Diet And Feeding
Cats are classified as obligate carnivores, because their physiology is geared toward efficient processing of meat, and lacks efficient
processes for digesting plant matter.
The cat cannot produce its own taurine (an essential amino acid) in its own body and as it is contained in flesh, the cat must eat flesh to survive. Similarly as with its teeth, a cat's digestive tract has become specialized over time to suit meat eating, having shortened in length only to those segments of intestine best able to break down proteins and fats from animal flesh. The trait severely limits the cat's ability properly to digest, metabolize, and absorb plant-derived nutrients, as well as certain fatty acids.
For example, taurine is scarce in plants but abundant in meats. It is a key amino sulfonic acid for eye health in cats. Taurine deficiency can cause a condition called macular degeneration wherein the cat's retina slowly degenerates, eventually causing irreversible blindness.
Despite the cat's meat-oriented physiology, it is still quite common for a cat to supplement its carnivorous diet with small amounts of grass, leaves, shrubs, houseplants, or other plant matter anyway. One theory suggests this behavior helps cats regurgitate if their digestion is upset; another is that it introduces fiber or trace minerals into the diet. In this context, caution is recommended for cat owners because some houseplants are harmful to cats. For example, the leaves of the Easter Lily can cause permanent and life-threatening kidney damage to cats, and Philodendron are also poisonous to cats. The Cat Fanciers' Association has a full list of plants harmful to cats.
An unsupplemented vegetarian diet cannot meet a cat's dietary requirements. Nevertheless, there are several vegetarian or vegan commercially-available cat foods supplemented with chemically-synthesized taurine and other added nutrients that attempt to address nutritional shortfalls.
Additionally, cats have been known to develop a fondness for prepared human foods, normally such entrees which are rich in proteins or fats. However, a diet consisting only of human food (even if high quality meat) is unlikely to contain the balanced nutrition required by the cat. Cats normally are good self-regulators of diet; however, unlimited access to food, or excessive human-food 'treats', will often lead to the cat becoming obese, particularly if it is older or more sedentary. This may lead to several health complications, such as diabetes, especially in neutered males. Such health conditions can be prevented through diet and exercise (playing), especially for cats living exclusively indoors.
Cats can be selective eaters (which may be due in some way to the aforementioned mutation which caused their species to lose sugar-tasting ability). Unlike most mammals, cats can voluntarily starve themselves indefinitely despite being presented with palatable food, even a food which they had previously readily consumed. This can happen when the vomeronasal or Jacobson's organ becomes accustomed to a specific food, or if the cats are spoiled by their owners, in which case the cat will reject any food that does not fit the pattern it is expecting. It is also known for cats to merely become bored with their given food and decide to stop eating until they are tempted into eating again. Although it is extremely rare for a cat to deliberately starve itself to the point of injury, the sudden loss of weight can cause a fatal condition called hepatic lipidosis, a liver dysfunction which causes pathological loss of appetite and reinforces the starvation, which can lead to death within as little as 48 hours.
Some cats have a fondness for catnip, which is sensed by their olfactory systems. While they generally do not consume it, they will often roll in it, paw at it, and occasionally chew on it. The effect is usually relatively short, lasting for only a few minutes. After two hours or less, susceptible cats gain interest again. Several other species of plants (such as mint and valerian) cause this effect, to a lesser degree.
Cats can also develop pica. Pica is a condition in which animals chew or eat unusual things such as fabric, plastic or wool. In cats, this is mostly harmless as they do not digest most of it, but can be fatal or require surgical removal if a large amount of foreign material is ingested (for example, an entire sock). It tends to occur more often in Burmese, Oriental, Siamese and breeds with these in their ancestry.
Toxic sensitivity
The liver of a cat is less effective at detoxification than those of other animals, including humans and dogs; therefore exposure to many common substances considered safe for households may be dangerous to them. In general, the cat's environment should be examined for the presence of such toxins and the problem corrected or alleviated as much as possible; in addition, where sudden or prolonged serious illness without obvious cause is observed, the possibility of toxicity must be considered, and the veterinarian informed of any such substances to which the cat may have had access.For instance, the common painkiller paracetamol or acetaminophen, sold under brand names such as Tylenol and Panadol, is extremely toxic to cats; because they naturally lack enzymes needed to digest it, even minute portions of doses safe for humans can be fatal and any suspected ingestion warrants immediate veterinary attention. Even aspirin, which is sometimes used to treat arthritis in cats, is much more toxic to them than to humans and must be administered cautiously. Similarly, application of minoxidil (Rogaine) to the skin of cats, either accidental or by well-meaning owners attempting to counter loss of fur, has sometimes proved fatal.
In addition to such obvious dangers as insecticides and weed killers, other common household substances that should be used with caution in areas where cats may be exposed to them include mothballs and other naphthalene products, as well as phenol based products often used for cleaning and disinfecting near cats' feeding areas or litter boxes, such as Pine-Sol, Dettol (Lysol), hexachlorophene, etc. which, although they are widely used without problem, have been sometimes seen to be fatal. Ethylene glycol, often used as an automotive antifreeze, is particularly appealing to cats, and as little as a teaspoonful can be fatal.
Many human foods are somewhat toxic to cats; theobromine in chocolate can cause theobromine poisoning, for instance, although few cats will eat chocolate. Toxicity in cats ingesting relatively large amounts of onions or garlic has also been reported. Even such seemingly safe items as cat food packaged in pull tab tin cans have been statistically linked to hyperthyroidism; although the connection is far from proved, suspicion has fallen on the use of bisphenol A-based plastics, another phenol based product as discussed above, to seal such cans.
Many houseplants are at least somewhat toxic to many species, cats included and the consumption of such plants by cats is to be avoided.