Cats do not possess the ability to separate meat from fur bones or feathers before eating like we do so they have to eat the digestible parts along with the indigestible parts. When the cat has finished digesting what he can the rest has to be removed from the system. To try and pass this through the digestive tract could cause obstruction and severe illness so this is where grass eating comes in.
The habit of eating grass triggers the natural process of regurgitation of indigestible foods including its own fur balls which would otherwise obstruct the system. This can appear to us humans as if the cat is being violently ill but this is not the case and in fact is very beneficial to the cat.
What about other plants and vegetables?
Cats cannot produce the enzymes needed to break down plant matter. Even when pureed or powdered, vegetables simply pass through the digestive tract without leaving any of their nutritional content. This alone is not harmful to your cat, but when undigested plant matter mixes with highly digestible food such as meat, indigestion and other complications may occur. Grass is an exception however; some cats compulsively eat grass when you leave them outdoors. This will cause them to regurgitate the indigestible matter, including raw vegetables.
If your cat is unable to get access to grass he may attempt to eat your house plants instead. This could be dangerous because some house plants and flowers are toxic to cats, so if you have a house cat like a Ragdoll cat for example you should provide a regular supply of potted grass to prevent him looking elsewhere. And most importantly do not punish your cat for regurgitating on to you carpet because he will not know why you are punishing him. Regurgitation is an involuntary action of your cat’s stomach so he could not stop it happening even if he tried. Cleaning up after the occasional regurgitation should be an accepted part of being a loving cat owner as they are only doing what comes naturally.
by Drew Pilton