Your cat should be eating a good quality food formulated for his age (kitten food for kittens, senior food for older cats, etc.). There are certain vitamins and amino acids that must be added to cat food by the manufacturer because the cat’s body can not make them for himself.
I used to think that cats could probably eat canned tuna that’s packaged for people, and I didn’t know there was any difference. But it turns out that if we tried to have our cat live off of canned tuna that isn’t fortified with the nutrients that cats need, he would develop severe vitamin deficiencies and become very ill. Mindy Bough, veterinary technician for the ASPCA Pet Nutrition and Science Advisory Service, says this: “An occasional tuna treat for your cat is generally harmless. However, if a large part of the cat’s diet consists of tuna–or if the cat is fed tuna exclusively–some problems are likely to arise.” She added, “I recommend premium commercial food for domestic cats. These foods are formulated to meet all of a cat’s dietary needs. Then you can feed an occasional tuna treat for your pet’s enjoyment. And remember, no more than five to ten percent of a cat’s diet should ever be table food.”
We’ll discuss food and feeding some more later, but for now I’ll say that cats should not eat dog food – they are obligate carnivores and must have a high percentage of meat. (Dogs do not need the same high percentage of meat, so dog foods have less meat than cat foods do). Likewise they must have the right vitamins yet not dangerous quantities of vitamins like they might get if given the wrong foods (such as the Vitamin A overdose they can get from too much liver) or supplements. So a good quality commercial cat food is likely to provide what your cat needs each day.
UPDATE: I have started a new online magazine about cats, Cats Digest, and you can find more about feeding cats people food on the new site.