Salmon farmers use TMG to protect the fish from osmotic stress when they go from fresh to salt water. Made from sugar beets, it's a very non-toxic supplement. There are 599 known carotenoids besides beta-carotene. One of the jobs of these brightly-colored substances is to protect against the sun. As such, they have strong antioxidant action against UV radiation.
Carotenoids appear to work synergistically.
In a study on humans, beta-carotene deficiency caused suppressed immunity. Beta-carotene alone did not restore immunity. However, when a mixture of carotenoids was taken, immunity was restored. They can sicken a dog as well. Onions contain substances that destroy red blood cells.