Feline nutrition

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Protein and amino acids

A key difference between the nutritional needs of cats and omnivores (e.g. dogs and humans) is;

Cats also show a greater tolerance for excess crude protein and several essential amino acids; and a lesser tolerance for glutamic acid than other animals. Cats also require taurine and niacin which can be synthesised by most animals from cysteine and tryptophan respectively. Cats cannot.

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.

The minimum requirement of bioavailable crude protein (CP) for an adult cat is about 160g/kg of diet (16% of dietary intake) whereas the dog is approximately half this. The CP needs does not vary significantly amongst age groups in cats, with kittens requiring a minimum 18% CP in diet with geriatric cats requiring approximately 14%. Most commercial cat foods reflect this protein need by containing between 28-30% crude protein. The reason why adult cats require as much crude protein in their diet compared with kittens is due to the nature of catabolic enzymes in cats. These enzymes, which are active continuously, move nitrogen into the liver for the urea cycle and for synthesising urea for excretion. These enzymes do not downregulate when cats are fasting or given a low protein diet as occurs in omnivores and herbivores. Cats however, can function well on diets with higher than 30% crude protein. This explains the critical nature of cats that are anorexic, and how they can quickly lose muscle mass, since the cat will break down muscle stores to meet its daily protein requirements if they are not provided in the diet. In addition, on low protein diets, cats will lose excessive amounts of nitrogen in the urine because it cannot be converted to urea, resulting in increased ammonia in urine.

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. An enzyme, arginase, is used by the cat to metabolise arginine to ornithine and urea. The highest activity of arginase is found in the liver, the only organ containing all the enzymes of the urea cycle, which bespeaks its important role in ammonia detoxification, and the importance of arginine in the diet. Significant arginase activity has also been detected in other organs which lack a complete urea cycle such as the kidney, prostate, lactating mammary gland, pregnant uterus, nervous system, intestine, skeletal muscle and in cancers of the stomach, prostate and colon. This widespread distribution of arginase in many tissues both normal and cancerous suggests many other functions besides its role in ureagenesis in the liver. These include biosynthesis of ornithine as a precursor to polyamines (spermine, spermidine and putrescine), glutamate (precursor of GABA) and proline. Polyamines are vital for cell proliferation and have been described as being modulators of ion channels and neurotransmitter receptors.
 

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.
 

 
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.