The reasons vets say tuna is bad are:
1. Human Tuna is high in poly-unsaturated fatty acids. If a cat is fed this as their main diet, these fatty acids oxidize and destroy vitamin E, which can lead to a disease called “steatitis” otherwise known as “yellow fat disease”. This disease is very painful for cats. It causes lumps in the fatty tissue which harden, and the cat feels pain when moving.
2. Human Tuna contains mercury. Mercury is toxic and can cause brain and neurological damage. If a cat is fed canned tuna as their primary food, mercury can build up in their systems.
3. Human Tuna lacks the proper nutrients for cats such as taurine. Taurine is an amino acid which is normally added to cat food (dry and canned). Cats can’t manufacture it from themselves – but it is needed for their heart, digestion, eyes and reproductive systems.
In the wild, cats get taurine from birds and mice (it is found in muscle tissue and it is abundant in brains). Human Tuna also lacks copper, iron, calcium, vitamin A, vitamin B and vitamin E.
4. If you think raw tuna is better – think again. Raw fish contains thiaminase – an enzyme that destroys the thiamine vitamin (B1). Be aware that cooking destroys the thiaminase enzyme so canned tuna doesn’t contain it.
Onions, Garlic, Chives
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.
Milk and Other Dairy Products
What could be wrong with offering your cat a saucer of milk, or a piece of cheese? Although kittens, with their undeveloped digestive system, are able to tolerate milk, most adult cats cannot. Their digestive system is simply not designed to process rich dairy foods, and the result can be digestive upset, and can lead to food allergies (which often manifest as itchiness).
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
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.
Coffee, Tea, and Other Caffeine
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, fits, and bleeding. 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 pain killers.
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.
Candy and Gum
Candy, gum, toothpaste, baked goods, and some diet foods are sweetened with xylitol. Xylitol can cause an increase in the insulin circulating through your cat's body. That can cause the cat's blood sugar to drop and lead to liver failure. Initial symptoms include vomiting, lethargy, and loss of coordination. Eventually, the cat may have seizures, and liver failure can occur within just a few days.
Fat Trimmings and Bones
Table scraps often contain fat trimmed off of meat and bones. Both are dangerous for cats. Fat trimmed from meat, both cooked and uncooked, can cause pancreatitis. 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 that an enzyme in raw eggs, avidin, interferes 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 Fish
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.
Liver can be healthy for a cat if the cat doesn't get too much. 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.
Sugary Foods and Drinks
Too much sugar can do the same thing to cats that it does to humans. It can lead to obesity, dental problems, 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.
Reacting to a drug commonly 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.
Mushrooms can contain toxins, which may affect multiple systems in the body, cause shock, and result in death.
Almonds and nuts in general
Large quantities can lead to stomach upset. Limit salted nuts. Just like with babies, whole nuts can pose a choking hazard.
There are some worries that avocados contain a toxic principle known as Persin. There have not been any clinical tests on what consititutes a toxic dosage and it may be best to avoid avocado completely.
Mace, nutmeg, paprika and turmeric should be avoided completely.
Tomatoes and tomato plants
Tomatoes of all kinds are toxic to cats, as are parts of the tomato plant. Ingesting as little as a cherry tomato can cause severe gastrointestinal upset.