By Kymythy R. Schultze, CN
This is by no means a list of everything on the planet that's bad for cats. But it's a few of the things that should be avoided themselves or as an ingredient in something else.
Alpha lipoic acid. Also referred to as lipoic acid. It's an antioxidant that increases production of glutathione and can cause gastrointestinal distress and low blood sugar in cats.
Chocolate. This contains the alkaloid theobromine which is toxic to cats. It also contains caffeine and may contain sugar.
Dairy. Dairy products include milk, cream, butter, cheese, cottage cheese, yogurt, whey, sour cream, kefir, casein, and ice cream. Milk is a hormonal growth fluid produced by a mother for her young of the same species. Cats do not "milk" cows (or mice) and, after weaning, have no need for dairy products. A weaned cat isn't equipped with the enzymes needed to digest the protein and sugar in dairy products. Plus, studies with cats show that casein (a protein in milk) interferes with the absorption of other nutrients.
Drugs. No, not all drugs of course. But there are many drugs that are safe (relatively speaking) for some other animals that are very harmful to cats. Examples include aspirin, acetaminophen, antihistamines, decongestants, ibuprofen, NSAIDs, salicylates, and sodium phosphate enemas. Always be certain that any drug you are considering is specifically safe for cats. And always check the drug insert or the Internet for possible side effects and contraindications.
Essential Oils. Cats are very sensitive to the potent essential oils that may be used around other animals, you included. If you wish to use aromatherapy for your cats, look into the more dilute hydrosols. Use caution with cleaning products that contain essential oils. Also make certain your cat doesn't get into potpourri.
Grapes and Raisins. These are tricky because no one is certain yet why there have been recent reports of toxicity due to grape and raisin ingestion. Until we know more, they're best avoided.
Houseplants. There are too many houseplants that are toxic to cats to name here. Some aren't toxic but, personally, I presume one is until I find out otherwise. Let your cat know the only plant it may call its own is a nice pot of wheatgrass or "cat grass".
Herbs. Let me be very clear that not all herbs are bad for cats. But, because there are quite a few that are not recommended for them, I want to make sure you check an herbal reference book for cats to be certain the ones you're interested in are safe for felines. There are many wonderful herbs that can be very useful in cat health. But, be sure they are specifically safe for cats before using.
Onions and Garlic. Ingestion of onions and garlic are related to destruction of red blood cells. They may also irritate the gastrointestinal system. There's still debate about the harmful effects of garlic as many people including holistic vets have used it in cats without problem, but I do want to inform you of the potential risk. Our cats could probably ward off a vampire without the aid of garlic anyway!
Raw Salmon. Salmon poisoning is an infectious disease caused by a rickettsia that uses a parasitic fluke on salmon as a host. It can cause serious illness and death.
Soy. Soy is found in various forms in many products. It contains compounds that may negatively affect cats by interfering with nutrient absorption, normal growth, thyroid function, and hormonal development.
Sugar. Much research concludes that cancer cells thrive on sugar as do many other disease processes. And sugar comes in many forms, including beet, raw, brown, cane, fructose, corn sweetener, corn syrup, date, dextrin, dextrose, glucose, lactose, maltose, manitol, polydextrose, sorbital, sorghum, sucanat, sucrose, turbinado, barley malt, molasses, honey, and maple syrup. Xylitol, a sweetener made from carbohydrate should also be avoided.
Yeast. This is a fungus that many cats cannot tolerate. It may cause allergic reactions, bloating, digestive and urinary problems. Different forms include brewer's, nutritional, baker's, torula, and primary yeasts.
Article courtesy of Pet360 
Kymythy Schultze has been a trailblazer in animal nutrition for over two decades. She is also a human nutritionist and has helped thousands of people and their pets live happier, healthier lives. To learn how to easily prepare healthy meals for dogs, cats, and humans, check out her best-selling books: Natural Nutrition for Dog and Cats: The Ultimate Diet, The Natural Nutrition No-Cook Book: Delicious Food for You and Your Pets! and her new book Natural Nutrition for Cats: The Path to Purr-fect Health. Visit her website at www.kymythy.com .