The short length of the digestive tract of the cat causes cats' digestive system to weigh less than other species of animals, which allows cats to be active predators. While cats are well adapted to be predators they have a limited ability to regulate catabolic enzymes of amino acids meaning amino acids are constantly being destroyed and not absorbed. Therefore, cats require a higher protein proportion in their diet than many other species. Cats are not adapted to synthesize niacin from tryptophan and, because they are carnivores, can't convert carotene to vitamin A, so eating plants while not harmful does not provide them nutrients.