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.