It has become normal that animals can follow a vegetarian diet. However, neither cats, nor dogs are vegetarians by nature: cats are carnivores and dogs are omnivores. At the same time, both cats and dogs belong to the class of carnivores. But this doesn't mean a lot.
Speaking about vegetarian cat food, it's necessary to take into consideration that by nature cats wouldn't eat anything, which can generally be found in cans with pet food. Cats require special diet full of taurine (an amino acid found in animals' muscles). Can taurine be found in vegetarian cat food? Thanks to the recent achievements in cat food production synthetic taurine has been developed and already been used in non-vegetarian (or commercial) cat foods. As for vegetarian cats, they should be fed such food as a supplement. Be aware that taurine deficiency may result in cat blindness and sometimes even death. It's worthy mentioning that one of the taurine sources is cow's milk, however it's poorer than meat.
So, cats require much taurine, minerals, arachidonic acid and vitamins (particularly A and B12). It's worthy admitting that all known vegetarian cat food usually contains the mentioned essential ingredients. If you're still concerned it's necessary to ask a qualified veterinary regarding changing your cat's diet and feeding it with vegetarian cat food. At the same time, don't forget that cats fed on vegetarian food are likely to hunt for their preferred meat and kill birds or rodents.
As it has already been mentioned all cats require a dietary source of arachidonic acid (fatty aids) which convert with time into other vital substances. As a matter of fact, arachidonic acid isn't necessary for humans because they are able to synthesize it from linoleic acid found in vegetable fat. However, this dietary source is extremely essential for cats. Milk and fatty tissue contain almost no arachidonic acid and only small amounts of this acid can be found in eggs. The only major source is meat. Be aware that deficiency of arachidonic acid takes some time to develop. But with time its influence on cat's health is profound.
All cats need vitamin A, but they are not able to utilize it that's why they require that their food contains performed vitamin A (or retinol) occurring primarily in animal food (one of the richest vitamin A sources is liver). It's important to know that only small quantities of this vitamin can be found in dairy products and eggs. The same problem is with vitamin B12 which is present only in animal products. Cats need a dietary source of vitamin B12 because they cannot synthesize their own vitamin of this group.
Among other essential ingredients which should be present in cat daily diet are thiamine, niacin and, of course, protein. The rich source of the mentioned partials is meat. That's why very often vegetarian cat food doesn't fit all the cat's needs. Often vegetarian cats are given vegetarian cat food recipes in combination with special supplements which contain all the necessary vitamins and minerals for good maintenance of cat's health.