Unique Nutritional Needs of Cats
Dogs are no longer treated as strict carnivores; they are now omnivores that eat a variety of foods. Cats are different with metabolic processes of a strict carnivore. Their requirements constrain the formulation of diets for cats more so than for dogs. Cats' diets must be formulated differently from dogs' diets.
Digestibility of Foods
Because of the cat's anatomy and physiology it may seem that the digestibility of many natural food ingredients, particularly those of vegetable origin, should be lower in cats than it is in most other animals. This is based on the cat's shorter small intestine, lack of cecum with its bacterial population, and generally shorter transit time. Design of the feline intestine is for a high-fat, high-protein diet that is, a high-energy, low-bulk diet. Despite this, most commercial cat foods contain many vegetable products, mainly cereal grains which are more incompletely digested than animal products. These diets cause no problems due to nutritional deficiency because essential amino acids, primarily taurine, and fatty acids are added. In addition, the diets contain salt and mineral contents for maintaining acid-base balance.
Unique Vitamin Requirements
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.
Protein and Amino Acid Requirements
A cat's diet must provide proteins and amino acids containing a source of essential amino acids for which cats have an absolute requirement. Dietary proteins must also provide a greater source of nitrogen than that needed by 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. Feeding low protein diets is always wrong for cats.
Many animals can survive on a low protein intake 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 the cat needs 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 integrity for the eyes’ rods and cones, 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. Proteins from plants such as soybeans and from animal products such as cottage cheese provide no taurine. These foods also greatly reduce a cat's ability to maintain normal plasma taurine concentrations. 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.) Taurine deficiency results in other important diseases. Only some of these problems can be reversed with taurine supplementation. Since some problems cannot be corrected, it is important to assure that the content of taurine is adequate for 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.
Fatty Acid Requirements
Cats have a deficiency of the enzyme needed to synthesize arachidonic acid from linoleic acid. Therefore, the diet's supply of unsaturated fatty acids needs to include arachidonic acid.
The natural diet of a true carnivore, such as the cat, is normally very low in carbohydrate, being mainly protein and fat. Blood glucose levels remain normal in carnivores as well as in omnivores but not principally because of the carbohydrates they eat. Carnivores convert amino acids and glycerol for glucose. A carnivore's liver does not receive high amounts of glucose from a carbohydrate meal. Therefore, the carnivore's liver is not prepared to store much glycogen made from carbohydrates. Cats can maintain blood glucose levels on starvation diets better than other starving animals usually fed high protein diets. Cats are also able to store more glycogen in the liver than others when fed a high protein diet.
Tolerance to Food Additives and Some Unique Foods
Cats are more sensitive to food additives than other animals. Cats are intolerant to the common food preservative, benzoic acid. Benzoic acid occurs naturally in plants but is not found in animal tissues. Cats have a limited ability to metabolize the chemical to a form that the body can excrete. Aspirin is a form of benzoic acid that cats excrete poorly.
Cat foods can contain propylene glycol which can damage erythrocytes. The result is a shorter life span that can lead to anemia. Relatively small amounts of onions can poison cats. Documented cases appeared after eating onion soup and baby foods containing onion as a flavoring agent. Cocoa and chocolate can poison dogs and the toxic derivative, theobromine, can be lethal for cats.
Cats may have other unique nutritional needs that are currently unknown. Feline hyperthyroidism has been associated with feeding commercial canned cat food. This disease may be related to cats' unique needs that are not being met.
The cause of feline hyperthyroidism is still unexplained. Many articles on this disease do not discuss any possible causes but some do indicate that feeding commercial canned cat food is still the most likely possibility but no one has ventured to investigate this as a cause.
Baker DH, Czarnecki-Maulden: Comparative Nutrition of Cats and Dogs. Annu. Rev. Nutr. 11:239-263, 1991.