International Congress of the Italian Association of Companion Animal Veterinarians

Close this window to return to IVIS www.ivis.org International Congress of the Italian Association of Companion Animal Veterinarians 29-31 May, 2009 Rimini, Italy Next Congress : 65th SCIVAC International Congress May 28-30, 2010 - Rimini, Italy Reprinted in IVIS with the permission of the Congress Organizers

Feline nutrition: facts, fun and physiology, cats are different than dogs! Margie Scherk DVM, DABVP (feline practice), Vancouver, BC, Canada Cats are obligate carnivores. They diverged from canids approximately 30 million years ago, evolving metabolically into carnivores with unique strategies for the utilization of amino acids, fats and vitamins. This concept must be at the center of trying to understand the nutritional needs of cats and planning dietary therapies for health disorders. Behaviour Domestic cats have not evolved far from the wild cat model; they display a much narrower diversity of phenotype than dogs. They are anatomically and physiologically adapted to eating 10-20 small meals throughout the day and night. This allows them to hunt and eat when their prey are active. Small rodents make up the majority of their diet, with rabbits, birds, insects, frogs and reptiles making up a smaller proportion. The average mouse provides 30 kcal of energy, which is about 8% of an average feral (i.e. active) cat s requirements. Repeated hunting behaviours thoughout the 24 hour period are needed to meet this need; this has evolved into the normal grazing feeding behaviour of domestic cats. Cats eat their prey head first. This is a tactile response to the sensation from the direction of the hair. Cats are very sensitive to the feel of a food (physical form), its odour and taste. When offering novel foods, this should be kept in mind. Most cats prefer foods that are solid and moist, like flesh. They prefer their food at fresh-killed body temperature rather than room temperature or out of the refrigerator or hot. They dislike foods that are powdery, sticky or greasy. Flavour preferences include those which are similar to those of prey, namely fat, meat extracts, protein hydrolysates ( digest ), and certain amino acids that are abundent in muscle (alanine, proline, lysine, and histidine). Cats cannot taste sweet; they lack the second gene required to do so. Generally, cats avoid eating plant materials, even expressing the ingesta from entrails before eating them. Variations on these basic preferences occur and are a result of early experience. Under stressful situations, cats will refuse a novel food; under other circumstances, the same cat may be very adventuresome and chose a new diet over their familiar food. Anatomy and Physiology As a true carnivore, cats teeth are for tearing, not for chewing. Cats have 30 adult teeth; they have fewer premolars and molars than dogs. They don t have fissured crowns, which is a hallmark of omnivores. Their jaws have limited latero-medial and cranio-caudal movement, resulting in limited grinding ability. Additionally, they lack occlusal surfaces for grinding. The scissor-like action of the caranassial teeth is ideal for delivering the cervical neck bite used to transect the spinal cord and immobilize or kill prey. The grooves on their canine teeth are called blood grooves. Feeding dry food has little impact on dental health as cats do not actually chew. Unless the diet has an enzymatic formulation for dental hygiene, or is so large that the tip of the tooth penetrates the kibble deeply before the kibble fractures, dry food does not make much of a difference. Cats lack salivary amylase used to initiate the digestion of starches; this adaptation also reflects the nutritional composition of typical prey i.e., minimal starch content, thus digestion does not start until food reaches the stomach in cats. The stomach is small and serves to mix ingesta with secretions. Its size limits the storage period to 1-4 hours. Initially, the post-prandial period results in an alkaline tide. As HCl acid is excreted into the stomach, blood becomes alkaline and the kidneys respond by excreting bicarbonate ion, producing alkaline urine. Once the food enters the duodenum, the pancreas secretes intrinsic factor, enzymes to break down protein, fat and carbohydrate. Bicarbonate is secreted to buffer the gastric acid entering the duodenum, resulting in a transient acidemia/acid tide. The kidneys respond by excreting H+ ion, creating an acidic urine. A cat s intestinal tract is shorter than that of other species allowing for the rapid transit of ingesta. They have a shorter intestinal:body length than is seen in other species. To some extent, they make up for this with greater villus height, but the net effect is about 10% lower absorptive capacity than the dog. The length of the small intestine in the domestic cat is approximately 1.3 meters. Intestinal length (intestinal: body length) is markedly shorter in cats than in omnivores and herbivores. The sugar transport system of the small intestines of cats is not adaptive to varying levels of dietary carbohydrate; also, cats have very low activities of intestinal disaccharidases (i.e., sucrose, maltase and isomaltase). In cats, pancreatic amylase production is about 5% of that in dogs. Certain amino acid transporters in the small intestines of cats are highly adaptive, particularly the transporter responsible for arginine uptake. This finding underscores the importance of amino acids in foods for cats. In fact, cats down-regulate their enzymes for protein metabolism with difficulty and are therefore considered OBLIGATE carnivores. They need protein on an ongoing basis. The purpose of the small intestine is to complete digestion (started in the mouth and stomach), and to absorb the diges- 495

tive products along with vitamins and fluid. The intestinal phase of digestion is mediated by pancreatic enzymes as well as the mucosal disaccharidases, lipases and peptidases secreted by the enterocytes. After digestion, monosaccharides, free amino acids, di-and tripeptides, short and medium chain fatty acids are all absorbed into the portal circulation where they are transported to the liver. Triglycerides are absorbed in the small intestine and are transported via lymphatics. Bile acids, produced by the liver, enter the intestine to aid in digestion of fats, and then are recycled via the enterohepatic pathway. In the cat, a significant amount of taurine, an essential amino acid needed for conjugation of bile acids, is lost in the feces through degradation by colonic microflora. The cecum is poorly developed in carnivores. One of the functions of the colon is to further digest unabsorbed nutrients (carbohydrates, proteins and fiber) by intraluminal bacterial flora. Cats have a short colon; this limits their capability to use poorly digestible starches and fiber by microbial fermentation in the large bowel. The end result of bacterial degradation of this material by bacteria is short-chain fatty acids (SCFA), CO 2, water, methane and hydrogen gas. SCFAs include acetate, propionate and butyrate. Beneficial effects of SCFAs include acidification of the intestinal environment, which may keep potentially toxic compounds in an un-ionized form. They are precursors for lipid synthetic and gluconeogenic pathways in the liver; they help to maintain normal fluid and electrolyte balance in the colon. Finally, they help to maintain a healthy luminal bacterial flora, preventing invasion of potentially pathogenic organisms. Butyrate is a fuel for colonocytes and is preferred over glucose, glutamine, and other SCFAs. Colonocyte lifespan is a mere 4-7 days, thus a ready source of fuel needs to be consistently available. (In the small intestine, the preferred fuel for enterocytes is glutamine.) The most important source of butyrate is fiber, especially soluble (highly fermentable) fibers. Nutrients There are six classes of nutrients: water, protein, fats, carbohydrates, viatmins and minerals. Of these six, three can be used as sources of energy: protein, fat and carbohydrates. Water Water is the most important nutrient for all species. It provides the biochemical milieu required for all metabolic reactions needed for life. Without water, we will die within a few days. Our bodies cannot tolerate an acute loss of 20% of water, yet we can lose 50% of our protein reserves and/or nearly all of our glycogen and fat reserves and survive. Water is obtained from ingestion of liquids, metabolism of food as well as the Kreb s (TCA) cycle. Oxidization of food results in approximately 10-13g H2O/100 kcal of metabolizable energy (ME) produced. Water is lost from the body through urine and feces (75%) as well as via evaporation, respiration, mucous membranes and skin (insensible losses). Energy: Carbohydrates, Protein and Fats When considering any discussion about energy, it is important to understand the terms gross energy (GE), digestible energy (DE) and metabolizable energy (ME). 496 Gross energy is the energy produced by food when it is burned in a calorimeter (i.e., the total energy released by oxidation to CO2 and water). Digestible energy is the energy that is digested and absorbed (i.e., GE-fecal energy losses). Metabolizable energy is the energy that is actually utilized by the body (i.e., DE urine losses). These values vary with species for any given food. A diet needs to be balanced relative to its energy content so that when a cat consumes enough energy, enough protein, essential fatty acids, vitamins and minerals are also obtained. If this isn t the case, the result may be an energylimited diet: one which is very energy dense so the cat stops eating before the other nutrient needs have been met. If the diet is bulk-limiting, then a cat will feel satiated before the energy and other nutrient requirements have been met. It is difficult to determine accurately what the energy requirements are for any given individual. In fact, there is even a range in energy determinations for the species depending on which formula is used, what life stage, what environmental circumstances cats are in. It is now recognized that spaying and neutering decreses energy expenditure by 7-33%, so it is very important to counsel clients to change from a growth to an adult formulation and to restrict the caloric intake after surgical altering. In general, a rule of thumb for unaltered cats is that they need 60-80 kcal/kg/day and after altering, they need about 4-50 kcal/kg ideal body weight/day. As obligate carnivores, cats have no dietary requirement for carbohydrates. They lack salivary amylase and have only 5% of the pancreatic amylase activity and 10% of intestinal amylase activity of dogs. Cats derive less energy per gram of carbohydrate than humans or dogs do because eof their shorter colon and vestigial cecum. A critical difference in cats is that, while other species are able to rest their metabolic pathways from the efforts of glucose (energy) synthesis when they have been fed, cats must continue gluconeogenesis in both the fed and fasted states. Cats lack the hepatic enzyme glucokinase which, in other species, becomes active when glucose levels increase. Glucokinase is the initiating enzyme in all pathways that utilize glucose. This limits cats in their ability to metabolize large glucose loads. Cats use hexokinase instead for this role. Hexokinase is less specific for type of sugar. This may be significant when we consider that glucokinase facilitates postprandial glucose uptake by prompting insulin release from the pancreas as well as initiating the conversion of glucose into its storage forms: glycogen or fatty acids in the liver. Again, because cats are obligate carnivores and didn t evolve eating carbohydrate-rich (sugar and starch) diets, it makes sense to conserve energy by not making enzymes they don t need. This does not, however, imply that cats cannot use carbohydrates. They can use carbohydrates quite efficiently despite a lack of a dietary requirement for this energy source. Carbohydrates are a good energy source and have been shown to be necessary for lactating queens. If there is too much lactose or other sugars in the diet, then bloating, diarrhea and flatulance may result. These points are of clinical significance when considering the role that dry formulations play in the way we feed cats.

We really don t know yet what impact long-term carbohydrate intake plays in predisposing cats to obesity and diabetes mellitus, but research is currently being done to look at these questions. When cats are anorectic, they catabolize body proteins. Protein supplementation during fasting will slow hepatic lipid accumulation. Urea cycle enzymes in the liver of cats are always turned on. The rate of the urea cycle is slowed down when ornithine levels decline and resumes when arginine levels (the dietary source for ornithine) are replenished. In extreme cases of feeding arginine deficient foods or human casein-based liquid enteral products, hyperammonemia may develop with severe, and potentially lethal, neurological signs. As other species, cats require nine amino acids which are considered essential, those which must be consumed preformed because the body isn t capable of making sufficient quantity of to sustain tissue growth and repair. Additionally, along with lysine, tryptophan, histidine, valine, methionine, isoluecine, leucine, threonine, phenylalanine, cats have a requirement for arginine and taurine. When considering minimum protein requirements in a food, it is critical to know the essential amino acid content rather than just the protein levels of that diet. Cats are capable of synthesizing taurine, however the enzyme required for transforming cysteine to taurine is limited in amount and required also for felinine and glutathione pathways. In addition, cats require taurine in order to be able to conjugate (and excrete) bile acids. Taurine deficiency results in central retinal degeneration and blindness, reproductive failure and abnormal growth of kittens and dilated cardiomyopathy. More taurine is required in canned diets (2000 mg/kg dry matter basis) than in dry formulations (1000 mg/kg). This is believed to occur because enteric microbes deconjugate bile acids breaking down taurine and canned diets promote bacterial growth. However, other factors may be involved, as acidified, low potassium diets may also result in low serum taurine levels, regardless of formulation. The sulfur containing amino acids methionine and cysteine are required in higher amounts by cats than most other species. Methionine may be limited in many diets, especially home-prepared or vegetable based diets. Resulting clinical signs include crusting dermatitis at the mucocutaneous junctions, and poor growth. Kittens require 19% of their diet to be animal protein in order to meet methionine requirements. Proteins from different sources vary widely in their biologic value/availability. Egg protein is considered 100% available. Proteins from animal sources generally have a higher biologic value than proteins from plant sources, however, even animal source proteins may be less biologically available if they are improperly processed or stored during food manufacturing. Grain proteins are limited in their amounts of methionine, lysine, leucine and tryptophane resulting in biologic values of 67 for soybean and 45 for corn. Gelatin (animal collagen) has a biologic value of zero. Other factors which affect the value of protein in a product include digestibility, amino acid balance, the energy density and palatability of a diet and the health, physiological demands and environment of the cat eating the diet. 497 Adult cats have a much higher requirement for protein than dogs or humans, needing approximately 4.5-5 g good quality protein/kg ideal weight/day. Expressed as a percentage of diet, adult cats need 29% vs the adult canine requirement of 12% or the human need for 8%. Cats have the ability to digest and utilize high levels of dietary fat. High fat, low carbohydrate diets are more suitable for cats with chronic diarrhea than are high fiber diets. The paleolithic diet of the cat, rodents and small birds, is high in protein and fat and low in carbohydrate, so it is not surprising that cats are adapted to this type of balance. Cats are unable to synthesize arachidonic acid from linoleic acid, and are therefore, dependent on dietary intake to meet their needs for both of these essential fatty acids. Cats have limited hepatic delta-6 desaturase which is the enzyme required for elongation of linoleic to arachidonic acid. Diets rich in animal tissues provide these. An arachidonic acid deficiency results in platelet aggregation problems and poor reproductive performance. It is still not clear what the ideal ratio of n-3 to n-6 fatty acids is. It is thought that n-3 fatty acids are antiinflammatory, but whether this is true in all conditions and what the ideal ratio of n-3: n-6 is unclear. Vitamins: Fat soluble Vitamin A occurs only in animal tissue. Its precursors (e.g., beta carotene) are synthesized by plants and can be utilized by omnivores and herbivores as Vit A source. Cats are unable to utilize beta carotene. Cats lack sufficient 7-dehydroxholesterol in their skin to meet the needs for Vitamin D photosynthesis. They require a dietary source of Vit D such as liver or animal fats. Vitamin E is an important antioxidant. If cats eat red tuna or fish oil that is Vit E deficient, peroxidation of the bonds in the polyunsaturated oils results, leading to a release of reactive peroxides in the cat s body fat causing severe inflammation (pansteatitis) and hyperesthesia. Cats absorb their Vitamin K2 from bacterial synthesis in the ileum and colon. They absorb plant source Vit K1 from the small intestine. A deficiency of Vit K from either malabsorption or anorexia or severe liver disease, results in coagulopathy related to a deficiency of factors II, VII, IX and X. Water soluble: Thiamine (Vitamin B1) deficiency occurs when high levels of fish viscera are fed or when fish is poorly processed. Cats require four times as much niacin as dogs do, because they lack the ability to convert tryptophan to niacin. Niacin deficiency results in oral ulceration and nonspecific malaise. Pyroxidine (Vitamin B6) is required for all transaminase activity which is key in protein metabolism. Cats require four times as much pyroxidine as dogs Biotin deficeincy occurs if cats are fed raw egg whites as these contain avidin which destroys biotin. It results in a scaley dermatitis.. Cobalamin (Vitamin B12) is important in carbohydrate and fat metabolism and myelin synthesis. It is absorbed in

the ileum and requires a protein called intrinsic factor which is secreted from the pancreas unlike other species in which it is produced in the stomach. Minerals Excessive micromineral concentration can lead to serious adverse effects. It is important to counsel caution to clients regarding the use of supplements when they are using an already balanced commercial diet. Providing adequate calcium in the form of bone meal is a concern with any homemade diet. Calcium deficiency causes nutritional secondary hyperparathyroidism or tetany. Many plant ingredients contain fibers and phytates that interfere with the availability of certain trace minerals (iron, copper, zinc). Magnesium excess can predispose to struvite urolith formation in susceptible cats being fed a urine alkalinizing diet. Potassium deficiency results from nephropathy and causes muscle weakness. References Biourge, V: Feline Nutrition Update. World Small Animal Veterinary Association Congress, 2001. Buffington, CAT: Nutritional Requirements and Feeding Recommendations. In Sherding, RG (ed): The Cat, Diseases and Clinical Management (2nd ed). Philadelphia, WB Saunders Co., 1994, pp 133-151. Buffington, CAT: Nutritional Diseases and Nutritional Therapy. In Sherding, RG (ed): The Cat, Diseases and Clinical Management (2nd ed). Philadelphia, WB Saunders Co., 1994, pp 161-190. Fooshee, SK: The cat as a medical species. In August, JR (ed): Consultations in Feline Internal Medicine. Philadelphia, WB Saunders Co., 1991, pp 3-11. Kirk, CA, Debraekeleer, J, Armstrong, PJ: Normal Cats. In Hand, MS, Thatcher, CD, Remillard, RL et al (ed): Small Animal Clinical Nutrition (4th ed). Topeka, Mark Morris Institute, 2000, pp 291-347. Laflamme, DP: Nutritional Management and Nutrition-related Disease in Feline Populations. In August, JR (ed): Consultations in Feline Internal Medicine 2. Philadelphia, WB Saunders Co., 1994, pp 653-662. Michel, KE: Feline Nutrition: Fundamentals and Clinical Relevance. ABVP Symposium, 2002. 498