The balance of bacteria and other organisms in the rabbit's digestive tract must be just right to maintain normal motility, digestion, absorption, and elimination. Fibre is an essential aid to this process. An adverse change in the balance of organisms in the digestive tract can lead to diarrhoea. Proper nutrition plays an important role in this balance. Practices of feeding just alfalfa pellets and treats leave a rabbit prone to a variety of ailments such as obesity, foot problems, hairballs, diarrhoea, etc. Grass hay is very important to a healthy digestive tract. Alfalfa pellets have too much calcium and the fibre is not coarse enough for a rabbit's needs.
The cat, in comparison, has a shorter GI tract than that of the rabbit. This means that they do not retain food for very long in the gut and has little capacity for fermentation, due to its almost non-existent cecum.To compensate for this, they have a very efficient rate of absorption. The cat's body does not produce the enzymes necessary for the break down of plant cellulose. The cat's digestive tract is designed to deal with highly digestible foods of animal source; any ingested plant matter is normally passed "unharmed". The cat's digestive juices may be potent, but it is digestion time that breaks down plant matter. The cat passes food much too quickly from stomach to the small intestine and then to the colon, for any digestion of plant matter to happen. Because of this fast absorption of nutrients the cat needs a diet which is rich in energy. They have special requirements for protein and amino acids, which are thought to be a result of their evolution as strict carnivores. Cats need more protein in their diet than dogs and many other mammals. The way they break down amino acids is relatively wasteful, and they are unable to adjust this process even if their diet is low in protein.
Simple differences between animals as herbivores and cats as true carnivores make it impossible for the cat to break down nearly any type of plant matter. Digestion of carbohydrate and cellulose containing plant matter begins in the mouth. The substances are ground up by our chewing motion and fortified with enzymes, in order to break them down manually as well as chemically. The cat possesses no molars, nor the ability to chew or grind foods. Also the lower jaw can only be moved up and down and possesses no ability for a lateral grinding motion. However the rabbits’ teeth consist of four elongated incisor teeth used for biting and gnawing, and several large flat molars, used for grinding up the cellulose rich food. A high protein diet will largely keep the cats mouth free from dental disease as it will keep the cat's body acidic and prevent mineral deposits. The absence of carbohydrates, which are broken down by enzymes in the saliva to sugars, inhibit plaque causing bacteria growth. Any plaque that may naturally accumulate is wiped away by the biting and shearing through animal tissue.
It is possible that grains could be cooked as a means of “pre digesting". The cat's food could be fortified with carbohydrates, which is converted by the cat's liver to glucose as a source of energy. The problem is, however, that the cat converts a large portion of dietary protein (glycogenic amino acids) to glucose for energy, and any excess of glucose from carbohydrate source is stored as body fat which in the long run could be harmful as this may cause obesity and Heart disease. Also starch which is poorly digested may give the cat problems such a flatulence, indigestion, vomiting and diarrhoea. Therefore it is possible to feed a cat a carbohydrate free diet in which energy is derived exclusively from non carbohydrate sources.
Carnivores have been defined through evolution by eating meat raw. Cats fed on a heat processed diet will become deficient in vital nutrients and suffer from innumerable ailments ranging from low immunity, irritability, and allergies; to skeletal deformation, organ malfunction; poor development during kitten hood, low birth rate, birth defects, infertility, and shortened life-span. Meats will supply the cat with nearly everything it needs. As carnivores, cats can not synthesize Taurine and Arginine - amino acids that are only found in animal tissue. Also, the cat lacks the ability to de-saturate polyunsaturated fatty acids and can not convert Linoleic acid into Arachidonic acid as the cats’ liver contains no delta-6-desaturase enzyme to convert linoleic to Arachidonic acid, and therefore must be provided through animal fat.
In particular, organ meats are an important source of essential nutrients - including vitamins and minerals. The liver, for example, presents a rich source of vitamin A - a key vitamin for cats because they can not use carotanoids found in plant matter for vitamin A conversion. As a fat-soluble vitamin it can be stored by the body, and if overdosed may accumulate to toxic levels resulting in hypervitaminosis A - causing severe disease. Because of this risk, cats are often deprived of this vitamin by restricting the feeding of organ meats which leads to severe deficiency, mainly causing as low resistance to disease. Vitamin A is an important antioxidant and immune booster for the cat and essential for the utilisation of protein. Therefore, cats must have a balanced source of vitamin A for them to be able to efficiently use the protein rich diet they need. Cats process little or no enzymes that will break down the plant-produced carotenoids. They must eat preformed active Vitamin A (that is, Vitamin A that already has been converted from carotenoids to its active form by some other creature such as a mouse or rabbit). Rabbits on the other hand have these enzymes which allow for the breakdown of the carotenoids to retinol. Rabbits have the enzymes needed to perform this task in the lining of their intestine.
Another vitamin which a cat can not synthesise is niacin (B vitamin). They can not convert the amino acid tryptphan to niacin, and must also obtain this vitamin through eating animal tissue. The rabbit on the other hand is able to carry out this conversion.
The diagrams below illustrates the fact that the cat has a very small cecum, which is not capable of digesting plant materials, whereas the rabbit has a very large cecum, ideal for the digestion of such material.
