Domestic cats are often more adventurous in their tastes and will often sample vegetables and fruit. There is also a long, but erroneous, tradition of giving cats milk. Cats have not evolved to eat these things. With the exceptions of fruit and vegetables listed below, a little of these does not harm and can address constipation or obesity by adding bulk to the cat's diet. Other food enjoyed by owners should not be given to cats at all. The stimulants in tea, coffee and chocolate that make them so agreeable to us makes them toxic to cats.

The important thing is to know whether your cat has eaten enough to suffer ill-effects. Some foods are toxic if only a small amount is eaten while others won't cause illness unless eaten repeatedly or in a large amount. Compared to humans and dogs, cats have poor liver function. In evolutionary terms, they don't need good liver function as they rely on their prey to have broken down any harmful substances found in vegetable matter.

The following common human foods are bad for cats. Some foods are harmful in excess and should only be given in moderation or as treats. Many of the lists of harmful foods online are combined dog and cat lists. Cats are not scavengers to the same extent as dogs and are less likely to eat many of the foodstuffs listed as harmful. I found that some of the data in other lists comes from cattle and laboratory rats - two creatures with very different digestive systems to cats - that were deliberately fed excessive amounts of a foodstuff. Where applicable I have included the information, but with notes questioning its usefulness related to cats.

Alcohol

Compared to humans, cats have poor liver function and are less able to detoxify substances. Cats have become tipsy on red wine sauce from meal leftovers. While a single occasion might not cause long term ill-effects, large amounts or repeated amounts of alcohol will cause liver damage. As well as intoxication and gastrointestinal irritation; alcohol poisoning will result in breathing difficulties, coma and death.

Bach Flower Remedies are popular with many cat owners, but the main ingredient of these is alcohol used as a solvent (brandy according to the ingredients list). If used, they should be given very sparingly and infrequently. There are now alcohol-free versions of the remedies that are safer for pets.

Avocado

All parts of the avocado contain a toxin known as persin. It causes gastrointestinal irritation, vomiting, diarrhoea, respiratory distress, congestion, fluid accumulation around the tissues of the heart and even death. Cats may be attracted to the oily texture, but should not be fed avocado pieces (in salads) or avocado dips.

Broccoli

Broccoli toxicity was noted in cattle fed excessive amounts of it. Where the amount of broccoli in the diet exceeded 10% it caused gastrointestinal upsets. Where it exceeded 25% it was fatal. Since no-one is likely to feed a cat this amount of broccoli in the diet there are no reports of broccoli toxicity in cats. Their very different digestive system means they wouldn't digest broccoli as thoroughly as a cow and might not absorb the isothiocyanate toxin. Fed in small amounts, broccoli contains beneficial bioflavinoids.

Cats are not cattle and, being carnivores not herbivores, are not likely to consume sufficient broccoli to cause illness.

Chocolate, Cocoa

Chocolate contains theobromine, a substance very toxic to cats. Theobromine is a cardiac stimulant (makes the heart beat faster and irregularly) and a diuretic (makes the animal pee more). Once in the bloodstream, it causes hyperactivity and thirst. After several hours it may cause vomiting and diarrhoea. It can lead to a fatal heart attack within 24 hours of eating the chocolate.

Weight for weight, cocoa powder, cooking chocolate and high cocoa brands of chocolate (gourmet brands) are most dangerous as these are concentrates and contain far more theobromine. Ordinary bittersweet chocolate and dark/plain chocolate are the next most dangerous forms, with milk chocolate being the least dangerous (but still not safe to give to cats).

Don't be tempted to share chocolate cake or confectionery with your pet. There is no reason to give your cat chocolate - it doesn't need it! Although there are special chocolate-style treats, these are made to be attractive to owners who like to treat their pets as children. They are not healthy, have no place in the normal feline diet and cats don't need them.

In my personal experience, I have seen possible neurological effects in a cat that ate a small piece of chocolate confectionery. Shortly after eating the chocolate, the cat showed signs of disorientation and walked backwards. Luckily the effect was temporary.

Coffee and Tea

Coffee grounds, coffee beans and tea all contain caffeine, a stimulant. Some cats are attracted to milky tea and coffee. Cats are not children - there is no reason to give them tea or coffee to drink. It's not cute and it's not safe. I have seen a cat become hyperactive after licking espresso from a cup.

Grapes and Raisins

The toxicity of grapes and raisins to cats isn't known, but they are known to be toxic to dogs. The ASPCA Animal Poison Control Center (A USAnian body) advises not giving grapes or raisins to cats or dogs in any amount. That means not sharing your breakfast muesli or fruit cake, however much your cat is begging.

Hops (Spent)

There is a lot of confusion over hop toxicity. In 1995, the National Animal Poison Control Centre in Illinois recorded 8 cases of fatal hop toxicity in dogs that consumed spent hops from home-brew kits. The result was an uncontrollable rise in body temperature. For many years, cats at an cat shelter near me have been able to nibble fresh green hops on a vine growing up the side of a fence with no ill-effects.

While hop residue from beer-making can cause raised temperature, increased heart rate, seizures and even death (at least in dogs), there's no evidence that fresh or dried hops are harmful.

Liver

Large amounts of liver can cause Vitamin A toxicity. This affects muscles and bones and can cause abnormal bone growth, particularly noticeable on the spine and neck region.

Milk

Milk has adverse effects on most cats. Milk is a baby-food and is not a natural food for adult cats. Cow's milk is suitable for cows (and many humans), but contains lactose which adult cats cannot digest. This results in stomach upsets, diarrhoea and intestinal discomfort. Many cats appear to like milk and there is a longstanding tradition of giving milk to cats.

The higher the fat content in the milk, the less lactose it contains so a small treat of cream may be safe for some cats. Some cats can tolerate evaporated milk as the high temperatures have changed the composition. Alternatively, use a lactose-free milk such as Lactolite (made for humans), Whiskas or Felix (made for cats). Evaporated milk may also be tolerated. Goats milk and sheep milk are also alternatives.

Cats that have a balanced diet and water available really don't need milk and it's best not to get into the habit of giving them milk.

Nuts

Many nuts are not good for cats and the high phosphorus content may cause problems. Walnuts are reported to be toxic to pets.

Raw and roasted macadamia nuts are harmful to pets. There is no data for cats, but there are toxicity cases in dogs. Dogs that ate between 6 and 40 macadamia kernels or macadamia nut butter developed temporary muscle tremors and hindlimb weakness/paralysis. Some had swollen and painful limbs. They became unable to get up and were visibly distressed. Although the effects were temporary, they were painful and distressing. With their less effective livers, cats are also at risk from macadamia nuts and related products.

While cats may be attracted to the oily texture of some nuts, it is wisest not to allow them to eat nuts; especially macadamia nuts.

Onions, Shallots, Spring Onions, Garlic etc

Onions contain disulphides (or thiosulphate) that destroy red blood cells in the cat, causing a form of anaemia called Heinz body anaemia (a haemolytic or red-cell-bursting anaemia). All forms of onion are toxic: raw, dried, powdered or cooked. Ready meals, takeaways and baby foods containing onion or onion powder must not be fed to pets as treats. Baby food such as "beef dinner" or "lamb dinner" is sometimes fed to cats as a tonic, but the onion powder used to add flavour will do more harm than good. Onion powder may be used as a flavouring in instant gravy.

Garlic contains a similar substance in a lesser amount and much larger quantities would be needed to cause illness. In some continental countries owners have added a little garlic to a pet's meals on the basis that garlic is good for humans so it must be good for pets as well. With their very different liver function, foods harmless to humans can be deadly to cats.

Onion toxicity results in haemolytic anaemia, where the red blood cells burst while circulating in its body. Symptoms occur a few days after eating onion. The first symptoms are generally gastroenteritis with vomiting and diarrhoea, loss of appetite and lethargy (because the oxygen-carrying red blood cells are damaged). The red pigment from the burst blood cells is excreted in the urine and it becomes breathless as there are fewer cells to transport oxygen around the body.

Onion poisoning can result after a single meal containing a large amount of onion relative to body size or repeated meals containing smaller amounts of onion.

Chives are sometimes used as a flavouring in commercial cat foods. Although potentially toxic to cats (in large quantities), chive leaves are eaten rather than the bulbs. Some cats are attracted to chives and will eat them from the herb garden. However, I would prefer that the Co-Op did not produce a chicken and chives variety as it gives a mixed message to owners. For safety, the onion family is best avoided altogether.

Pips and Kernels: Pear/Apple Pips, Apricot/Plum/Peach/Nectarine Kernels

Pear pips, apple pips and the kernels of drupes (plums, peaches, nectarines, apricots and related fruits) all contain cyanogenic glycosides that can result in cyanide poisoning. Cyanogenic glycosides interfere with the ability of the blood to release oxygen into the tissues, resulting in suffocation even though there is oxygen in the bloodstream.

Apricot kernels are currently a trendy food for humans, but are dangerous to humans in large amounts (most health shops recommend eating no more than 6 kernels per day). They are even more dangerous to cats due to their liver function and smaller bodyweight. There is no truth in the claim they prevent or cure cancer in humans or any other creatures. The same applies to other drupes.

Raw Eggs

Raw eggs contain an enzyme called avidin and excessive amounts of raw egg white could cause a biotin (B vitamin) deficiency resulting in skin and hair problems. This data came from laboratory rats that were fed abnormally large amounts of raw egg white proportional to their bodyweight. It is unlikely cats would consume dangerous quantities of avidin unless you fed them raw eggs at every meal.

In practice, wild cats will include eggs in their diet. As long as the cat has a varied diet and sources of biotin and is not fed excessive amounts, an occasional raw egg will do no harm. Raw egg yolk has long been considered a tonic.

The quality of eggs is variable and eggs may be a potential salmonella risk. If you're worried about feeding occasional eggs, it can be lightly scrambled. I have fed whisked raw egg (sometimes whisked with goat's milk) as a treat two or three times a month with no ill-effect.

Raw Fish

Feeding raw fish regularly can result in thiamine (vitamin B1) deficiency due to the enzyme thiaminase in the fish. It results in loss of appetite, seizures and (in severe cases) death. Although an occasional treat of raw fish is generally not harmful, over-consumption will cause problems. Cooking the fish will destroy the enzyme.

Raw salmon in parts of North America may carry flukes and Neorickettsia helminthoeca. This can cause salmon poisoning disease in pets. It has been reported in dogs rather than cats, but results in lethary, high temperature, vomiting and bloody diarrhoea and may mimic infectious enteritis. The mortality rate is around 90% if untreated. It is preventable by cooking the salmon.

Some fish also contains environmental pollutants e.g. PCBs or mercury; these are not removed by cooking, but their levels may be higher in raw fish that have not been gutted (the cat may eat the fish organs as well as the flesh). Mercury poisoning has been reported in cats in a coastal region of Japan where fish was a main ingredient of the diet. If fed in excess, oily fish such as tuna can lead to steatitis (Yellow Fat Disease, pansteatitis).

Many feral cats near fishing ports supplement their diets by scavenging raw fish. As long as the raw fish was not eaten in excess and contained no harmful contaminants the cats did not appear to suffer ill-effects.

Ready Meals, Prepared Foods

As with all treats, they should only be given in small amounts (once the ingredients have been determined cat-safe). Too much liver will cause vitamin toxicity and abnormal bone growth (these can be lethal). Too much canned tuna can cause a painful inflammation of the fatty tissue.

Rhubarb

Rhubarb leaves contain oxalates. Eating rhubarb leaves leads to oxalic acid poisoning which can cause kidney failure. The cooked rhubarb stem is safe to eat, but is very high fibre (that's why humans eat it for the laxative effect) and may cause indigestion, intestinal discomfort and diarrhea.

In practice, cats are unlikely to eat rhubarb leaves due to the taste. Some cats will chew on leaves without swallowing them.

Spoiled Food, Mould and Bacteria

Spoiled foods harbour harmful bacteria and their breakdown products. Many of these can cause illness in humans (e.g. Clostridium perfringens, E coli, Bacillus cereus to name just a few). Moulds on food can also contain toxins.Unless starving, cats will generally avoid stale or spoiled foods. When they do eat spoiled or stale food, their first line of defence is to vomit it back up again. If the food smells "off" to you, don't even think of giving it to your cat.

Tomatoes and Potatoes

Tomatoes are members of the Solanaceae family of plants and are related to Deadly Nightshade. They contain a bitter, poisonous alkaloid called glycoalkaloid solanine that can cause violent lower gastrointestinal symptoms. Generally cats aren't attracted to tomatoes, but there have been reports of a single cherry tomato causing a near-fatal reaction. Green tomatoes and the leaves and stems are all toxic.

The toxin is destroyed by cooking so the tomato juice in cans of sardines, pilchards and other fish is safe to eat. Some cats also like the juice from cans of baked beans, but these may contain harmful preservatives. As with all canned treats - read the label.

Like tomatoes, potatoes are members of the Solanaceae family of plants and are related to Deadly Nightshade. They contain a bitter, poisonous alkaloid called glycoalkaloid solanine that can cause violent lower gastrointestinal symptoms. Uncooked or green potatoes and raw potato peelings are all toxic.

Once cooked, the alkaloid is destroyed making the potato safe. Cooked mashed potato can be safely mixed into canned food as a bulking agent for overweight cats. It goes without saying that green potatoes should be discarded.

Tuna

If fed in excess, tuna can lead to steatitis (Yellow Fat Disease, pansteatitis). This painful inflammatory condition results from a diet high in unsaturated fatty acids & deficient in Vitamin E; over-consumption of oily fish is the main cause in cats. Tuna seems addictive to cats, but should be limited to special treats only. Tuna contains little vitamin E and the excessive unsaturated fatty acids further deplete vitamin E in the body.

Cats with steatitis develop flaky skin and a greasy, dull coat. They show signs of severe pain when touched and are reluctant to move. They also lose their appetites and develop fever. If untreated, it results in death.

Yeast Dough

There is an urban legend that yeast-based bread dough will expand so fast in the warmth of the stomach that it rises into the throat and causes suffocation. Unless a huge amount is eaten (more of a problem in indiscriminate eaters such as dogs than in fastidious eaters such as cats), the stomach acids prevent this. Yeast dough can cause bloating, indigestion and discomfort, but is unlikely to expand so much it will rupture the stomach (there is only one case on record, this in a dog). The TV show Mythbusters showed just how much a stomach can expand before it ruptures - you or your cat will vomit from discomfort before the stomach gets close to rupturing! Even so, yeast dough is not a suitable foodstuff for a cat.

OTHER HARMFUL THINGS

Medication

Keep human medication well away from cats. Common drugs such as aspirin and paracetamol are poisonous to cats. Warfarin and other commonly used blood thinners are dangerous; even if the dosage itself is non-fatal, it can prevent blood clotting if the cat scratches itself. The sugar coating on tablets can be attractive to some cats.

Smokers' Items/Nicotine

Although not a foodstuff, it should go without saying that snuff, tobacco, cigarettes and cigars should also be kept well away from cats. The same applies to smoking substitutes such as nicotine patches.

PORK

It is often claimed pork (and ham, bacon etc) shouldn't be given to cats. In fact pork products are used in pet-foods - sometimes this appears on the label ("Chicken and Ham", "Chicken and Bacon", "... and Pork") and other times it is hidden as part of the meat byproducts and derivatives.

The idea of pork being unhealthy (as opposed to unclean) relates to tapeworm that may be carried by pigs. Historically, undercooked pork was a source of infection to humans. As with all meats, the pork should be cooked before feeding it to your cat. Bacon is often too salty to safely feed to a cat. Cooked pork or ham is safe to feed as a treat. If you allow your cat to eat pieces of sausage, make sure it doesn't contain onion powder or leeks as these are toxic to cats.

Pig meat products can be found in commercially prepared pet foods. Ham and pork passed fit for human consumption and properly cooked is safe for cats to eat. Raw pork is best avoided.