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The statement “cats are not small dogs” is nowhere more appropriate than when it comes to the nutritional requirements of this species.

The major difference between dog and cat nutrition is that the cat is a carnivore (hunts on demand) versus a dog that is omnivorous. Cats would take very long to eat one meal (up to 3 hours) where as a dog will eat the whole meal within minutes.

With this in mind it is thus quite normal for a cat to eat about 20 times a day.

Cats are also very sensitive when it comes to taste and smell (palatability) of a food.


The differences between canine and feline digestion and absorption

Digestive Anatomy

The differences between these two monogastrics in their food assimilation, stems from the fact that dogs are omnivores, whereas cats are carnivores.

There are no significant variations between the dog and cat’s digestive anatomy, with the exception of a few characteristics. These include the cat’s intestinal lengths being notably shorter than the dog’s, the cat having a uniform gastric mucosa and the canine stomach having a distinct proximal and distal compartment, and the cat having a slightly less developed caecum. The first mentioned is due to the obvious body size limitation and the fact that carnivores tend to have lower intestinal length to body size ratios than omnivores. The latter are allowed more time for digestion of vegetative foods by their longer intestines.

Even though the cat has a shorter intestinal length, it has a higher potential absorptive capacity than the dog. Burger mentioned that Barry (1976) attributed this to the fact that higher surface area to bodyweight ratios are seen in animals with carnivorous diets than those exhibiting herbivorous habits. The dog, being an omnivore therefore falls between the two.

Regarding the caecum, the dog caecum exists as a more developed, blind-ended sac (diverticulum), whereas the cat’s is merely a rudimentary attachment.


Oral digestion

The dog and the cat’s saliva differs from humans in that they lack the enzymes áamylase, which is responsible for the initiation of starch digestion in the mouth.

This explains why dogs tend to swallow most foods rapidly with minimal chewing and why cats select for a low starch diet (Burger, 1996). Cats and dogs have the same number of incisors and canines, but the differences in their dentitions are most evident by the lack of grinding molars in the cat, consistent with that of a true carnivore. The dog also has a greater total number of molars and premolars than the cat. Dogs therefore have 42 permanent teeth, whereas the cat only has 30.


Digestion in the stomach

Vomiting tends to be quite a common occurrence in dogs, due to their well-developed vomiting centre. This allows for an effective defence mechanism whereby toxins can be expelled from the gut. Dogs tend to be “meal” feeders and cats tend to be “snack” feeders, therefore, the fact that the proximal stomach is capable of great expansion for temporary food storage, is highly significant in dogs. On the issue of enzymatic differences, the cat relies more on pepsin than the dog, since pepsin is involved in collagen digestion (Burger, 1996).


Differences in carbohydrate digestion and absorption in the dog and cat

The activity of pancreatic amylase is about three times higher in the dog than the cat, hence dogs adapt to high levels of dietary starch more rapidly than cats. Cats also exhibit lower activity of the brush border enzymes. Burger (1996) illustrated the effects of this by quoting Kienzle’s (1989) finding that cats can therefore only tolerate starch levels up to 4 g/kg bodyweight before diarrheoa results, whereas dogs are able to consume up to 2,5 times that level without any side-effects. A further difference is that cats are relatively unresponsive to varying levels of carbohydrate intake, while dogs are able to regulate the rate at which their small intestines absorb monosaccharides, in response to different starch levels.


The large intestine

Since the large intestine of the dog and the cat is not expected to digest polysaccharides, it is considerably shorter than in herbivores. The difference in caecum size between the dog and the cat has already been discussed. Ileo-caecal flow of water in the dog and cat is relatively high due to the low dry matter content of the ileal chyme (approximately 20%) (Burger, 1996). Residence time of undigested food residues in the large intestine is approximately 12 hours in the dog, with only 8% of total food digestion occurring in the large intestine. Due to the limited length of the canine’s large intestine, fibre fermentation is only ± 7-35%, while starch digestion varies from 15-100%, depending on the nature of the starch reaching the lower intestines.

Fat reaching the lower intestine is usually minimal, due to the efficient pre-caecal fat digestion in dogs and cats. This is a benefit considering fat inhibits bacterial fermentation. Sufficient levels of fibre in the dog and cat tends to prevent diarrheoa and constipation, with the normal water content of faeces being in the range of 65-75% (Burger, 1996). Bacterial fermentation and the presence of micro-organisms in the large intestine will be reserve for the section on pro- and prebiotics.


Water requirements

Cats can withstand acute dehydration slightly better than dogs. Homeostatic control of water balance in cats differs in some important respects from that of dogs.

Cats make less precise and rapid compensatory changes in voluntary water intake than dogs in response to change in the water content of their food. This apparent weakness of the cats thirst drive to respond to changes in their state of hydration has led to the conclusion that feeding moist food assures adequate hydration (Max’s House, Feline Nutrition, 2001).


Other Eyes

Cat eyes are well adapted to hunting. Their visual acuity is greater than that of dogs because of their larger optic cortex (Kirk, Debraekeleer and Armstrong, 2000).


Limbs

The retractable claws of cats are a unique adaptation of hunting. In contrast, the claws of dogs play only á secondary role in capturing prey.


Oral cavity

Cats and dogs have the same number of incisor, prominent canine and carnassial teeth. However, cats have fewer premolar and molar teeth and they do not possess tissured crowns. The jaws of cats have limited lateromedial and craniocandal movement, thereby limiting grinding ability.

Cats lack salivary amylase used to initiate digestion of dietary starches.


Stomach

Because cats eat small but frequent meals, the stomach is less important as a storage reservoir compared to the stomach of dogs (Kirk, et. al.,2000).


Small and large intestine

Intestinal length, as determined by the ratio of intestine length to body length, is markedly shorter in cats (4:1) versus dog (6:1). The absorptive capacity is about 10% less than that of dogs (Kendall, et. al., 1982). The sugar transport system of the small intestines of cats is not adaptive to vary amounts of dietary carbohydrate.

Certain amino acid transporters in the SI of cats are highly adaptable, particularly the transporter responsible for arginine uptake. This demonstrates the need for higher protein inclusion level in cat foods.

Cats cannot synthesize sufficient amounts of ornithine or citrulline within the intestine, who are precursors to arginine, thus making an absolute requirement for arginine in cat food. Due to a vestigal caecum and short colon, limits the cats ability to use poorly digestible starches.


Protein metabolism

The high protein requirement of cats is due to the high activity of hepatic enzymes responsible for ureagenesis (conversion of ammonia to urea in the liver). The cat cannot decrease the activity of these enzymes when it is fed a low protein diet, thereby continually breaking down proteins for shunting through the urea cycle regardless of the levels of dietary protein.


Amino acids

Amino acids which can be assimilated in the liver from carbon and nitrogen precursors are labelled non-essential and those which must be obtained from dietary sources are called essential amino acids.

Taurine is an essential B-aminosulphonic acid which the cat is unable to synthesise sufficiently. A taurine deficiency can lead to dilated cardiomyopathy, central retinal degradation and reproductive failure.

Taurine is well-supplied in a meat-based diet and is therefore not a limiting factor in the natural diet of cats. Cats with taurine deficiency should be supplemented with 250-500mg of taurine twice daily to replete tissue stores (Kirk et. al., 2000). Usually 50mg per day is required.

Arginine is an essential amino acid that is needed for the conversion of ammonia to urea. The cat lacks adequate levels of the enzyme responsible for the synthesis of ornithine, an arginine precursor. Deficiency of arginine in the diet results in hyperammoneamia with severe and acute clinical symptoms. The deficit is unlikely to occur in cats eating a meat-based diet and is only seen in experimental animals fed a diet containing casein as the sole source of protein (Johnston, 1996; Lewis et. al., 1992; MacDonald et. al., 1984; Morris & Rogers, 1991). The suphur containing amino acids methionine and cystine are required in higher amounts by cats compared to other species during growth (Kirk et. al., 2000).


Carbohydrate metabolism

Enzymes of gluconeogenesis are maintained at high levels of activity to allow metabolism to proceed normally after ingesting carbohydrate-free meals. In most mammals, the hepatic enzymes hexokinase and glucokinase work together to maintain blood glucose levels within the physiological range. In the cat, however, it is unusual for the body to encounter high levels of simple sugars from carbohydrate metabolism and glucokinase is virtually absent in this species. Hexokinase is present in normal amounts but has optimal activity at low concentrations of glucose.

Which is consistent with metabolic adaptation to a high protein, low carbohydrate diet. An alternative gluconeogenic pathway exists in the feline liver which utilises the amino acids serine and glycine, to compensate for the limited ability of the cat to utilise carbohydrates (Johnston, 1996; Morris & Rogers, 1991; Lewis et. al., 1992).


Fat metabolism

Cats have the ability to digest and utilise high levels of dietary fat (as is present in animal tissue). They have a special need for arachidonic acid since they cannot synthesise it from linoleic acid as can the dog. A deficiency of archidonic acid can produce poor growth, a starting coat, skin lesions, fatty liver and congenital defects.

Animal source fats are high in arachidonic acid (MacDonald et. al.,1984; Lewis et. al., 1992).


Vitamin metabolism

Cats do not convert tryptophan to niacin, due to increased activity of an enzyme which diverts trypophan away from the niacin-synthesising pathway. As a result, the niacin requirements of cats is four times higher than that of dogs. Animal tissue is high in niacin. Performed vitamin A is a dietary essential for the cat because they cannot beta-carotene (present in plants) to vitamin A (Morris & Rogers, 1991; MacDonald et. al., 1984).


References

BURGER, I.H., 1996. The Waltham Book of Companion Animal Nutrition. Butterworth – Heinemann, Oxford, UK.

JOHNSTON, K.L., 1996. Peculiarities of protein metabolism in the cat. Vetmed Profiles (Pet Nutrition) 9 (1):20-21.

KENDALL, P.T., Holme, D.W and Smith, Ph.M., 1982. Comparative evaluation of net digest and absorptive efficiency in dogs and cats fed a variety of contrasting diet types. J. Small Anim. Prac. 23:577.

KIRK, C.A., DEBRAEKELEER, J. and ARMSTRONG, P.J., 2000. Normal cats. Chapt. 11 In: Small Animal Clinical Nutrition, 4th Ed. Hand, M.S., Thatcher, C.D., Remillard, R.L., Rondebush, P., Morris, M.L. and Novotny, B.J. Mark Morris Institute, Walsworth Publishing Company Marceline, Missouri, USA.

LEWIS, L.D, MORRIS, M.L., HAND M.S., 1992. Small Animal Clinical Nutrition III. Morris & Ass.

MACDONALD, M.L., ROGERS, Q.R., MORRIS, J.G., 1984. Nutrition of the domestic cat, a mammalian carnivore. Annual Review of Nutrition 4:521-562.

MORRIS J.G., ROGERS, Q.R., 1991. Why is the nutrition of cats different to that of dogs? 64S-67S

The previous article is a special collaboration from AFMA South Africa (Animal Feed Manufacturers Association) and their magazine AFMA Matrix. We thank AFMA for their continuous, kind support!

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