Pet Health Pictures Slideshow: Harmful Foods Your Cat Should Never Eat

Because they're such picky eaters, we sometimes think cats know what's best for them when it's time to eat. But the fact they'll walk away from a piece of bad meat doesn't mean they'll bypass an open can of tuna. And, that can of tuna can be just as dangerous. In fact, you may be surprised to learn some of the common foods your cats should never eat.

Cats can be addicted to tuna, whether it's packed for cats or for humans. Some tuna now and then probably won't hurt. But a steady diet of tuna prepared for humans can lead to malnutrition because it won't have all the nutrients a cat needs. And, too much tuna can cause mercury poisoning. Remember the saying, "Honest as a cat when the meat's out of reach." Your cat will see an open can of tuna next to the sink as a dinner invitation.

Onion in all forms - powdered, raw, cooked, or dehydrated - can break down a cat's red blood cells, leading to anemia. That's true even for the onion powder that's found in some baby foods. An occasional small dose probably won't hurt. But eating a large quantity once or eating smaller amounts regularly can cause onion poisoning. Along with onions, garlic and chives can cause gastrointestinal upset.

What could be wrong with offering your cat a saucer of milk or a piece of cheese? Most cats are lactose-intolerant. Their digestive system cannot process dairy foods, and the result can be digestive upset with diarrhea.

Beer, liquor, wine, foods containing alcohol -- none of it is good for your cat. That's because alcohol has the same effect on a cat's liver and brain that it has on humans. But it takes far less to do its damage. Just two teaspoons of whisky can cause a coma in a 5-pound cat, and one more teaspoon could kill it. The higher the proof, the worse the symptoms.

Grapes and raisins have often been used as treats for pets. But it's not a good idea. Although it isn't clear why, grapes and raisins can cause kidney failure in cats. And, a small amount can make a cat ill. Repeated vomiting and hyperactivity are early signs. Although some cats show no ill effects, it's best not to give your cat any grapes and to keep grapes and raisins off countertops and other places accessible to your cat.

Caffeine in large enough quantities can be fatal for a cat. And there is no antidote. Symptoms of caffeine poisoning include restlessness, rapid breathing, heart palpitations, muscle tremors, and fits. In addition to tea and coffee -- including beans and grounds -- caffeine can be found in cocoa, chocolate, colas, and stimulant drinks such as Red Bull. It's also in some cold medicines and painkillers.

Chocolate can be lethal for cats. Although most cats won't eat it on their own, they can be coaxed to eat it by owners and others who think they are giving the cat a treat. The toxic agent in chocolate is theobromine. It's in all kinds of chocolate, even white chocolate. The most dangerous kinds, though, are dark chocolate and unsweetened baking chocolate. Eating chocolate can cause abnormal heart rhythm, tremors, seizures, and death.

Table scraps often contain fat trimmed off of meat and bones. Both fat and bones may be dangerous for cats. Fat, both cooked and uncooked, can cause intestinal upset, with vomiting and diarrhea. And a cat can choke on a bone. Bones can also splinter and cause an obstruction or lacerations of your cat's digestive system.

There are two problems with giving your cat raw eggs. The first is the possibility of food poisoning from bacteria like salmonella or E. coli. The second is rare problem in which a protein in raw egg whites, called avidin, could interfere with the absorption of the B vitamin biotin. This can cause skin problems as well as problems with your cat's coat.

Raw meat and raw fish, like raw eggs, can contain bacteria that cause food poisoning. In addition, an enzyme in raw fish destroys thiamine, which is an essential B vitamin for your cat. A lack of thiamine can cause serious neurological problems and lead to convulsions and coma.

An occasional bite of dog food won't hurt your cat. But dog food is not a substitute for cat food. They do have many of the same ingredients. But cat food is specially formulated for a cat's needs, which include more protein as well as certain vitamins and fatty acids. A steady diet of dog food can cause your cat to be severely malnourished.

Small amounts of liver are OK, but eating too much liver can cause vitamin A toxicity. This is a serious condition that can affect your cat's bones. Symptoms include deformed bones, bone growths on the elbows and spine, and osteoporosis. Vitamin A toxicity can also cause death.

Eating too much too often can do the same thing to cats that it does to humans. It can lead to obesity and even diabetes.

Before it's baked, bread dough needs to rise. And, that's exactly what it would do in your cat's stomach if your cat ate it. As it swells inside, the dough can stretch the abdomen and cause severe pain. In addition, when the yeast ferments the dough to make it rise, it produces alcohol that can lead to alcohol poisoning.

Ingesting a drug prescribed for humans is one of the most common causes of poisoning in cats. Just as you would do for your children, put all medicines where your cat can't get to them. And never give your cat any over-the-counter medicine unless advised to do so by your vet. Ingredients such as acetaminophen or ibuprofen are common in pain relievers and cold medicine. And they can be deadly for your cat.

Many other items commonly found on kitchen shelves can harm your cat. Keeping food items where your cat can't get to them and keeping pantry and cupboard doors closed will help protect your cat from serious food-related illness.

No matter how cautious you are, it's possible your cat can find and swallow what it shouldn't. It's a smart idea to always keep the number of your local vet, the closest emergency clinic, and the ASPCA Animal Poison Control Center -- (888) 426-4435 -- where you know you can find it in an emergency. And, if you think your cat has consumed something that's toxic, call for emergency help at once.

Cats are carnivores, and need meat. Talking with your vet about the cat food you provide and following the directions on the label will help ensure your cat's diet is balanced and your cat stays healthy. An occasional taste of cooked boneless beef or brown rice can be an OK treat. But it's best to keep it small and infrequent.