The following is a list of foods that cats should never eat:
Onions, Garlic, & Related Root Vegetables
Onions contain a substance (N-propyl disulphide) which destroys red blood cells in the cat, causing a form of anemia called Heinz body anemia. Garlic contains a similar substance in a lesser amount.
Tomatoes, Green (raw Potatoes)
These foods are members of the Solanaceae family of plants, which includes the Deadly Nightshade, and contain a bitter, poisonous alkaloid called Glycoalkaloid Solanine, which can cause violent lower gastrointestinal symptoms. The Feline Future web site offers a rare description of a cat which was close to death from ingesting just one cherry tomato (See the link on the sidebar).
Chocolate
It's becoming more widely known that chocolate is very toxic to both cats and dogs. Theobromine is the offending substance here. Janet Tobiassen Crosby, D.V.M. has an excellent article on the symptoms, effects, and treatment of chocolate toxicity.
Grapes and Raisins
These foods' toxicity has mainly been found in dogs, in quantities of varying amounts. The ASPCA advises: "As there are still many unknowns with the toxic potential of grapes and raisins, the ASPCA Animal Poison Control Center advises not giving grapes or raisins to pets in any amount." That's good enough for me.
Milk
Although milk is not toxic to cats, it may have adverse effects. Simply put, adult cats fed a nutritious diet don't need milk, and many cats are lactose-intolerant, which means that the lactose in milk and milk products produces stomach upset, cramps, and gassiness. If your cat loves milk, and begs for it, a small amount of cream may be okay, two or three times a week. (The more fat in the milk, the less lactose.) Another compromise is CatSip, a product made from skim milk with an enzyme added that helps the digestion of lactose. Catsip is available in supermarkets such as Safeway, Albertson's and A&P, as well as pet products chains, such as PetSmart and Petco.
These are the most commonly seen "people foods" that are potentially harmful to cats. The bottom link is to feed your cat nutritious food developed with his needs in mind and choose treats designed for cats instead of table scraps.
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