Cats have 30 teeth compared with 42 teeth in the dog. They have the same number of incisors, canine, and carnassial teeth but fewer premolar and molar teeth with fissured surfaces. Their dentition is for cutting and shearing flesh rather than grinding it. Grinding food requires sideways jaw movements as well as fissured surfaces, both of which are lacking in cats. Similar to dogs, cats lack salivary amylase, the enzyme involved in early starch digestion.5

Because cats evolved to eat small, frequent meals throughout the day, their stomach capacity is smaller than that of dogs. The cat’s maximum stomach capacity is between 45 and 75 mL/kg of body weight (BW) compared with 90 mL/kg in the dog.⁶

Relative intestinal length is determined by the ratio of intestinal length to body length. Intestinal length is one of the factors that influence amount of time for digestion and absorption to occur. Cats have a shorter relative intestinal length than dogs and other omnivores and herbivores with a ratio of 4:1 compared with 6:1 in dogs.⁵ In herbivorous species this ratio is higher due to the lower digestibility of foodstuffs: 12:1 in the horse, 20:1 in the ox, and 27:1 in the sheep.⁶ Short intestinal length and transit time in cats requires higher digestible diets compared to other noncarnivorous species.

Cobalamin (vitamin B12), requires binding to intrinsic factor for absorption and uptake in the ileum. Most mammals manufacture and secrete intrinsic factor from both the stomach and pancreas. In the cat, intrinsic factor is produced only in the pancreas.⁷

While the gastrointestinal microbiota play an essential role in an animal’s well-being, composition of the feline microbiota is poorly understood and the impact of diet on the gut microbiota is relatively unknown. The human microbiome contains approximately 10¹⁴ microorganisms, ten times the number of human cells.⁸ The metabolic functions of the microbiota within the gastrointestinal tract include production of metabolites such as short-chain fatty acids used as an energy source for colonocytes, degradation of potentially toxic compounds, enhanced metabolism of AAs and nondigestible carbohydrates, and synthesis of vitamins and lipids.⁹ A study in clinically healthy cats demonstrated that cats have large numbers of bacteria in their proximal small intestine compared to the dog. In this study, the total bacterial counts from feline duodenal cultures ranged from 10⁵ to 10⁸ colony-forming units (CFU)/mL, compared with a maximum of 10⁴ CFU/mL in dogs.¹⁰ Higher bacterial levels in the gut may be another adaption to a carnivorous diet. While this study relied strictly on culture methods, newer molecular techniques are showing great promise in evaluating these ecosystems in both the cat and dog.¹¹

In the wild, cats hunt small prey such as mice, rats, rabbits, birds, frogs, reptiles, and insects. Mice are the most common prey for the cat with a caloric density of approximately 30 kcal per mouse. The small size and low caloric density of prey dictates the necessity to eat many small meals throughout the day to meet energy and nutritional requirements. Domestic cats eat 7 to 20 small meals a day if given free access to food.12

Predatory behavior is a strong drive in the cat and will take precedence over feeding. Cats will stop eating a meal to kill prey and then go back to eating the original meal rather than the freshly killed prey.¹³ In the wild, cats prefer to ingest freshly killed prey rather than carrion.¹⁴

Food preferences are influenced by the diet of the queen during pregnancy and lactation. The flavors kittens experience between 1 and 6 months of age are particularly important to later food preferences and choices.¹⁵ Encouraging owners to feed kittens a variety of flavors and textures at this stage may lead to more flexibility in the adult.

Smell, taste, and texture all play important roles in the cat’s dietary preferences. The most abundant taste receptors (neurons of the facial nerve) are those for AAs, particularly those AAs that are described as sweet.¹⁴ These include proline, cysteine, ornithine, lysine, histidine, and alanine. Cats reject bitter-tasting AAs such as arginine, isoleucine, phenylalanine, and tryptophan if given in purified form. The second most abundant taste receptors are for acidic foods. These receptors are stimulated by phosphoric acid, carboxylic acids, and nucleotide dipeptides and tripeptides.¹⁴ Cats will avoid monophosphate nucleotides, which accumulate in tissues after death. This may be why cats dislike carrion. Cats do not have functional taste receptors for sucrose/sugar.^14,16 Temperature is also important, with cats preferring food at body or room temperature. Cats will generally reject foods at temperatures colder than 15°C (59°F) or greater than 50°C (122°F).¹⁵

Food preferences are strongly correlated with the amount of protein in the diet, particularly animal protein. Liver, blood, and red meat are highly palatable to cats. In addition to protein, fat has been shown to have positive palatability in cats. Fats applied to the outside of dry kibble are positive flavor enhancers, but it is believed that the positive influence of fat is more related to textural changes rather than flavor.¹⁷ While fats composed of long-chain fatty acids are highly palatable to cats, cats have a strong aversion to medium-chain fatty acids.³ Cats will generally select moist foods similar in water content to animal tissue when compared with dry, extruded diets; however, cats that have been fed exclusively dry foods for an extended period of time can develop a strong preference for only dry foods.¹⁸ Stressful situations can result in learned aversions to new or novel foods; therefore, starting a new therapeutic diet in hospitalized cats is inadvisable. Compared to rats, cats are intermediate in their ability to avoid foods that can result in deficiencies over a long period of time; for example, cats will consistently eat taurine-free diets despite development of cardiac, reproductive, and retinal diseases.¹⁴ Similar to other species, cats will learn to avoid foods that are toxic.

A study evaluated whether macronutrient regulation occurs in adult domestic cats, and if so, what is the target nutrient profile in terms of percentages of calories from protein, fat, and carbohydrate in the diet.¹⁹ The study found cats consistently regulated their macronutrient composition when given varying nutrient distributions. Their target nutrient profile was 52% of energy from protein, 36% of energy from fat, and 12% energy from carbohydrate (Fig. 18.1). When the cats were fed the high carbohydrate dry diets, researchers found a carbohydrate ceiling of 72 kcal per day. Diets with levels higher than these resulted in diet refusal with subsequent energy and protein deficiencies. These results are similar to two studies that summarized the typical prey composition of feral cats. The estimated feral cat’s average daily energy intake from crude protein was 52%, 46% from crude fat, and 2% from nitrogen-free extract.²⁰ Significant differences were also found in fatty acid composition and mineral composition. Higher levels of both omega-3 fatty acids and minerals were found in the feral profile when compared with domestic cat food profiles. In another study examining nutrient profiles of feral cats in northern California, estimated average composition of typical feline prey was 63% protein, 25% fat, and 12% carbohydrate on an energy basis.²¹