12 micrograms dietary beta-carotene or 24 micrograms dietary alpha-carotene or beta-cryptoxanthin. These factors depend on form and source; supplemental beta-carotene uses a different factor, so the label's basis must be identified.