Fiber Content of Pet Foods
The type of fiber in commercial diets has been unimportant other than whether it causes diarrhea. High-fiber content causing large-volume bulky bowel movements has not concerned most manufacturers. The 20 percent fiber level of some pet foods is excessive. Some manufacturers now add less fiber and use a poorly soluble (so that it does not affect digestive function) and moderately fermentable (so that it provides nutrients for the colonic mucosa but not enough fermentation of fiber to cause diarrhea) fiber. Beet pulp and rice bran are examples of such fibers. If the need for fiber is small why give an excess that can cause impaired digestive tract function, bulky bowel movements, and diarrhea?

Fiber is the source of energy for colonic mucosal cells.  Bacteria accomplish that by producing short chain fatty acids (acetate, proprionate and butyrate). More than 70 percent of colonic cells’ energy is dependent on these fatty acids. Fiber levels needed to meet these requirements is probably low. Many animals live on very low fiber diets for years without developing colonic disease.

Fiber for Management of Disease
Fiber is added to commercial pet foods designed for weight reduction. Companies claim this to be effective because fiber reduces digestibility of other nutrients and supposedly reduces appetite or hunger by filling the stomach. Sometimes veterinarians vary the dietary content of fiber for other reasons. They often feed low fiber diets to dogs with chronic diarrhea. Some recommend feeding high fiber diets to dogs with colitis, however. High-fiber diets can help manage some animals with diabetes mellitus. Although veterinarians promote additional fiber in pet foods to treat these medical problems, there is little scientific evidence that additional fiber is of any value.

Fats^5,9,10
Animals eat primarily to satisfy their caloric needs. A strictly carnivore's diet contains very little carbohydrate, but is rich in fat and protein. Carbohydrates and fats usually provide most of the calories in pet foods; proteins are a minor source of calories. Some dogs are unable to maintain normal weight with low dietary fat. Fats contribute to a diet’s palatability and acceptable texture. They also aid absorption for fat-soluble vitamins. Triglycerides are not essential nutrients for dogs or cats which have requirements for two or three unsaturated fatty acids, however. Fats are not expensive but increasing a pet food’s fat content increases its cost; therefore, most commercial pet foods are low-fat. Fat in moderation is the most digestible nutrient.

Essential Fatty Acids
Three essential unsaturated fatty acids required by animals are linoleic acid, linolenic acid and arachidonic acid. Plants but not animals produce linoleic and linolenic acids. Animals, except cats, produce arachidonic acid by chemical modification of linoleic acid. Deficiency of these three fatty acids results in reduced growth rate, skin problems and inability to reproduce. Rich sources of unsaturated fatty acids include egg yolks and vegetable oils.

Linoleic acid is the source of omega-6 fatty acids, and linolenic acid is the source of omega-3 fatty acids. Animals need more omega-6 than omega-3 fatty acids for health. Some scientists believe that the omega-3 fatty acids are important in maintaining health for older individuals, however. Fish contain higher levels of omega-3 fatty acids (see table) than most other dietary sources. People who consume large quantities of fish have lower blood cholesterol levels and a lower incidence of heart disease than people consuming foods with lower levels of omega-3 fatty acids.

Omega-3 Fatty Acid Content of Omega-3-rich Foods

Omega-3 fatty acids (grams) in 100 grams of food
Salmon    0-1.5	Mackeral     2.2	Herring     1.7
Albacore tuna     0.5	Trout     0.6	Whitefish     1.5
Canola oil     11.1	Soybean oil     6.8	Cod liver oil     19.2
Wheat germ oil     6.9	Butter     1.2	Lard     1.0
Flax oil     57.0

Even with fish diets animals consume more omega-6 fatty acids than omega-3 fatty acids. Some manufacturers suggest the optimal omega-6 to omega-3 ratio to be between 4 and 10 to 1, and a diet with a ratio of greater than 50 to 1 is likely to result in omega-3 fatty acid deficiency. No studies prove that omega-3 fatty acid deficiency causes any problems, however.¹¹ Requirements  are unknown for these fatty acids and also for any ratio of the omega-6 to omega-3 fatty acids. The body uses omega-6 fatty acids to make chemicals that promote inflammation. It also uses omega-3 fatty acids to make similar chemicals that may be important in reducing inflammation.

Dietary Fat Requirements
Low fat diets for dogs―less than 5 percent of dry matter total fat and 1 percent essential linoleic acid―leads to dry, scaly skin and harsh coat. High fat content introduced abruptly also may cause problems. Undigested fat causes steatorrhea. Excess dietary fat reduces food consumption, which can cause deficiency of other nutrients unless their dietary levels increase. Protein, iodine and thiamin levels should especially be augumented when dietary fat increases. Excess fat consumption is also likely to cause obesity. There are no clearly defined optimal ranges for dietary fat levels. A minimum recommendation of 5 percent is usual. The ranges preferred by most dog owners involved with breeding and show dogs or working dogs are considerably higher (15 to 35 percent of dry matter), however. The normal dog requires linoleic acid at a dietary level of about 1 percent (this is about 2 percent of the calories). Cats have the same requirement. In addition, cats need a source of arachidonic acid. Linoleic acid does not provide that source. Dogs can eat a meat-free diet and receive their nutritional requirements for all unsaturated fatty acids. Cats must eat meat to obtain their arachidonic acid requirements. (There are few exceptions, notably borage oil, red current seed oil and evening primrose oil which contain arachidonic acid.)

Requirements for a High-Fat Diet
High fat diets are important for many dogs. Milk fat is the most important source of calories for unweaned animals. Bitch's milk contains about 10 percent fat, which is much greater than cow’s milk. Puppies double their body weight rapidly and need calories to sustain that growth and activity. Fat content of queen's milk is greater than cow's milk but much less than that of the dog. With fats providing caloric needs, carbohydrates are less important for energy. Lactose in milk is tolerated well unless for some reason there is insufficient intestinal lactase activity. High fat diets are needed to maintain normal body weight in most active larger breed dogs. Many such individuals do not receive enough calories from poorly digested commercial pet foods. They lose weight or are unable to gain weight no matter how much they eat. Dogs working at strenuous activities, such as sled dogs, maintain normal weight by eating high-caloric density diets.

Problems With Feeding a High-Fat Diet
High fat diets are relatively safe for dogs and cats. Feeding a high fat diet is thought to cause acute pancreatitis, but that has never been proven.¹² Many dogs with pancreatitis have a history of eating a fat-rich meal or raiding garbage containing fatty meat scraps, however. Normal dogs fed a 70 percent fat diet do not develop acute pancreatitis. Some working dogs consume that amount. High fat diets cause problems in animals with maldigestion or malabsorption where unabsorbed fat enters the colon; normally very little fat reaches this point. Bacteria that normally live in the colon transform dietary fats to fatty compounds that are essentially the same as the active ingredient in castor oil (ricinoleic acid). Bacteria make one small change in normal fat and convert it to a potent laxative. Ricinoleic-like compounds damage the colonic mucosa, stimulate colonic water secretion, and stimulate intestinal motility, all of which contribute to diarrhea. Therefore most diets recommended for the management of chronic diarrhea are low in fat, even for dogs and cats with no loss in their ability to digest and absorb fats.

Dietary Fats and Palatability
Dietary fat is important for enhancing palatability. Low fat diets are unpalatable. Industry works harder to improve palatability than anything else. For dogs and cats the most important nutritional problem today is not any nutritional deficiency but obesity. Feeding diets with greater palatability and neutering contribute to the problem. Dogs and cats that once ate to satisfy their caloric needs now eat to satisfy their appetite.

Proteins^13,14

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. Animals 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. Dog and 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. Careful balancing of proteins from plant sources can improve a diet’s protein quality and make them suitable for meeting pets’ needs. The biological values of pet food proteins are largely unknown, however. Their value or availability changes when combined with other ingredients and after processing. A nutrient’s adequacy and availability can be known only through feeding trials, something the pet food industry wants to avoid.¹⁵

Protein Requirements
The minimum requirement for a protein with an ideal amino acid profile―not generally available on the supermarket shelf―is probably only 5 percent for dogs at maintenance(not working, pregnant, etc.). This estimate is based on a zero nitrogen balance (the animal is neither gaining nor losing nitrogen). Dogs need much more protein, 13 percent for young adults and 19 percent for older dogs to maintain tissue reserves of protein. Cats need much more protein in their diet. The minimum requirement is 24 percent protein. Most cat foods contain higher levels of protein. Diets must contain more proteins when their quality (biological value) is lower or when they have less than an ideal amino acid profile (Graph below). Dog foods contain mixed animal and plant proteins. Because plant proteins are poorer quality, dog foods usually contain proteins at levels of 15 percent for maintenance, 25 percent for growth and breeding, and 30 percent for severe stress.

Protein needs with proteins of different biological values. Graph shows protein intake in grams/10 lbs. body weight of animal being fed.

Essential Amino Acids and Nitrogen Intake
Dietary protein is essential for two reasons. It must supply amino acids which animals cannot synthesize which makes them essential. Amino acids essential for all animals are leucine, isoleucine, valine, tryptophan, phenylalanine, histidine, methionine, threonine, and lysine. Two additional amino acids essential for cats are arginine and taurine. A complete definition of an essential amino acid is that which the body cannot synthesize at a rate fast enough (from constituents normally in the diet) for normal growth or maintenance. Animals can synthesize seven of the nine essential amino acids (i.e., all except lysine and threonine) from chemical structures normally found in the animal but not in the diet. The body cannot make lysine and threonine; the diet must provide them. Protein is also essential in the diet to supply nitrogen for synthesis of the dispensable amino acids and other products containing nitrogen such as the building blocks of DNA, RNA, hemoglobin, etc. The body also uses amino acids to make chemicals that serve as regulators of neurologic function and cell growth.

Dynamic State of Body Proteins
Adult animals have ongoing requirements for dietary protein because body proteins are in a dynamic state; the body continually degrades and rebuilds them. Some proteins such as in gastrointestinal mucosa turnover rapidly while others such as in tendon and ligament turnover slowly. During protein degradation and rebuilding, the body does not completely reuse the amino acids released. Dietary amino acids replace those lost during degradation. When caloric intake is inadequate to meet energy requirements, body proteins are catabolized and used for energy. When the diet provides all nutrients needed for energy requirements, amino acids requirements are less and minimal protein is degraded. During disease with fevers, infection and trauma, extensive body protein can be lost―much greater than with starvation. Release is augmented for hormones such as cortisone to promote body protein breakdown. Except during growth the amount of protein or nitrogen in the body should remain stable―the animal is in nitrogen balance. When the intake of nitrogen exceeds excretion there is positive balance and the animal stores nitrogen. With a negative balance, body protein is degraded. Nitrogen balance is not only the result of having the proper amount of protein in the diet. The balance also depends on the quality of protein. To maintain balance more protein is required when its biological value is low and less when that value is high

Formulation of Diets to Meet Amino Acid Needs
Requirements for dietary protein are based on (a) an animal’s nitrogen requirements, (b) an animal’s amino acid requirements, and (c) dietary protein’s amino acid composition. Nitrogen requirements are expressed as crude protein requirements. When a diet satisfies that requirement, it is necessary to find the first limiting amino acid. Dietary levels of that amino acid decide the least amount of protein necessary to satisfy all essential amino acids. When a diet satisfies requirements for the first limiting amino acid, it satisfies all other amino acid requirements. One can evaluate some commonly used cereal proteins for their amino acid content and decide which ones are limiting. Graph below shows the amino acid content of wheat grain protein and the ratio of the amount contained to the amount required for a growing puppy. The smallest ratio represents the limiting amino acid. Lysine and the sulfur containing amino acids (methionine is also sulfur containing) are deficient for puppies. A protein containing lysine and the sulfur amino acids or the amino acids themselves must be added. To feed wheat without any other protein or amino acids, the amount fed would have to more than double. 

Besides calculating the protein and amino acid requirements, It is necessary to determine their availability. The minimal protein requirement is known only after calculating dietary protein’s digestibility. Feeding trials give information on protein digestibility; otherwise, it is merely a guess. Moreover, it is not possible to know the efficiency of use for a protein, or its biological value, from amino acid or protein analysis of a food. That is done only by evaluating performance during feeding studies. Digestibility and utilization vary greatly for different proteins. A diet's nonprotein nutrients, such as fiber, influences some of that. Cooking improves the digestibility of proteins. Sometimes heat can decrease digestibility, however, especially if protein is cooked with sugars. Heating reduces lysine availability and further reduce its low levels. Processing meat meal and finished commercial pet food products can reduce protein and amino acid availability. The calculated composition can change so the product does not meet an animal’s nutritional requirements. Only feeding studies can verify that a diet is complete and balanced.

Wholesomeness
The digestibility and biological value of any pet food depends mostly on its raw ingredients. Government and industry also call for pet food ingredients to be “wholesome.” The definition of wholesome is “good for one’s health” or “healthful.” Wholesomeness has an important effect on digestibility and biological value. Owners preparing pet food can easily select wholesome ingredients to make up their diets. Owners feeding commercial pet foods have little control over wholesomeness of the product. Because plant proteins are not complete for providing essential amino acid needs some animal proteins or amino acids must be fed. Animal proteins are more expensive than plant protein sources. The cost of pet foods is lessened by using no more animal protein than is necessary. Using sources of animal proteins not fit for human consumption also reduces the cost. For example, animal meat meals are unfit for human consumption. Some meat meal protein is from animals that died and were beginning to decompose. Protein sources are the most likely food ingredients to be unwholesome. A pet food is not wholesome when animal products are used that have been produced from dead and rendered animal carcasses. Ingredients listed (and defined by AAFCO¹⁶)on a pet food label that are not likely to be wholesome include:

Meat meal is rendered product from animal tissues exclusive of any added hair, hoof, horn, hide trimmings, manure, stomach and rumen contents, except in such amounts as may occur unavoidably in good processing practices.

Meat and bone meal is the rendered product from mammal tissues including bone, exclusive of any added hair, hoof, horn, hide trimmings, manure, stomach and rumen contents, except in such amounts as may occur unavoidably in good processing practices. It shall not contain added extraneous materials not provided for in this definition.

Animal by-product meal is rendered product from animal tissues exclusive of any added hair, hoof, horn, hide trimmings, manure, stomach and rumen contents, except in such amounts as may occur unavoidably in good processing practices.

In contrast protein can be produced from slaughtered animals rather from carcasses of animals dead for days by using the following AAFCO defined ingredients:

Meat by-products derived from slaughtered mammals is non-rendered, clean parts, other than meat. It includes lungs, spleen, kidneys, brain, livers, blood, bone, partially defatted fatty tissue, and stomach and intestines freed of their contents. It shall be suitable for use in animal food.

Poultry by-products must consist of non-rendered clean parts of carcasses of slaughtered poultry such as heads, feet, viscera, free from fecal content and foreign matter except in trace amounts as might occur unavoidably in good factory practice.

Meat is the clean flesh derived from slaughtered mammals and is limited to that part of the striated muscle which is skeletal or that which is found in the tongue, in the diaphragm, in the heart, or in the esophagus with or without accompanying fat, skin, sinew, nerve, and blood vessels.

Poultry by-product meal consists of the ground, rendered, clean parts of the carcass of slaughtered poultry, such as necks, feet, undeveloped eggs, and intestines, exclusive of feathers, except in such amounts as might occur unavoidably in good processing practices.

Slaughtered animals can also provide protein for animal feed when a diseased carcass is condemned for human use. In other cases, protein contaminated with fecal material is condemned but not for animal food. Some protein in pet foods is from animal parts that humans do not consume for esthetic reasons. Some meat sources of protein have nutritional limitations in diets for pets.

Analyses of protein ingredients such as meat and bone meal show great variability. There are virtually no standards so that some products may contain greater amounts of bone than is needed. There is also no analysis on amino acids that meat and bone meal provides when mixed with other foodstuffs. Most nonmeat proteins in pet foods cannot by themselves provide a completely balanced diet. For example, vegetable proteins are deficient in the amino acid lysine. Pet foods need meat products to provide enough lysine. Meat and bone meal is a source of lysine. Heat during rendering to make meat meal destroys lysine, however. Pet food must be analyzed to determine if it contains adequate amounts of lysine. Pet food manufacturers do not say if that is done. Manufacturers can add lysine to meet a pet's nutritional requirements. Larger manufacturers may do that. Smaller ones are not likely to do either the analysis or the addition. Small companies produce a considerable amount of pet food. Many have a primary interest in making foods for the livestock industry, not in making pet foods, however. Foods with predictably high levels of microorganismas or toxins can produce signs of gastrointestinal disease and cannot be considered wholesome. Meat meal is often contaminated with pathogenic bacteria.

Unique Feline Needs ^17,18
Cats have a number of unique nutritional needs and their diets must be formulated differently from canine diets. Feline diets must provide proteins containing essential amino acids for which cats have an absolute requirement. Dietary proteins must also provide more nitrogen than for most other animals. Cats do not conserve nitrogen as well as other animals. Their enzyme activities for metabolizing amino acids are greater than in other animals and that activity does not decrease when they eat a low protein diet. Excess amino acid destruction continues, leaving insufficient amounts for making protein. Many animals can survive on protein intakes of 4% to 8% of the total dietary calories. In contrast, cats need 18% to 20% of total calories as protein for growth and 12% to 13% for adult maintenance. Thus cats need two to three times more protein than most other animals under comparable circumstances. The 18% to 20% of total calories for a growing kitten represents about 25% of the dry weight of the diet. Commercial cat foods contain 30% to 35% protein on a dry basis, which is an excess because cats poorly digest the proteins in these diets. Diets formulated for dogs contain too little protein for feeding cats. Cats cannot synthesize the essential amino acid citrulline that is low in any food. Cats can convert arginine to citrulline, however, and that means that feline diets must contain arginine to meet the need for citrulline. Cats fed a diet lacking arginine develop hyperammonemia and show clinical signs of illness within several hours. Ammonia accumulates because it it is not converted to urea; arginine and citrulline are needed for that conversion. Feline diets must contain arginine. Cats have only a limited ability to synthesize the essential amino acid taurine from sulfur-containing amino acids. Therefore, a cat's diet must provide taurine. It also helps for the diet to be rich in sulfur-containing amino acids. Diets low in protein, and therefore sulfur-amino acids, are more likely to induce taurine deficiency. Taurine is the most abundant free amino acid in the body. It is not incorporated into body proteins. Its many important functions include being a precursor for bile salts (both cats and dogs have an obligatory and continuous requirement for taurine to make bile salts to replace bile salts lost continuously in the feces). Taurine is also involved in growth and maturation of nervous tissue, maintenance of normal vision, normal heart function, and female reproduction. Taurine is found in all animal tissues but not in plant materials.  Since taurine is free, not incorporated in proteins, in animal tissues, it readily leaches out in water. Cooking meat in water and discarding the water can greatly reduce its taurine content. Feeding proteins from plants such as soybeans and from animal products such as cottage cheese provide no taurine. Canned diets require higher concentrations of taurine to maintain normal levels than dry foods. No reason is known for this difference other than the two diets have very different formulations. Within the last decade two diseases, dilated cardiomyopathy and central retinal degeneration, appeared in cats fed commercial diets containing insufficient taurine. (Surprisingly the foods' taurine concentrations were those recommended by the NRC.) Only some animals’ problems can be reversed with taurine supplementation, so it is important that taurine is adequate in any feline diet. Taurine deficiency does not appear in cats living under natural conditions, catching their own food, or where the animal is eating what nature designs a carnivore to eat, meat. Dry expanded cat foods have a safe taurine concentration if it exceeds 1200 milligrams taurine per kilogram dry matter. In contrast, canned foods need at least 2000 milligrams taurine per kilogram dry matter to maintain adequate plasma concentrations.

Cats show low tolerance for the amino acid glutamic acid. Excess amounts cause sporadic vomiting and thiamin deficiency. Glutamic acid is abundant in vegetable proteins and is comparatively low in animal proteins.

Cats also have some unique vitamin needs. Cats do not have the ability to convert carotene to vitamin A. Lacking the enzyme for that conversion makes it necessary for cats to have vitamin A in their diet. Cats also do not have the capacity for converting tryptophan to the niacin. Cats metabolize tryptophan too rapidly to other compounds. The diet must provide niacin. Cats and dogs cannot manufacture vitamin D or its precursor 7-dehydrocholesterol. Thus, the diet must provide vitamin D.

Summary
Dietary carbohydrates are not necessary for most pets. They provide the most calories in commercial pet foods, however. Carbohydrates are the least costly of all nutrients. Carbohydrate rich foods vary greatly in digestibility and if they are not properly cooked digestibility can be poor. The cost rather than the quality of carbohydrate sources dictates which ones are used in commercial pet foods. Low cost pet foods containing poor quality sources of carbohydrates frequently cause a variety of health problems. Pet owners have little control over carbohydrate quality unless owner-prepared diets are fed. Fats are necessary for some essential fatty acids. Commercial pet foods provide these needs. Fats also can be an important source of calories. Very active larger breed dogs often do not receive enough calories on low to moderate fat diets. In these cases it is necessary to feed relatively high fat diets to maintain normal body weight and the stamina needed for their physical activity. Obesity is an important medical problem for a large number of pets. Feeding high fat diets contributes to the problem. The pet food industry prepares low fat diets to feed obese pets. Industry prepares some pet foods with high levels of omega-3 fatty acids that are claimed to be effective for treating inflammatory and allergic diseases. No one has shown any benefit from feeding these foods however. Pet owners completely control the kinds and amounts of fats when feeding owner-prepared diets. Proteins are the most costly ingredient in any pet's diet. Pet foods are inexpensive because they are made with cheap sources of protein. The least expensive are grains. Cereal proteins are of the lowest quality, however. Cereals do not provide all the required amino acids and their digestibility is poor. Pet food manufacturers add meat proteins to provide amino acids deficient in grains. Meat sources used are the cheapest available and often are ones that cannot or will not be used for human consumption. It is important to select wholesome protein sources to feed a pet. That is best achieved by feeding an owner-prepared diet.

References