Vitamin A is actually a family of substances called retinoids that includes retinol, retinal, and retinoic acid. These are called preformed vitamin A because they are in a form that your body readily uses. Retinol is the most usable of the three forms and can be converted to both retinal and retinoic acid in your body.
Preformed vitamin A is found only in foods from animal sources, such as liver and eggs, and is added to all processed milk. Plant food sources do not contain preformed vitamin A, but some do contain provitamin A carotenoid, which can be converted to retinol in your body. Carotenoids are the yellow-red pigments that give carrots, butternut squash, and cantaloupe their vibrant, deep orange color.
There are over 600 different carotenoids, but only 3—beta-carotene (β-carotene), beta-cryptoxanthin (β-cryptoxanthin), and alpha-carotene (α-carotene)—can be converted to vitamin A. These three provide approximately 25 to 35 percent of the dietary vitamin A consumed by adults in the United States, with the majority of it coming from beta-carotene. Other nutritionally significant carotenoids, including lycopene, lutein, and zeaxanthin, may function as antioxidants or provide health benefits, but cannot be converted to vitamin A.