Max's House
Feline Nutrition

African wildcat Felis silvestris lybica.
Genetic investigations indicate that this is the true ancestor of the domestic cat.

The cat's dependence upon eating meat has been at the core of their evolution.

Of all the carnivores, the felids are the most specialized meat-eaters.  The cat thrives with very little, if any, direct ingestion of plant material. Of course, just as an herbivore such as the cow needs specialized digestive and metabolic processes to deal with converting grass to flesh or milk, the cat too has enhanced or eliminated certain biochemical mechanisms to deal with a diet rich in protein and fat, but with little or no carbohydrate. 

The cat also needs proportionally more protein in its diet compared to other mammals - one reason is that certain liver enzymes that break down proteins are always functional (they are turned "on" and "off" in other animals) and so cats use some energy from protein just to fuel this process.  Other mammals use most of their protein for growth and body maintenance. While an adult dog's protein requirement will drop to about one third of its requirements as a growing puppy, the kitten only needs about one-and-a-half times the protein of an adult cat because the adult level is still relatively high.

Unlike an omnivore, whose digestive system consists of a fairly large small intestine and relatively large stomach, the carnivore's system consists of a fairly short, small intestine and relatively small stomach. Thus, a carnivore's optimum diet must be concentrated, highly digestible, and low in residue..
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The cat also needs certain nutrients made by the metabolic processes of other animals and not available in plant material. Dogs have a range of biochemical processes that convert nutrients from plant and animal sources into what they require - for example dogs can convert the carotenes found in fruit and vegetables into vitamin A. The cat cannot do this and must obtain vitamin A already preformed in animal sources.  Cats ingest not only the flesh and organs of their prey but also the partially and wholly digested vegetable foods the prey had eaten. With the assistance of the prey's own digestive processes, the cat then is able to derive nutrition from various vegetable sources. Thus the cat is more than a carnivore -- the cat is an obligate carnivore. To survive the cat must eat meat.

Hence, when some people want to feed their cats a diet consisting largely of vegetable matter for either economy or convenience or to fit in with their own preferences or ethical beliefs, they need to consider that the cat they love for its looks and behavior is as it is because it is a carnivore - a vegetarian cat would probably have developed to look like a rabbit!

NUTRIENT REQUIREMENTS

Cats, like all animals, require six classes of nutrients: water, energy, protein, essential fatty acids, minerals, and vitamins.  Cats do not have a dietary requirement for carbohydrates.  The metabolic requirement for glucose in the cat is derived from protein (glucogenic amino acids) and fat (glycerol).  Cats are adapted to a protein- fat-rich, carbohydrate-poor diet.

TERMS USED TO DESCRIBE FELINE NUTRITION

1. "Digestibility." This is the relationship between the amount of a nutrient or food eaten and the amount absorbed expressed as a percentage. For example, a cat consuming a pound (454 g) of a food that is 80% digestible has only 12.8 oz (384 g) 0 6 oz [454 g] X 80%) available to the body for actual use. The difference in the two amounts represents the waste matter that is excreted.

2. "Utilization." This term expresses the relationship between the quantity of a nutrient or food eaten and the actual amount retained by the body. Like digestibility, the ratio is expressed as a percentage. Food utilization is the best overall way to determine the actual nutritional value of a food. Scientific analysis of food disposition in the body can provide this information. However, since food utilization figures are often not readily available to pet owners, food digestibility is often substituted for it in discussions of nutrition.

3. "Energy" - "Kcal".  The chemical energy of foods is most often expressed in units of calories or kilocalories (kcal). A calorie refers to the amount of heat energy necessary to raise the temperature of 1 gram (g) of water from 14.51 Celsius (C) to 15.50 C. Because a calorie is a very small unit, it is not of practical use in the science of animal nutrition. The kcal, which is equal to 1000 calories, is the most commonly used unit of measure for energy in pet foods in the US.  A second unit of measurement for energy is the kilojoule (kj), which is a metric unit. A kilojoule is defined as the amount of mechanical energy required for a force of I newton (N) to move a weight of 1 kilogram (kg) by a distance of 1 meter (m). To convert kcal to kj, the number of kcal is multiplied by 4.18.

4. "Metabolizable energy" (ME). This term represents the number of calories available to the body from food. It is conventional among nutritionists to specify nutrient concentration requirements for pet foods as quantities needed per each 1,000 calories of metabolizable energy (Kcal ME) provided by the food, since some nutrient requirements change when the calories available from a given quantity of food increase or decrease. When comparing calories provided by food to calories required by the animal, it is important to be sure that both are expressed in the same energy units. Metabolizable energy units specify the actual energy available. Other units such as gross energy or digestible energy are less accurate measures of the actual calories provided by food.

5. "Dry Matter Basis" (DM). The guaranteed analysis numbers represent the nutrient amounts that are present on an "as fed" (AF) basis, which does not account for the amount of moisture that is present. Dry foods can contain between 8-12% water, and canned foods contain between 70% and 80% water. To make a accurate comparison between various types and brands of cat foods, the AF numbers must be converted to "dry matter" (DM) percentages, which represent the actual percentages of nutrients consumed.
AAFCO recommendations use  DM basis.

A simple formula can be used to convert AF values into DM basis.  The formula calculates the percentage of a nutrient on an AF basis with the proportion of DM in the diet. For example, compare two different types of foods:

 

 
Adapted from Stephen W. Crane, Companion Animal Clinical Nutrition

Calculate the dry matter percentage of the food:  Total minus moisture content

 

Convert As Fed to Dry Matter

 

Multiply the quotients by 100 to get a percentage of the nutrient on a DM basis.

In this example, the canned food contain more protein than the dry food.

Water Requirements

Water is the single most important nutrient necessary to sustain normal function of all living cells. Water helps regulate body temperature, cushion the joints and internal organs, digest food, eliminate waste, lubricate tissue and allow salt and other electrolytes to pass through the body.  The nonfat component of mammals contains about 73 percent water (about the same amount found in canned food). Cats can lose nearly all their reserves of glycogen and fat, half the body protein stores, and 40 percent of their body weight and survive. However, cats are much less tolerant to losses of body water.

Cats can withstand acute dehydration slightly better than dogs.  Although cats can tolerate some depletion of their body water for a short period, they must in the long run remain in water balance.  The losses of water from the body must be offset by an equal intake of water. At normal temperatures water is lost from the body via the lungs, skin, urine, milk, and feces. At high temperatures, an additional loss may occur via saliva, which is used to wet the fur and provide evaporative cooling. The body gains water from "free water" present in liquids and solid foods and "oxidation water" arising from the catabolism of carbohydrates, fats, and protein.

The cat has evolved to obtain her water requirements almost entirely on the moisture content in her food - inherited from her desert-dwelling ancestors. Cats can live for long periods without drinking water when receiving food containing 67-73% water but become dehydrated when the water content of the food is 63% or less. Canned diets contain enough water that cats consuming them rarely need to drink.  Daily water needs, in milliliters, often are "guesstimated" as equal to the metabolizable energy requirement in kilocalories or approximately 60 ml/kg.  Once the diet is consumed, oxidation of nutrients produces an additional 10 to 13 grams of water for each 100 kcal of metabolizable energy.   Thus a 4 kg cat consuming a 240 kcal canned diet containing 78% moisture will consume 237 ml or 98% of its daily water need directly from the diet.  Thus the cat needs to drink less than 1 oz. of additional water per day whereas a cat consuming a 240 kcal dry diet needs to drink over 7 oz. of water per day.   This can be difficult because cats are not naturally big drinkers.  Feeding a canned diet containing 78% moisture virtually guarantees homeostatic control of water balance in the cat.

The water content of the commercial foods commonly fed to cats varies from 8% in dry foods to over 75% in canned foods; thus the amount of drinking water required is affected substantially by the water content of the food.  When fed canned food (80% moisture)  with access to drinking water, cats obtain over 90% of their total water intake from the diet, whereas on dry food, 96% of the total water intake is obtained by drinking. The total free water intake (from food and drinking water) decreases when cats are fed dry food only, so that the water to dry matter intake ratio when fed on commercial dry foods varies from 2.0 to 2.8: 1 whereas on canned foods it varies from 3. 0 to 5.7: 1.  Thus for any given dry matter intake cats have a higher water turnover on canned than on dry foods.  (National Research Council [National Academy of Science] Nutrient Requirements of Cats).
Diet moisture content is related to the observation that cats fed dry food drink more than six times more water than cats fed canned food but that much of this water contributes to fecal moisture so that urine volume is lower and urine specific gravity higher in cats fed dry food.  The urine concentration of all solutes, including potentially calculogenic crystalloids, depends on urine volume. 

Cats increase voluntary water intake when fed dry food but not in sufficient amounts to fully compensate for the lower moisture content of the food.  In a recent study, cats consuming a diet containing 10% moisture with free access to drinking water had an average daily urine volume of 63 milliliters (ml). This volume increased to 112 ml/day when fed a canned diet with a  moisture content of  75%. Urine specific gravity was also higher in cats that were fed the low-moisture food.  Decreased urine volume may be an important risk factor for the development of urolithiasis in cats. Diets that cause a decrease in total fluid turnover can result in decreased urine volume and increased urine concentration, both of which may contribute to urinary tract disease in cats.  Several studies have shown that dry cat foods contribute to decreased fluid intake and urine volume.

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 changes in the water content of their food.  Similarly, their compensatory drinking response to dehydration due to increased environmental temperature is less effective than dogs.  This apparent weakness of the cat's thirst drive to respond to changes in her state of hydration has led to the conclusion that feeding canned food assures adequate hydration at all times.

In addition to ensuring adequate hydration, a high water turnover helps eliminate crystallogenic substances before they grow to sufficient size to interfere with normal urinary function.  This is a very important consideration for male cats.   Cats that cannot urinate for more than 24 hours due to urinary tract obstruction can die from acute renal failure and/or severe damage to the urinary bladder.  In addition to the removal of crystals, benefits of increased water intake include dilution of any noxious substances in urine, and more frequent urination to decrease bladder contact time with urine that may reduce the risks of urinary tract disease.  For that reason, canned diets are usually prescribed as the first-line therapy for feline lower urinary tract disease.

Energy Requirements

Energy is used to perform muscular work, processes such as breathing and physical activity to maintain body temperature.   Energy needs are met in the order of priority for survival.   Energy expenditure can be divided into two parts: basal metabolic rate (BMR), which keeps the body 'ticking', and thermogenesis. BMR includes processes such as respiration, circulation and kidney function and it may be affected by many factors including body weight and composition, age and hormonal status. Thermogenesis is simply an increase in metabolic rate over the basal level and includes the cost of digesting, absorbing and utilizing nutrients (sometimes called the 'thermic effect of food' or 'dietary induced thermogenesis'), of muscular work or exercise, of stress, or of maintenance of body temperature in a cold environment.  In contrast to BMR, the degree of thermogenesis can vary widely and may cause large variations in daily output. Thermogenesis is capable of rapid adaptive response to changes in the internal or external environment.  The basal energy requirement makes up approximately two-thirds of the total amount of energy needed for maintenance of the cat. It is only after maintenance energy needs are met that growth and reproduction can occur.

When energy needs are met, cats stop eating (unless satiety cues are overridden by exceptionally palatable diets or behavioral factors). All other nutrients must thus be present in the diet in amounts sufficient to meet the cat's needs for them before energy needs are met.  Thus only high-quality, nutritionally complete foods should be fed to cats.

Energy intake is  considered at three different levels: gross energy (GE), digestible energy (DE) and metabolizable energy (ME). Gross energy of ingested food is the amount of heat released when the food is totally oxidized in an environment of pure oxygen. This is the maximum amount of energy that can be released.   No animal is able to utilize all the energy from its food.  Although a substance may have a high GE content, it is of no use unless the animal is able to digest and absorb it. Digestible energy is the energy available from a food when it has been absorbed into the body after digestion in the digestive tract and is calculated as GE minus fecal losses.

Some of the absorbed food energy may be only partially available to the tissues, with the remainder (mainly in the form of end products of protein metabolism) being lost in the urine. The energy which is ultimately utilized by the tissues is known as metabolizable energy (ME) and is calculated as DE minus urinary losses.  Metabolizable energy (ME) is the value that is most often used to express the energy content of pet food ingredients and commercial diets and is usually expressed as ME (kcal/g), ME (kcal/lb), ME (kcal/kg), ME (kcal/oz.), ME (kcal/cup), or ME (kcal/can).  Carbohydrate and protein in commercial pet foods provide about 3.5 kcal of ME per gram nutrient, while fat provides approximately 8.5 kcal/g.  These values are called Modified Atwater Factors and are slightly lower than the Atwater Factors assigned for human foods.

FOOD ENERGY

The body obtains energy by oxidizing food but the energy is released gradually by a series of complex chemical reactions, each regulated by an enzyme. Many of these enzymes require the presence of vitamins or minerals in order to function properly.

Energy needs for a cat range from 45 kcal to 80 kcal of metabolizable energy per kilogram body weight per day for adult neutered cats. The energy requirements for a typical, inactive, 4 kg. cat at maintenance is about 180 kcal/day (45 kcal/kg/day), while the energy requirement for active 4 kg. cat is about 240 to 320 kcal/day( 60-80 kcal/kg/day).   Cats have no dietary requirement for carbohydrates for neither energy or glucose.  Typical of carnivores, cats can receive all their energy and nutritional needs from protein and fat.

The DE and ME contents of foods depend both upon their composition and upon the species which consumes them.  For example, because of the length and structure of its gastrointestinal tract, a nonruminant herbivore such as a horse can derive a greater amount of energy from grass than can a cat.  Therefore the ME value of grass for a horse is higher than the ME value of grass for a cat.  This principal applies to all of the nutrients in the cat's diet.  Although much of the carbohydrates commonly found in dry cat foods may have high ME values for herbivores and omnivores, those same carbohydrates may have much less ME value for a carnivore such as the cat.
 
 

      (Adapted from Nutrient Requirements of Cats, National Research Council)

*Neutered animals generally require 25% to 30% less carlories than intact animals.

* A lactating queen's energy requirements vary according to the number of kittens in the litter and they increase each week of lactation. Amounts given   in  the above table are for a lactating queen nursing four kittens in week 6 of lactation. The best method for feeding lactating queens and growing kittens is to allow free access to food at all times. This "free-choice" method of feeding allows the queen and kittens to adjust their intake to individual needs. Uneaten canned food should be replaced with fresh food after 30 minutes.  Neonates and young kittens and pregnant/lactating queens should not be fed dry food  because of the low moisture content and possible damage to young teeth.

*Overeating and obesity are not usually problems in this class of cats because of the high energy need In contrast, the nonlactating cat should be fed the measured amount of food that causes it to maintain optimum body weight.

Protein

Protein is required to maintain the supporting structure of the animal: the muscle, bone, ligaments, and tendons. Many of the functional components of the body, including enzymes, plasma proteins, many hormones, and some neurotransmitters also are proteins. Body proteins are in a "dynamic steady state" of constant synthesis and breakdown. The greater the importance of a protein in metabolic regulation, the more rapid its turnover will be, so proteins such as enzymes and hormones turn over more rapidly than do structural proteins.

Rapid turnover ensures prompt response to changing situations and enables a limited amino acid pool to be used with optimal efficiency.  Reutilization of amino acids into new protein is not completely efficient however, so some protein is constantly lost from the body. The more rapid the turnover rate, the more rapidly amino acids are lost.  During growth or healing, protein turnover and related processes can account for as much as 40 percent of total energy expenditure. Unlike dogs and humans who can who can adapt to using carbohydrates in place of protein to supply energy, cats must always use a portion of the protein they eat for energy. This is one important reason why cats must have a diet high in protein.

Cats require 20 amino acids to synthesize all the needed body proteins. Ten can be synthesized in the liver from carbon and nitrogen. These are called dispensable amino acids (nonessential amino acids) because they need not be present in the diet. The other 10 amino acids are indispensable (essential amino acids) in diet, because they cannot be synthesized in sufficient quantities to meet the animal's needs, or cannot be synthesized at all. For two of the indispensable amino acids, phenylalanine and methionine, approximately one-half the requirement may be met by the dispensable amino acids tyrosine and cystine, respectively.

Arginine

Feline requirements for most of the essential amino acids are similar to those for other species with the exception of some essential amino acids.  Cats require more arginine than most other animals do, however, because they lack an intestinal enzyme, pyrroline-5-carboxylate synthase, required for synthesis of the arginine precursor, ornithine (a urea cycle pathway intermediate).  Arginine is required for normal protein synthesis and ammonia detoxification.  Arginine enables conversion of ammonia to urea.  Cats can develop severe hyperammonemia from anorexia or ingestion of an arginine-free meal. Arginine has other important roles that include increasing endocrine secretagogue activity, improving nitrogen retention, acting as a substrate for nitric oxide production, reducing nitrogen loss in postoperative patients, enhancing collagen deposition in wounds, enhancing T-cell function, and the growth of lymphocytes.
 

Taurine

Cats also require a dietary source of the B-amino acid taurine which is present only in animal tissues.  Cats cannot synthesize enough taurine from dietary precursors to meet obligate intestinal loss. The cat uses only taurine for bile salt synthesis (in comparison to dogs, that can substitute glycine), causing an ongoing obligate loss of taurine with excreted bile salts.  Most animals produce both glycine and taurine conjugates of cholesterol for secretion as bile acids, but cats can only use taurine. Intestinal reabsorption of bile acids is not 100 percent efficient, so some taurine is continually lost in the feces. Although not incorporated into protein, taurine is required for normal cardiovascular (taurine deficiency has been proved to cause dilated cardiomyopathy in cats), reproductive, and visual function (taurine deficiency has also been proved to cause retinal degeneration).   AAFCO Nutrient Profiles for Cats require that canned cat food contain a minimum of 2000 mg of taurine/kg diet and that foods contain a minimum of 1000 mg/kg.

The nutritional value of protein depends on its amino acid composition as well as on the efficiencies of its digestion, absorption, and utilization. The use of amino acids for protein synthesis depends on the availability to cells of all amino acids in the right proportion and at the right time. The diet must provide these amino acids; otherwise, the body mobilizes them from protein in its tissues. Plants can make all the amino acids they require by synthesizing them from simple nitrogenous compounds such as ammonia and nitrates.  Cats require most of their dietary nitrogen to be as specific amino acids.
 

Biological Value of Proteins

Biological value describes how efficiently a protein is used. This value is high for proteins from meat, most meat by-products, eggs, and dairy products.   Cats digest these proteins efficiently, and they provide amino acids in proportions suitable for tissue protein synthesis. In contrast, the biological value of most plant proteins is low, due to insufficiencies of specific amino acids and lower digestibility

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The digestibility of pet foods is about 80 percent for dry foods, 85 percent for semimoist and canned foods containing large amounts of cereal grains, and more than 90 percent for canned diets with meat as the primary protein source. Digestibility is influenced both by the source of the protein and by how it is processed. Protein in cat foods comes from both animal and plant sources. Animal protein is generally more expensive and often of higher quality than plant protein.

Minimum protein requirements of cats are 30 and 26 percent of calories for growth and maintenance, respectively. Protein needs for late gestation and lactation are at least as great as growth requirements.  Thus 26% - 40% (DMB) of the daily kcal derived from protein, depending on the quality of the protein, should meet the needs of nearly all adult cats at maintenance.

The notion that dry food contains more protein than canned food is a gross myth attributed to confusing and often misleading pet food labels.

For example, the amount of protein guaranteed on the label of dry cat foods is about 35 percent, and in canned foods about 10 percent. Canned foods, however, contain more water. A more appropriate way to compare the nutrient content is on a "dry matter" basis. To compare nutrient contents on a dry matter basis, one divides the nutrient of interest by the total dry matter (100 percent minus the percentage of moisture on the label). In the above example, dry foods are about 90 percent dry matter, so 35 percent protein divided by 90 percent dry matter equals 38 percent protein on a dry matter basis; canned foods are closer to 25 percent dry matter, so 10 percent protein divided by 25 percent dry matter is 40 percent protein on a dry matter basis. Calculated on a dry matter basis, the canned food in this example contains more protein than the dry food.
 

 

Carbohydrates

All animals have a metabolic requirement for glucose.  Carnivores, such as the cat, convert glucogenic amino acids and glycerol to glucose for the maintenance of blood glucose, and therefore, have no established dietary requirement for carbohydrates. 

Because cats have adapted to diets high in protein and low in carbohydrate, continuous activity of amino acid catabolic enzymes provides a continuous source of carbon skeletons for glucose or energy production and nitrogen for synthesis of dispensable amino acids and other nitrogenous compounds. This continuous metabolic state causes the cat to catabolize a substantial amount of protein after each meal, regardless of its protein content. The cat does not have the capability to conserve nitrogen from the body's general nitrogen pool leading to an obligate nitrogen loss and a higher dietary protein requiremerit than omnivores.

In general, an absence of dietary carbohydrate in the feline diet will not affect blood glucose levels or cause an energy deficiency; this is because the body can use protein and the glycerol portion of fat for glucose production, and fat and protein for energy.

VITAMINS

Although the required levels of all the essential vitamins that should be included in cats' diets have not been firmly established, many important facts about vitamins in the cat's diet are known and should be heeded when selecting a diet for your pet. The  currently recommended amounts of vitamins should be fully available to a cat from his or her food. As with other nutrients, these levels will generally be lower than those actually present in commercial foods, since manufacturers must include higher levels when the food is formulated and first mixed to make up for nutrients that are not fully bioavailable from foods and losses caused by processing or storage.

VITAMIN A

Cats cannot convert beta carotene (found in green vegetables) to vitamin A as can dogs and people, so you must be sure that other sources of fully formed vitamin A (found in animal tissues) are provided in the diet to prevent a deficiency that can result in skin, eye, and reproductive changes. On the other hand, hypervitaminosis A (too much vitamin A) is far more likely to be a problem. This condition is typically seen in cats whose diets have been oversupplemented with, for example, cod-liver oil, and those which have been fed excessive amounts of liver which is highly palatable to cats and may result in an apparent 'addiction'. Signs of toxicity, which usually develops from feeding the diet over a period of months or years, do not develop until the prolonged daily intake exceeds 17mg (57,000 IU)/kg BW.

Excesses of this fat-soluble vitamin are stored in the liver and a toxicity can lead to hepatic damage due to lipid infiltration. Clinically, the most recognizable, signs of hypervitaminosis A are those related to the skeletal changes that occur, particularly in the cervical vertebrae and the long bones of the forelimb. The periosteum appears to be particularly sensitive to high levels of vitamin A and subperiosteal hyperplasia occurs around the bony insertions of tendons and ligaments in response to physical forces exerted in these areas. Bony exostoses result and may invade joints, causing enlargement and ankylosis.

Initial signs may be of stiffness and pain, particularly of the neck and forelegs, and the owner may first observe the cat's reluctance to groom itself. This may be accompanied by anorexia, lethargy, weight loss and an unkempt appearance. The painful lesions may induce an affected cat to adopt a sitting 'kangaroo' posture in order to avoid weight bearing by the anterior regions.

Treatment consists primarily of dietary correction and the provision of a normal diet; supplementation with fish oils is strictly contraindicated. Non-steroidal anti-inflammatory drugs are useful in the initial stages for the control of pain, and food dishes may be elevated to facilitate eating and drinking. Early treatment may bring about a resolution of clinical signs and halt the progression of the disease, but established ankyloses are irreversible.
Feed an average-sized adult cat no more than 1 ounce (30 g) of beef liver twice weekly. If necessary, balanced vitamin-mineral preparations may also be used as dietary supplements, but avoid giving unbalanced supplements such as cod liver oil to cats, since 1 teaspoonful can contain more than 5000 IU vitamin A. Use only balanced vitamin-mineral supplements recommended by your veterinarian and follow directions for their use caref

B VITAMINS

(THIAMINE, RIBOFLAVIN, PYRIDOXINE, PANTOTHENIC ACID, NIACIN, B-12)

Cats have relatively high requirements for B vitamins in their diets. Foods for cats must contain at least twice the amounts of many B vitamins found in diets adequate for dogs-another good reason not to feed cats dog food. Several B vitamins are destroyed by heating, a process used in making commercial cat foods, so all good processed foods must be supplemented with B vitamins.

Several B vitamins are synthesized by bacteria in the intestines of healthy cats. Intestinal problems, e.g., diarrhea, can eliminate this source, and antibiotics may also interfere with it. Vitamin supplementation is often necessary during prolonged illnesses involving the intestine or during prolonged antibiotic treatment.

Niacin

The cat is unable to to convert the essential amino acid tryptophan to niacin.  The regular consumption of a carnivorous diet throughout the cat's evolutionary history has not resulted in selective pressure for the cat to synthesize niacin from precursor substances.. However the inability of the cat to convert tryptophan to niacin is of little practical significance to the feeding management of pet cats provided a carnivorous diet is fed because animal tissues are well supplied with nicotinamide.

Thiamin

Thiamin (vitamin B,) is a water-soluble vitamin, with limited storage in the body, which plays an essential part in energy metabolism and neural impulse transmission. It can be destroyed during prolonged storage; interaction with high levels of glutamate, such as those present in vegetable protein, can lead to a thiamin deficiency; and it is progressively, but not immediately, destroyed by high temperatures and under certain conditions of processing. Most cat food manufacturers supplement their products to compensate for possible losses, but some home prepared diets may require additional thiamin. A deficiency of thiamin can also occur when cats are fed large amounts of certain types of raw fish which contain the enzyme thiaminase, although this is destroyed by cooking.   Death may develop in cats fed inadequately cooked fish or soy-based food and/or cooked products inadequately supplemented with thiamin.

Initial signs of thiamin deficiency appear within 1-2 weeks of the introduction of a deficient diet and include salivation and a failure to eat despite being interested in food. Weight loss, vomiting and mild ataxia may also be apparent. This progresses to a critical stage in which there are severe neurological disturbances with impaired righting reflexes and short tonic convulsions with ventroflexion of the-neck. Mydriasis, circling, dysmetria and spinal hypersensitivity may also be observed. Eventually, there is a spasticity of all the limbs such that the cat appears to be 'walking on its toes' and there may be cardiac irregularities. The terminal stage of the disease is characterized by semi-coma, continuous crying, opisthotonos and, ultimately, death.
 

VITAMIN C (ASCORBIC ACID)

Ascorbic acid, commonly known as vitamin C, has a chemical structure that is closely related to the monosaccharide sugars. It is synthesized by plants and most animal species, including dogs and cats. Ascorbic acid is produced in the liver from either glucose or galactose through the glucuronate pathway. With the exception of humans and a few other animal species, all animals, including cats,  are capable of producing adequate levels of endogenous vitamin C and therefore do not have a dietary requirement for this vitamin.  Therefore, unless there is a high metabolic need or inadequate amounts are being synthesized by the body, a dietary source of ascorbic acid is unnecessary in cats. In addition to being unwarranted, ascorbic acid supplementation in cats may be detrimental. Excess ascorbic acid is excreted in the urine as oxalate, and high concentrations of oxalate have the potential to contribute to the formation of calcium oxalate uroliths in the urinary tract.
 

VITAMIN E

Vitamin E functions as a biological, chain-breaking antioxidant that neutralizes free radicals and prevents the peroxidation of lipids within cellular membranes. In the diet, it limits the peroxidation of dietary lipids and the development of rancidity; greater amounts of this vitamin are thus required when the diet contains high levels of polyunsaturated fatty acids, which are easily oxidized. The vitamin is preferentially oxidized before the unsaturated fatty acids, thus protecting them from rancidity. However, in this process, vitamin E is destroyed.  A cat's requirement for vitamin E depends on dietary levels of polyunsaturated fatty acids (PUFAs) and selenium, a trace mineral. Vitamin E and selenium function synergistically. Therefore, as the level of unsaturated fatty acids in pet foods increases, the amount of vitamin E should increase.  There is individual variation between cats in their dietary requirements for vitamin E, which can also be affected by dietary levels of selenium, sulfur aminoacids, other anti-oxidants and pro-oxidants in prepared foods, and by individual susceptibility to peroxidation.

A deficiency of vitamin E (a-tocopherol) in cats results in pansteatitis (yellow fat disease) which is a painful inflammatory condition of the subcutaneous fat.  The condition is usually associated with diets of oily fish (especially red tuna) which are rich in polyunsaturated fatty acids or with feeding rancid, oxidized fat (commonly found in dry foods). The clinical signs of vitamin E deficiency are related to the deposition of ceroid, the end product of lipid peroxidation, in adipose tissue. This provokes a foreign body reaction and results in inflammation, with massive neutrophilic infiltration, and fat cell necrosis. Fat thus affected is firm, painful and nodular on palpation; in the latter stages of the disease it assumes a dirty orange or mustard yellow color attributable to the ceroid pigment. Subcutaneous fat is most notably affected, but fat within the body cavities may have a similar appearance.

Initially, the affected cat will be inappetent and show pain and hypersensitivity to touch. A fever develops, which is related to the inflammatory and necrotic lesions, and is unresponsive to antimicrobial therapy. Abdominal pain is apparent and vomiting may occur. Subsequently, the nodular character of the subcutaneous fat may be detected on palpation but this procedure may be vigorously resented by the cat.

Dietary correction is required and additional supplementation with 75-100 mg a-tocopherol acetate per day PO may be given initially to replace body stores. Short-term therapy with prednisolone is valuable in controlling the inflammatory reaction. Parenteral nutritional support may be necessary, but this should be administered intravenously as subcutaneous fluid therapy is likely to be painful and absorption reduced. Recovery is slow and may demand several weeks or even months of continued therapy. Occasionally, severely affected cases do not respond to treatment and they continue to deteriorate until they either die or are euthanized. Rarely, sudden death occurs.  Therefore, foods with high levels of polyunsaturated fatty acids (PUFAs) should be avoided unless expressly ordered by a veterinarian.
 

Vitamin D

Cats cannot manufacture vitamin D or its precursor 7-dehydrocholesterol.  Cats appear to have an extremely low dietary requirement for vitamin D provided that they have exposure to some sunlight and are otherwise well nourished. However, vitamin D toxicity can be produced relatively easily and is usually the result of overzealous dietary supplementation with, for example, cod-liver oil. As with all fat-soluble vitamins, excesses are stored in the body and their effects are cumulative. The resulting hypercalcemia and hyperphosphatemia lead to soft tissue calcification, which may be demonstrated radiologically, and to multiple organ dysfunction. There may be neuromuscular abnormalities, typified by general weakness and poor motor reflexes, and resorption of bone resulting in pathological fractures. Cases are normally presented because of the most. obvious signs of renal failure, and the prognosis is always guarded. Treatment is symptomatic and the cat should be encouraged to eat a balanced diet without additional supplementation.

Vitamin K

The synthesis of vitamin K by bacteria in the large intestine of dogs and cats can contribute at least a portion, if not all, of the daily requirement in these species. Therefore a dietary supply of this vitamin only becomes significant when bacterial populations in the large intestine are reduced, such as during medical treatment with certain types of antibiotics, or when there is interference with the absorption or use of vitamin K from bacterial sources.

Minerals

There is a paucity of data available on the mineral requirements of the cat. Nevertheless calcium, phosphorus, sodium, potassium, magnesium, iron, copper, zinc and iodine are all indispensable in this species; their presence is essential for the maintenance of acid-base balance and tissue structure and as enzyme cofactors. The overall balance of the diet is affected not only by the finite levels of these minerals but also by the interactions between them. For example, the ratio of calcium: phosphorus is important in the maintenance of bone and cellular integrity; in cats, this should be within the acceptable range of 0.5:1 to 2:1. A mineral excess may therefore be as harmful as a deficiency. Home prepared, meat-rich diets may well require mineral supplementation as meat is a poor source of calcium but a relatively rich source of phosphorus.

If adequate amounts of calcium and phosphorus in the proper ratio are provided but without sufficient vitamin D, abnormalities of bone result again. Insufficient levels of vitamin D interfere with calcium absorption in the intestine. Excessive amounts of vitamin, D in the presence of adequate levels of calcium and phosphorus may result in excessive mineralization of bone, abnormal teeth, and calcification of the soft tissues of the body. The delicacy of these relationships is remarkable.

Unthinking or uninformed owners most often distort the calcium-phosphorus balance of their cat's diet by feeding a diet consisting almost exclusively of muscle meat or organ meats such as liver, heart, or kidney. All of these meats contain phosphorus but are devoid of calcium, which results in a calcium--phosphorus ratio of 1 to 15 or greater. Prolonged feeding of such a diet results in severe demineralization of bones, pain, and sometimes fractures or paralysis, a condition called nutritional secondary hyperparathyroidism. An adult cat may exist on such a diet for years without showing signs of disease, but the body changes are occurring nevertheless.  Remember that the wild ancestors and living relatives of the domestic cat relied on a variety of foods found in the entire body of their prey.

Magnesium

Magnesium is probably the mineral of the most concern in cat nutrition for owners, and especially to owners of male cats because of its role in the formation of struvite uroliths (magnesium ammonium phosphate). However, magnesium is not a "bad guy".  Magnesium is a macromineral, its amount in the body is much lower than that of calcium and phosphorus. Approximately 60% to 70% of the magnesium found in the body exists in the form of phosphates and carbonates in bone. Most of the remaining magnesium is found within cells, and a very small portion is present in the extracellular fluid. In addition to its role in providing structure to the skeleton, magnesium functions in a number of metabolic reactions; a magnesium ATP complex is often the form of ATP that is used as a substrate in many of these processes. As a cation in the intracellular fluid, magnesium is essential for the cellular metabolism of  protein. Protein synthesis also requires the presence of ionized magnesium. Balanced in the extracellular fluids with calcium, sodium, and potassium, magnesium allows muscle contraction and proper transmission of nerve impulses.

A commercial cat food should not be selected only on the basis of its magnesium content. The quantity of magnesium required to saturate urine with struvite at alkaline pH is very small.  As pH decreases below 6.4, the amount of magnesium required to saturate the urine with struvite increases exponentially.  Conversely, as urine pH increases above 6.9, the amount of struvite that forms in the urine increases markedly.  When urine pH is alkaline, the amount of struvite formed in urine is proportional to the dietary magnesium concentration. At urine pH values less than 6.1, struvite does not form regardless of the magnesium concentration of the diet.  Thus, the tendency of struvite to form is a function of urine pH. The magnesium content of the diet only becomes important when urine pH is greater than 6.1.

The food's caloric density, digestibility, and urine-acidifying properties should all be considered when selecting a commercial cat food for the prevention of struvite urolithiasis.  Constituents of foodstuffs exert major effects on urine pH. Sulfur-containing amino acids, phospholipids, and phosphoproteins naturally found in a carnivorous diet naturally acidify the urine, whereas salts of organic acids alkalinize it.  Salts of dietary organic acids, which come primarily from plant material and found in high quantities in dry food, have an alkalinizing effect.

The percentage of magnesium in the diet is not as important as the total amount of magnesium that a cat consumes. Diets that are moderate in caloric density and are highly digestible will be consumed in smaller amounts, thus lowering both DM and magnesium intake.  High-quality canned food is typically >90% digestible whereas equal quality dry food is ~80% digestible.  The lower DM intake results in decreased fecal matter and fecal water and increased urine volume. Feeding a canned diet with these characteristics further contributes to increased urine volume and decreased urine specific gravity reducing the risks of urolithiasis while supplying the cat's dietary magnesium requirements.  Although many  brands of dry cat food may contain relatively low concentrations of magnesium, they are often lower in digestibility than canned and contain high levels of cereal grains. Because the cat's requirement for dietary magnesium is substantially lower than the amount usually found in cat food, a general rule of thumb is to select a high-quality, highly digestible, canned food that contains 0.12 % magnesium or less.
 
 

 

Follow the dietary recommendations a knowledgeable veterinarian to prevent nutrition--induced disease in your cat.
 

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Dry Food vs Canned Food.  Which is really better?

Economy and convenience have made dry food the most popular product to feed to pet cats. But is what's good for us also good for our cats?

A carnivorous diet is comprised of primarily protein and fat from animal tissue.  They have teeth designed to tear flesh and a short and simple gastrointestinal  tract, one suited for digestion and absorption of a concentrated, highly digestible diet.  Dry foods typically contain 35-40% carbohydrate.  Carbohydrates are nearly absent in the cat's natural diet.  The cat obtains small amounts of carbohydrate through the stomach and intestines of her prey.  Commercial dry foods, however,  may contain as much as 45% - 50%  carbohydrates.  Since the cat metabolizes primarily fat and protein for energy, most of the excess carbohydrate is stored in the body as glycogen and fat.  The primary adverse effect of excess carbohydrate is obesity.  The effects of obesity are heart disease because of the increased workload on the heart; orthopedic problems are increased because of increased physical stress on the frame, leading to arthritis and early debilitation; diabetes mellitus, a condition in which the pancreas doesn't produce the amount of insulin that it should to help metabolize blood sugar, is one of  the most common problems in obese cats; several liver disorders occur more frequently in overweight cats. Surprisingly, the deadliest one, hepatic lipidosis, happens when the cat stops eating. Changes in the operation of the liver cause fat to be deposited there, which eventually can shut down the liver altogether.

In the cat's liver, gluconeogenic amino acids and fat in the diet are deaminated and converted to glucose for the maintenance of blood glucose levels.  The cat has evolved to maintain normal blood glucose levels and health on a carbohydrate-free diet, a capacity inherited from her desert-dwelling ancestors. This ability is related to its different pattern of gluconeogenesis.  In most animals, maximal gluconeogenesis for the maintenance of blood glucose levels occurs during the postabsorptive state, when dietary soluble carbohydrate is no longer available. However, carnivorous species, such as the cat, are similar to ruminant species in that they maintain a constant state of gluconeogenesis - the immediate use of gluconeogenic amino acids for the maintenance of blood glucose levels (these mechanisms are turned "on" and "off" in other animals).

There are differences between cats and omnivores in the relative importance of various gluconeogenic and carbohydrate-metabolizing pathways. Compared with omnivorous species, the cat has a high hepatic activity of the enzyme serine-pyruvate aminotransferase and low activity of the enzyme serine dehydratase. Thus the cat is able to convert the amino acid serine to glucose by a route that does not involve either pyruvate or serine dehydratase.

After glucose is absorbed into the body, it must be phosphorylated to glucose-6-phosphate before it can be metabolized. The liver of most omnivorous animals, including the domestic dog, has two enzymes that catalyze this reaction, glucokinase and hexokinase. Hexokinase is active when low levels of glucose are delivered to the liver, and glucokinase operates whenever the liver receives a large load of glucose from the portal vein. The feline liver has active hexokinase but does not have active glucokinase. Consequently, the rate of glucose metabolism in the liver of the cat cannot increase in response to high levels of soluble carbohydrate in the diet to the same degree as the rate in the liver of a species possessing both enzymes. Thus most of the carbohydrate in dry food ingested by the cat is converted and stored as fat.

Not all protein sources are of equal value to the carnivore, and the quantity of protein in a commercial dry cat food often says nothing about its quality. Before domestication, cats hunted their prey and consumed a diet very high in meat protein, low to moderate in fat, and very low in carbohydrates. This diet provided both the proper quantity and quality of protein for the carnivore's unique digestive system. Unlike an omnivore, whose digestive system consists of a fairly large small intestine and relatively large stomach, the carnivore's system consists of a fairly short small intestine and relatively small stomach. Thus, a carnivore's optimum diet must be concentrated, highly digestible, and low in residue because its body is designed to digest primarily protein. If an excess of carbohydrates is included in the diet, much of what the carnivore eats is only partially digested by the time it reaches the large intestine for fecal formation, overloading the digestive and excretory systems. ...

Protein digestibility in pet foods is about 80 percent for dry foods, 85 percent for semimoist and canned foods containing large amounts of cereal grains, and 90 percent for canned diets with meat as the primary protein source. Digestibility is influenced both by the source of the protein and by how it is processed. Protein in cat foods comes from both animal and plant sources. Animal protein is generally more expensive and often of higher quality than plant protein.  The composition of canned foods allows the use of protein and fat sources of higher biological value than can be used in dry food.  A recent survey (Morris, James G. and Quinton R. Rogers. 1994. Assessment of the nutritional adequacy of pet foods through the life cycle. Journal of Nutrition 124:252OS-2534S). compared a well-known canned food with the leading dry food, both of which claim to provide "balanced" nutrition.  The digestibility claim of the canned food was approximately 90%, while the digestibility of the dry food was rated at 80%.  The biological value of the protein content (in other words, how useful the protein is to the animal) was given as 70% for the canned food and 60% for the dry. Net utilization (the amount of food used by the animal in relation to the amount provided) can be calculated by multiplying digestibility by biological value. The results: 68 % net utilization for the canned  food and 48% for the dry. This means a cat would have to eat nearly twice the volume of dry food to achieve the net utilization that higher, more digestible sources of nutrients, found in canned food, would provide.  All those excess waste products must be filtered from the blood placing an extra workload on the kidneys.  This may explain the high prevelence of chronic renal failure in middleaged cats.

The cat's natural diet, live prey, contains between  65%-75% water. The cat, having evolved on the plains of Africa, has adapted to obtain  her water requirements almost entirely on the moisture content in her prey. Cats can live for long periods without drinking water when receiving food containing 67-73% water but become dehydrated when the water content of the food is 63% or less.  The water content of the commercial foods commonly fed to cats varies from 8% in dry foods to over 75% in canned foods; thus the amount of drinking water required is affected substantially by the water content of the food.

When fed canned food (80% moisture)  with access to drinking water, cats obtain over 90% of their total water intake from the diet, whereas on dry food, 96% of the total water intake is obtained by drinking. The total free water intake (from food and drinking water) decreases when cats are fed dry food only, so that the water to dry matter intake ratio when fed on commercial dry foods varies from 2.0 to 2.8: 1 whereas on canned foods it varies from 3. 0 to 5.7: 1.  Thus for any given dry matter intake cats have a higher water turnover on canned than on dry foods.  (National Research Council [National Academy of Science] Nutrient Requirements of Cats).

Diet moisture content is related to the observation that cats fed dry food drink more six times more water than cats fed canned food but that much of this water contributes to fecal moisture so that urine volume is lower and urine specific gravity higher in cats fed dry food.  The urine concentration of all solutes, including potentially calculogenic crystalloids, depends on urine volume.  Cats increase voluntary water intake when fed dry food but not in sufficient amounts to fully compensate for the lower moisture content of the food.  In a recent study, cats consuming a diet containing 10% moisture with free access to drinking water had an average daily urine volume of 63 milliliters (ml). This volume increased to 112 ml/day when fed a canned diet with a  moisture content of  75%.  Urine specific gravity was also higher in cats that were fed the low-moisture food.  Decreased urine volume may be an important risk factor for the development of urolithiasis in cats. Diets that cause a decrease in total fluid turnover can result in decreased urine volume and increased urine concentration, both of which may contribute to urolithiasis in cats.

Canned diets contain enough water that cats consuming them rarely need to drink.  Daily water needs, in milliliters, often are "guesstimated" as equal to the metabolizable energy requirement in kilocalories or approximately 60 ml/kg.  Once the diet is consumed, oxidation of nutrients produces an additional 10 to 13 grams of water for each 100 kcal of metabolizable energy.   Thus a 4 kg cat consuming a 240 kcal canned diet containing 78% moisture will consume 237 ml or 98% of its daily water need directly from the diet.  Thus the cat needs to drink less than 1 oz. of additional water per day whereas a cat consuming a 240 kcal dry diet needs to drink over 7 oz. of water per day.   This can be difficult becausecats are not naturally big drinkers.  Feeding a canned diet containing 78% moisture virtually guarantees homeostatic control of water balance in the cat.

In addition to canned food ensuring adequate hydration, a high water turnover helps eliminate crystallogenic substances before they grow to sufficient size to interfere with normal urinary function.  This is a very important consideration for male cats.   Cats that cannot urinate for more than 24 hours due to urinary tract obstruction can die from acute renal failure and/or severe damage to the urinary bladder.  In addition to the removal of crystals, benefits of increased water intake include dilution of any noxious substances in urine, and more frequent urination to decrease bladder contact time with urine that may reduce the risks of urinary tract disease.  For that reason, canned diets are usually prescribed as the first-line therapy for feline lower urinary tract disease.

The domestic cat is a carnivorous mammal. Compared with an omnivorous or herbivorous diet, a carnivorous diet has the effect of increasing net acid excretion and decreasing urine pH naturally.  This urine-acidifying effect is primarily a result of the high level of sulfur-containing amino acids found in meats. Oxidation of these amino acids results in the excretion of sulfate in the urine and a concomitant natural decrease in urine pH.  In addition, a diet that contains a high proportion of meat is lower in potassium salts than a diet containing high levels of cereal grains, which have been shown to produce an alkaline urine when metabolized.  Therefore the inclusion of high levels of cereal grains commonly found in high-carbohydrate (>35%)  dry cat foods has been shown to be a contributing factor in the development of struvite urolithiasis by producing an alkaline urine.

The "solution"  to "correct" the alkaline urine-struvite dilemma was the advent of acidified, magnesium-restricted "urinary tract health" diets.  Even though the maintenance of a urine pH of 6.4 or lower helps prevent the formation of struvite crystals, the production of urine that is too acidic can be detrimental to a cat's health. If more acid is consumed than an animal is capable of excreting, metabolic acidosis occurs. Several studies have shown that when some cats are fed an acidifying diet for several months, they develop metabolic acidosis, decreased levels of serum potassium, and depletion of body potassium stores. Other studies indicate that the long-term feeding of highly acidifying diets containing marginal levels of potassium cause hypokalemia and kidney disease in some cats.  For example, three out of nine cats fed an acidifying diet containing 40% protein and marginal levels of potassium developed chronic renal failure within 2 years. (J Am Vet Med Assoc 1993 Mar 1;202(5):744-51)  Feeding a diet that contains ingredients that naturally promote moderate urine acidification (sulfur-containing amino acids, phospholipids, and phosphoproteins, e.g., meat and animal fat - a carnivorous diet)  present less risk for overacidification than does supplementing a cat's diet with a urine-acidifying agent.   The alkalizing nature of carbohydrate-laden dry food requires more than twice to three times the amount of acidification than does canned food increasing the risk of acidosis and kidney damage proportionally.

Another effect of an acidified urine may be to promote the formation of another type of urolith. Although struvite is soluble in an acid urine, an acid pH may increase the likelihood of calcium oxalate formation. The prolonged feeding of a highly acidified diet leads to a loss of calcium in the urine, making this mineral available for the formation of calcium-containing uroliths. In addition, feeding a low-magnesium diet can exacerbate this problem because urine magnesium  inhibits calcium oxalate formation.  The incidence of calcium oxalate urolithiasis in cats has increased while struvite urolithiasis has decreased during the past several years.  It is theorized that the widespread feeding of acidifying diets that contain low levels of magnesium may be a contributing factor to this trend.

Early studies reported that more than 95% of uroliths in cats were composed of struvite.   However, the incidence of this type of urolith has changed significantly within the last 10 years since the advent of acidified, reduced-magnesium, "urinary tract health" diets.  A study conducted in 1981 found that 78% of feline uroliths analyzed at the Minnesota Urolith Center were composed of struvite and only 1% of calcium oxalate. By the mid 90s, the incidence of struvite urolithiasis decreased to 43% of the cases, while the incidence of calcium oxalate urolithiasis increased to 43%.  Struvite crystals and uroliths can be medically dissolved whereas calcium oxalate  cannot be medically dissolved requiring surgical removal in many cases.

Acidification of the urine is not without  potential toxicity. Dl-methionine causes hemolytic anemia, met hemoglobinemia, and Heinz body formation in cats.  Dl-methionine is commonly used in dry foods as a urinary acidifier. Additional concerns about chronic acidification are its potentially detrimental effects on renal function and bone development. Dietary potassium content also may be important because chronic metabolic acidosis can cause potassium depletion which can contribute to renal dysfunction. A syndrome of hypokalemic nephropathy occurs in cats fed an acidifying diet low in potassium.  Feeding a diet that has marginal amounts of potassium and that also contains excessive acidifying chemicals (e.g., dl methionine) may cause chronic metabolic acidosis and depletion of body potassium stores. Potassium depletion and hypokalemia may lead to renal dysfunction characterized by chronic tubulointerstitial nephritis (Chronic Renal Failure) and increased urinary fractional excretion of potassium, further aggravating potassium depletion.

Dry cat food is generally unpalatable to cats because of its dry nature.  Palatability of dry cat foods is enhanced by animal fats, protein hydrolysates, meat extracts, acid, and the amino acids alanine, histidine, proline, and lysine.  The preference for protein breakdown products and acidity may explain the use of "digest" as an ingredient in nearly all dry foods. Digest is "a microbiologically stable material resulting from digesting animal tissues. . . ."   It is produced by enzymatic hydrolysis of animal tissues and by-products, which yield a viscous solution of amino acids, peptides, and fatty acids. Digest also contains significant quantities of phosphoric acid, which is added to stop the enzymatic degradation process and to preserve the product. Digest is sprayed onto the outside of cat foods at 4 to 10 percent of the final finished product or is incorporated directly into the food. Digest can enhance the palatability of foods by as much as two- to threefold over the uncoated product. Once incorporated into cat food, the phosphoric acid increases the amount of acid ingested by the cat. Because of this manufacturing practice, urine acidifiers should not be given to cats fed commercial cat foods, however, nearly all dry foods contain urine acidifiers, most commonly, dl-methionine.  Chronic, overacidification leads to metabolic acidosis, demineralization of bone, calcium oxalate crystal formation and possibly renal damage.  This may be a contributing factor in increase of incidence of calcium oxalate urolithiasis and high prevelence of chronic renal failure in middleaged cats.

The only benefit of feeding dry food is the marginal dental benefit. However, as is typical of carnivores, the teeth of the cat are appropriately modified for grasping, puncturing, and tearing (cutting), rather than for true mastication. With the exception of "crunching" dry food, cats do little, if any, actual chewing.  The hinging of the lower jaw can only be moved up and down and possesses no ability for a lateral chewing motion.

The cat has no first premolars and no lower (inferior) first or second premolars; the molars consist of a single upper and lower tooth on each side. When the mouth is closed, the upper sectorial tooth slides across the vestibular surface of the lower sectorial tooth,  producing an effective scissor-like cutting action, rather than a chewing action. Thus the dental benefits of feeding dry food are grossly overrated.

It has long been felt that feeding a cat or a dog a dry kibble diet is better for the teeth than feeding them a canned diet. The logic goes that dry food leaves less residue in the mouth for oral bacteria to feed on and so plaque would accumulate at a slower rate. Despite that, many animals fed on commercial dry diets still have heavy plaque and calculus accumulations and periodontal disease. This is because most dry pet foods are hard but brittle so that the kibble shatters without much resistance and so there is little or no abrasive effect from chewing.  A small portion of dry food (no more than 25%) or so-called "tarter reducing" treats (no more than 10% of the cat's total daily caloric requirement) probably have the same slight dental benefit as an all-dry diet without the accompanying risks and adverse effects.
 

....to be continued

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