The good news is, if you feed a nutritionally balanced commercial cat food along with providing a source of clean, fresh water, then your cat is already getting all the essential nutrients.
You don't need to supplement the diet with anything. If you still choose to throw in some extras, make sure you avoid these foods:
Cartoons always show cats happily lapping up milk, as if it's their favorite food in the world. Actually, too much milk can cause diarrhea in cats, and it's certainly not a source of the complete nutrition your cat needs. Remember never to substitute milk for water.
With the salmonella that exists these days, you should never feed raw eggs to cats. For cats, repeated consumption of raw eggs can cause skin inflammation, loss of hair and poor growth due to biotin deficiency.
Certain types of raw fish can cause cats to suffer from a thiamine deficiency, which can cause anorexia, abnormal posture, weakness and even death.
Raw diets are gaining traction right now, but it's best not to experiment with raw meat when it comes to feeding your cat. Many raw meats contain parasites, and strictly raw meat can't provide the proper balance of nutrients a cat needs.
Cats, like dogs, shouldn't gnaw on bones. Small soft ones, like those that come in pork chops or chicken bones, may splinter and lodge in your cat's mouth or throat.
If your cat is eating a nutritionally-balanced diet, excess supplements mean wasted vitamins, or worse, vitamin toxicity. However, these supplements may provide a benefit to cats with certain health issues. You should check with your veterinarian before providing supplements to your cat.
Cats may enjoy a table scrap or two, but lots of times they'll be uninterested. That's because most human food doesn't provide the balanced diet cats need. Treats or table scraps should be limited to no more than 10% of your cat's total daily calories. Ask your veterinarian if these are OK for your cat, although we personally recommend avoiding them.