Tuna also contains high levels of unsaturated fats - something humans can, and should include in their diet, but not so much cats. Too much unsaturated fats in a cats diet can leave your cat developing a vitamin E deficiency, leading to an inflammation of the fatty tissue, a condition known as steatitis.
One of the more important reasons your cat shouldn't consume commercial canned tuna is the fact that this "tuna" is not just tuna, but also other added vitamins and minerals and LOTS of sodium - something which can lead to extreme dehydration in your cat.
2. Onions, Garlic & Chives: Onions contain a substance called N-propyl disulphide which destroys red blood cells in the cat. This will eventually lead to a form of anemia called Heinz body anemia. Garlic and chives contain a similar substance but in smaller amounts. Occasional small doses of these three foods will not hurt, but if consumed enough, and in large quantities, can be harmful to your cat.
3. Dairy Products: Milk is not toxic to cats, but it can have adverse effects. Many cats are actually lactose-intolerant and cannot digest the lactose in milk. This leads to symptoms like tummy upset, cramps, and diarrhea. Higher fat milks have less lactose, so if you want to treat your cat to less than a tablespoon once a week, it should not affect their health. There are also safer milk products for cats that are specially designed for them, and which are made without lactose.
4. Grapes & Raisins: There are still many unknowns with the toxic effects of grapes and raisins, however the ASPCA animal poison control centre advises against giving grapes or raisins to pets in any amount. Grapes and raisins have the ability for causing acute (sudden) kidney failure in dogs and cats.
5. Caffeine: Any product containing caffeine may be toxic to cats. The caffeine excites the nervous system and will cause shaking and vomiting or diarrhea.
6. Chocolate: Chocolate contains theobromine which can cause seizures and even death. Domesticated animals metabolize theobromine much more slowly than humans, which leads to complications of theobromine poisoning. Theobromine is also suspected to induce right atrial cardiomyopathy after long term exposure at levels equivalent to around 15 g of dark chocolate per kg of weight and per day. The toxic dose for cats is lower than that for dogs, however cats are less prone to eating chocolate than dogs because they are unable to taste sweetness.
7. Liver: Small amounts of liver are okay, but eating too much can cause vitamin A toxicity. Too much vitamin A in your cats diet can affect their bones (deformed bones, osteoporosis, excessive bone growth on elbows and spine).