Vitamin A is a general term for a group of related substances that the human body can use for good health. Beta carotene is a specific molecule in a group of molecules called carotenoids, some of which can be converted into one type of vitamin A by the body. Source of beta carotene are plant-based, whereas preformed vitamin A only comes from animal sources.
Beta carotene is only one of a variety of molecules that can be converted into vitamin A. The other molecules present in an average Western diet are alpha carotene and beta-cryptoxanthin, and all together, these substances fall into a group of molecules called the carotenoids. Carotenoids are pigments, which means that they give color to the plant they grow in. Carotenoids in food come only from plants, so vitamin A and beta carotene play a role in the ingestion and absorption of plant nutrients, and not animal nutrients.
Once the body ingests beta carotene, it has to alter it to turn it into vitamin A. It does this one of two ways, either chopping up the beta carotene molecule in the middle of the chain, or cutting it up from the outside in. The breakdown and production of vitamin A and beta carotene happens in the small intestine, after the food passes through the stomach. The process involves an enzyme called beta-carotene dioxygenase, and the specific product of this vitamin A and beta carotene process is called retinol. Retinol is only one form of vitamin A, and others can be produced from other types of vitamin A raw material, like other carotenoids.
Storage of this product of the vitamin A and beta carotene process occurs in the liver. This has implications for the potential animal sources of vitamin A, as the livers of certain predators, such as polar bears or fish, can contain high levels of vitamin A. If too much preformed vitamin A is ingested by a human, sickness can occur.
If a person eats high levels of beta carotene, though, poisoning does not typically occur, as the body only makes enough vitamin A as it needs, and simply stores the beta carotene in its original form. When the body needs more vitamin A, it makes it from the beta carotene stored in the fat of the body. Too much beta carotene can make the skin turn yellow, but this appears to be a relatively harmless condition. Conversely, too little vitamin A can produce developmental problems in children, result in blindness, and increase the risk of dangerous infections.