Appropriate diets for cats: Part 1 - Obligate Carnivore

The internet is a source of raging debates about suitable diets for cats with many people arguing very passionately that cats should only be fed raw meat because they are obligate carnivores and to feed them any other form of pet food is to endanger their health.

I think that these arguments stem from many people's misunderstanding of the term obligate carnivore.

Some definitions~
The mammal order Carnivora contains a diversity of species, some of which are carnivorous, like cats; many are omnivorous, such as dogs or black bears and a few are herbivorous, like panda bears. The distinguishing feature of carnivora is their dental structure, where the molar and premolar teeth are blade-shaped and adapted for shearing - these specialised teeth are called carnassial teeth.

The word carnivorous refers to organisms whose diet consists of animal tissue and includes many different kinds of animals and plants.
A hypercarnivore is an animal whose diet consists of more than 70% meat and this includes every species of the cat family who are all highly specialised predators.

An obligate carnivore must eat meat to derive certain nutrients, as these are not synthesised by their bodies. In the cat, these are:

Vitamin A - cats have little or no enzymes that will break down the plant-produced carotenoids into retinol.
Vitamin D - Cats also lack sufficient enzymes to meet the metabolic requirements for vitamin D photosynthesis in the skin; therefore they require a dietary source.
Arachidonic Acid is an essential fatty acid that plays a vital role in fat utilization and energy production. Cats cannot make their own even in the presence of adequate linoleic acid.
Vitamin A, vitamin D and arachiodonic acid are found in plentiful amounts in animal fats. Fat is important not only for provision of fuel for energy, but also for increasing palatability and acceptance of food.

Vitamin B3 - Cats obtain Niacin only by eating the preformed vitamin. They cannot convert Tryptophan to niacin and also have very high requirements for Vitamin B6 and B12. Because most water-soluble B vitamins are not stored (except cobalamin - B12, which is stored in the liver), a continually available dietary source is required to prevent deficiencies. Deficiencies are rare in cats eating appropriate diets because each of the B vitamins is found in high concentrations in animal tissue. The exception to this is fish: cats fed large amounts of raw fish can develop thiamine deficiencies.

Arginine - cats lack an intestinal enzyme, pyrroline-5-carboxylate synthase, required for synthesis of the arginine precursor, ornithine. Arginine is required for normal protein synthesis and ammonia detoxification.
Cysteine - cats require this amino acid in large amounts although they are unable to synthesise Taurine from cysteine as many other animals can.
Taurine - is essential for conjugation of bile salts, vision, cardiac muscle function, and proper function of the nervous, reproductive and immune systems. Cats can only conjugate bile acids with taurine to make bile salts.

Another way in which cats are adapted to a carnivorous diet lies in the taste buds of the facial nerve, which are highly responsive to amino acids and unresponsive to many starches and sugars, making diets high in vegetable matter unappealing to cats. In theory, it may be possible to supplement a vegetable-based diet with all the right nutrients to be nutritionally adequate. Good luck with getting a cat to eat it, though

Cats have a very short intestine which means that food is digested and excreted relatively quickly. Normal gut transit time is usually about 13 hours. Omnivores and herbivores have much longer intestines and are thus able to extract nutrients from plant fibres through bacterial fermentation in the colon, whereas cats cannot. Cats have very efficient absorption, even though they have shorter intestines - they are only about 10% less efficient in digestion of complex starches or fibers than dogs, which are omnivores and have longer intestines. Cats can use carbohydrates in their diets although large amounts of carbohydrate decreases protein digestibility and can change composition of bacterial populations in the gut and cause flatulence or diarrhoea.

Cats also lack salivary amylase, the enzyme used in digestion of dietary starches and glucokinase, which is used to transport glucose, but they use other enzymes such as hexokinase and have these enzymes in large amounts. Cats are thus able to utilise cooked carbohydrates in their diet.

Studies have concluded that cat's minimum maintenance requirement of highly digestible protein - partly, organ meat such as liver, and also muscle meat, is equivalent to 26% of metabolizable energy in the diet for kittens and 23% of metabolizable energy for adult cats. While this sounds like it is possible that cats can eat high carbohydrate diets, in practice, these are not palatable to them. However, the often-repeated phrase "cats can't digest or utilise carbs" is untrue.