Beta-Carotene
Beta-carotene is split into two units of retinol in the intestinal mucosa of most animals before absorption. Within animals, beta-carotene is converted into an active form of vitamin A. Beta-carotene in itself is non-toxic and any excess amounts of it will be stored in fatty tissues. This way, it cannot be converted to its active form and become toxic. People with high amounts of beta-carotene tissues may have a yellow or orange tint to their skin. Otherwise, there is no toxic affects.
Some animals can absorb beta-carotene intact (man, chicken, some cattle). Beta-carotene is then split into retinol in the liver. Cats cannot split beta-carotene. They cannot absorb beta-carotene at all. They have to be provided vitamin A in the retinol form.
6 beta-carotenes = 1 retinol