Cats: obligate carnivore (Proceedings)

In their natural environment, cats are an obligate carnivore, meaning that their nutritional needs are met by eating a diet that consists of animal-based proteins (i.e. mice, birds). How have our efforts to domesticate cats been affected by this dietary requirement?

Strictly speaking, cats and dogs are members of the order Carnivora and are therefore, classified as carnivores. From a dietary perspective, dogs are omnivores and cats and other members of the suborder Feloidea are strict carnivores. Domesticated cats (Felis catus) have evolved unique anatomic, physiologic, metabolic and behavioral adaptations consistent with eating a strictly carnivorous diet.

Feeding Behaviors

The evolutionary history of the cat indicates that it has eaten a purely carnivorous diet throughout its entire development. Feeding behaviors that have evolved to fit this lifestyle include searching, hunting and caching of prey as well as postprandial behaviors such as grooming and sleeping. Feral or outdoor cats feeding primarily on mice, voles and insects tend to live solitary lives when food is scarce and spread over a large area, but when food is plentiful and concentrated as with households, dumps and farms cats can be found living in large groups. Cats typically eat 10-20 small meals throughout the day and night. This eating pattern probably reflects the relationship between cats and their prey. Small rodents make up ~40% or more of the feral domestic cats diets, with small rabbits, insect, frogs and birds making up the remainder. The average mouse provides ~30 kilocalories or an estimated 8% of feral cat's daily energy requirements. Repeated cycles of hunting throughout the day and night are required to provide sufficient food for the average cat. House cats typically continue this pattern by eating 10-20 small meal throughout the day and night with each meal having a caloric content of ~23 kilocalories, very close to the caloric value of one small mouse.

For thousands of years the primary economic value of cats has been their hunting skills. Until recently, there has been little or no selective breeding done to alter their behavior or looks. The predatory drive is so strong in cats that they will stop eating to make a kill. This behavior allows for multiple kills, which optimizes food availability. Supplemental feeding may reduce the time spent hunting, but otherwise will not alter hunting behavior.

Cats are very sensitive to the physical form, odor and taste of foods. They consume live prey beginning at the head, this head first consumption is dictated by the direction of hair growth on the prey. Food temperature also influences acceptance by cats. They do not readily accept food served at either temperature extreme, but prefer food near body temperature (~ 38°C, 101.5°F) as would be found with freshly killed prey. House cats accustomed to a specific texture or type of food may refuse foods with different textures. An individual cat's preferences are often influenced by early experiences (good or bad). Many cats will choose a new food over a diet that is currently being fed. The reverse is true in new or stressful situations, such as illness or hospitalization, where cats tend to refuse novel foods. This can be important when trying to switch foods or forms of food fed.

Anatomic Adaptations

Cats have adapted physiologically to the life of a hunter. Their visual acuity is greater than that of dogs. In addition, their sense of hearing is well developed- their ears are upright, face forward and have 20 associated muscles to help them precisely locate sound. Their highly sensitive facial whiskers and widely dispersed tactile hairs are thought to help them hunt in dim light and to protect their eyes. Sharp and dagger-like, their retractable claws are ideal for capturing and securing prey, yet they are easily retracted to decrease noise when stalking.

The scissor-like carnassial teeth are ideal for delivering the cervical bite used to severe the spinal cord and immobilize or kill prey.

Their stomachs are smaller than dogs and simpler in structure. Because cats do not consume large meals, the stomach is less important as a storage reservoir.

Intestinal length, as determined by the ratio of intestine to body length is markedly shorter in cats than in omnivores and herbivores. The ratio for cats is 4:1, meaning that the intestinal length is 4 times longer than the length of the cat, for dogs this is 6:1 and for pigs 14:1. Cats do have greater villus height in their intestinal lining improving their absorptive capacity over that of dogs, so that overall they are only ~10% less efficient in digestion especially with complex starches or fibers even with their shorter intestinal length.

Physiologic Adaptations

Cats are not able to adapt to varying levels of carbohydrates in their diets due to various changes in the digestive and absorptive functions of intestine. Salivary amylase, the enzyme used to initiate digestion of dietary starches is absent in cats, and intestinal amylase appears to be exclusively derived from the pancreas. These enzymes were not necessary in a prey based diet with minimal starch content. The level of pancreatic amylase is only 5% that of dogs. The sugar transporter in the intestine is nonadaptive to changes in dietary carbohydrate levels. Disaccharide activity (i.e. the brush border enzymes responsible for sugar digestion) is also non-adaptive and only ~40% that found in dogs. These changes evolved because cats had little natural carbohydrate intake and it was not necessary to have systems intact that were not of little use to the animal.

Despite these adaptive changes, cats are able to still use carbohydrates in their diets, with a sugar digestibility of ~94% with a few exceptions. Lactose digestion declines sharply in kittens after 7 weeks of age. This is due to a decrease in intestinal lactase activity that is typical in mammals. Most adult cats can consume small amounts of milk without problems, but larger amounts (>1.3 gram/kilogram of body weight) can lead to signs of bloating, diarrhea and gas.

High amounts of dietary carbohydrate levels can negatively impact on diet digestibility. With high levels of dietary carbohydrates, decreases in protein digestibility are seen due to a combination of factors including reduced fecal pH caused by incomplete carbohydrate digestion and increased microbial fermentation in the colon with increased production of organic acids. Cats have a vestigial cecum and short colon, which limit their ability to use poorly digestible starches and fiber for energy through bacterial fermentation in the large bowel.

Metabolic Adaptations

     Energy

The liver of most animals has two active enzyme systems for converting glucose to glucose-6-phosphate (the first step to forming glycogen-the storage form of glucose within the cells); hexokinase and glucokinase. The glucokinase system is used primarily when a large load of glucose is received by the liver as would be seen with a high carbohydrate meal. Cats have very low liver glucokinase activity and therefore limited ability to metabolize large amounts of simple carbohydrates by this route. Blood glucose levels in carnivores are more consistent with less postprandial fluctuations because glucose is released in small continuous boluses over a longer period of time as a result of gluconeogenic catabolism of proteins. If the protein is not included in the diet, body muscle and organ tissue will be used. Cats did not need to handle large carbohydrate loads and therefore only use the hexokinase system for glucose metabolism.

     Water

Domestic cats are thought to have descended from the small African wildcat (Felis silvestris libyca), a cat naturally found in the deserts of Africa. Because of this ancient relationship, cats today maintain this adaptation to a dryer environment. Cats seem to be less sensitive to the stimulus of thirst, and are able to survive on less water than can dogs.

With this decreased response to thirst, cats may ignore minor levels of dehydration (up to 4% body weight). They are able to compensate for this reduced water intake by forming highly concentrated urine. 4 Cats adjust their water intake based on the dry matter content of their diet rather than the moisture content. They consume 1.5-2 ml of water/g of dry matter. This 2:1 ratio of water to dry matter is similar to that of their typical prey. Practically, this means that cats consuming a dry food diet will consume about half the amount of water through their diet and drinking, compared to cats eating a canned food diet.

     Protein

Protein metabolism in cats is unique; this is apparent because of their unusually high maintenance requirement for protein in the diet as compared to dogs or other omnivores. Cats have both a higher basal requirement for protein and an increased requirement for essential amino acids. Cats depend on protein not only for structural and synthetic purposes but also for energy. They will continue to use protein in the form of gluconeogenic amino acids for production of energy, even when inadequate protein is consumed in the diet. Although these changes impair the cat's ability to conserve protein when dietary sources are limited, on their natural diet it ultimately conserves energy by eliminating the cost of enzyme synthesis and degradation. In 1986, National Research Council recommended a minimum of 240 grams of protein/kg in the diets of growing kitten and 140 grams of protein/kg in the diets of adult cats. This is equivalent to 26% of metabolizable energy in the diet for kittens and 23% of metabolizable energy for adult maintenance. Keep in mind, these are minimum recommendations, and they assume a highly digestible protein source is provided in the diet.

     Taurine

Taurine, which is an essential amino acid for cats, is not incorporated into proteins or degraded by mammalian tissues, but 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. Taurine continues be lost in the gastrointestinal tract through this conjugation with bile, this coupled with a low rate of synthesis contributes to the obligatory requirement for cats. Carnivorous diet supplies abundant taurine; however cereal and grains supply only marginal or inadequate levels of taurine for cats. Therefore, diets based on these types of protein sources may be lacking or limiting in taurine. Taurine is either more available or better retained by cats fed dry food diets. Because of the wide spread uses of taurine within the body, changes from deficiency can be seen in virtually all body systems. Three syndromes have been identified related strictly to taurine deficiency; feline central retinal degeneration, reproductive failure and impaired fetal development and feline dilated cardiomyopathy. Clinical signs of taurine deficiency occur only after prolonged periods of depletion (from 5 months to 2 years).

     Methionine and Cystine

Methionine is an essential amino acid for cats; this species has a higher requirement than do dogs or other omnivores. Cystine is also required for production of hair and felinine, an amino acid found in cat urine. Felinine is found in largest amounts in intact male cats and is thought to be used for territorial marking. Can replace up to half of the methionine requirement in cats, methionine tends to be the first limiting amino acid in many food ingredients.

Nutritional deficiencies are possible, especially in cats fed home-made, vegetable based diets or human enteral diets. Clinical signs of methionine deficiency include poor growth and a crusting dermatitis at the mucocutaneous junctions of the mouth and nose.

     Vitamin metabolism

The cat is unable to convert beta-carotene to retinol (vitamin A) because of a lack of intestinal enzymes necessary for the conversion, and therefore this species requires a dietary source of pre-formed vitamin A. Vitamin A is necessary for the maintenance of vision, bone and muscle growth, reproduction and healthy epithelial tissues. Because vitamin A is a fat soluble vitamin and is stored in the liver, deficiencies are slow to develop, and are only seen in cats with severe liver failure or gastrointestinal disease resulting in fat malabsorption. Cats also are unable to convert sufficient amounts of the fatty acid linoleic acid to meet the requirements for arachidonic acid.

Cats also lack sufficient enzymes to meet the metabolic requirements for vitamin D photosynthesis in the skin; therefore they require a dietary source of vitamin D. The primary function of vitamin D is calcium and phosphorus homeostasis, with particular emphasis on intestinal absorption, retention and bone deposition of calcium. As with vitamin A, deficiency is rare and slow to develop.

Vitamin A, vitamin D and arachiodonic acid are found in plentiful amounts in animal fats. This fat is important not only for provision of fuel for energy, but also for increasing palatability and acceptance of food.

Cats require increased amounts of many dietary water-soluble B vitamins, including thiamin, niacin, pyridoxine (vitamin B6), and in certain circumstances cobalamin (vitamin B12). The requirement for niacin and pyridoxine is four times higher than that for dogs. Because most water-soluble B vitamins are not stored (except cobalamin, 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 cat may be seen as one of our most visible "specialists". As an obligate carnivore, they have evolved to such a point that many of the redundant systems that we have are no longer required. Instead of seeing cats as "inferior", I think we need to acknowledge that they have surpassed both us and dogs, and have streamlined their lives. We need to appreciate this unique and wonderful creature that continues to enrich our lives and protect our houses and yards.

References

Kirk CA, et al. Small animal clinical nutrition. 4th ed. 2000; pp 291-337.

Case LP et al, In Canine Feline nutrition. 2nd ed. 2000; pp 71-73, 217-224.

Voith VL et al, The Waltham book of Clinical Nutrition of the Dog and Cat. 1994; pp 119-127, 353-357.

Zoran DL. JAVMA 2002; 221, No 11: pp 1559-1566.

Wills, JM. Manual of Companion Animal Nutrition and Feeding 1996: pp 44-46.

Welborn MB et al. Clinical Nutrition Enteral and Tube Feeding. 1997: pp 61-80