African wildcat Felis silvestris lybica.
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 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.
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.
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. Guaranteed analysis listed on the label of commercial cat foods are minimums or maximums only. Guaranteed analysis values are not the actual nutrient content of the food. They are a guarantee by the manufacturer that the food contains not more or less than the stated amount. In many instances, label guarantees are more confusing than helpful because of the erroneous assumption that they represent the actual nutrient contents of the food. Two foods with the same label guarantee may contain nutrient levels that may vary by more than 100%, yet both meet their label guarantees.
6. "Dry Matter Basis" (DM). Dry foods
can contain between 8-12% water, and canned foods contain between 70% and 80% water. To
make a accurate comparison of nutrients between various types and brands of cat foods, the
as fed (AF) numbers
must be converted to "dry matter" (DM) percentages, which represent the actual
percentages of nutrients consumed.
AAFCO recommendations use DM basis.
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.
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.
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
(Adapted from Nutrient Requirements of Cats, National Research Council)
*Neutered animals generally require 25% to 30% less carlories
than intact animals.
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.
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
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.
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
provide the most concentrated source of energy (8.5 calories/g) of any of the necessary
dietary components. With fat providing caloric needs, carbohydrates are not needed for
energy. Fats carry fat-soluble vitamins (D, E, A, K) and supply linoleic acid
(linoleate) and arachidonic acid (arachiclonate) that are essential to health in cats.
Cats deficient in essential fatty acids grow poorly, have dry hair and dandruff, and may
be listless and have increased susceptibility to infection. Diets lacking arachiclonate
will not support reproduction and adversely affect blood platelet function.Fat
digestibility may depend on both the composition and method of processing involved in
production of the diet. The "ether extracts" (the analytical method
used to estimate the fat content of food) shows that the apparent digestibility of
the fat content of cat food is 96 percent for canned foods, 92 percent for semimoist
foods, and 79 percent for dry foods. Fat calories should account for between 20 and
40 percent (DM) of total caloric intake.Unlike dogs, cats cannot convert linoleate to
arachiclonate, a characteristic they share with other strictly carnivorous animals.
Therefore, both linoleic acid (found in plant oils and animal fats) and arachidonic acid
(found only in animal tissues) must be supplied preformed in the diet of the cat. A diet
that derives about 2.5% of its calories from linoleic acid and at least 0.04% of its
calories from arachidonic acid will provide adequate levels of fatty acids and enough fat
for absorption of the essential fat-soluble vitamins.
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
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.
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.
(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.
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 (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.
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 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.
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.
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.
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.
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.
Follow the dietary recommendations a knowledgeable veterinarian to
prevent nutrition--induced disease in your cat.
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
....to be continued
Subscribe to Max's House Feline Health & Behavior Free Monthly Newsletter
New Feline Health & Behavior Topic Each Month
Max's House does not rent or sell any email addresses or other contact information that you provide.