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Proteins are an essential part of the diet of all animals. They provide the material for building muscle and for tissue repair as well as being a source of energy.
Proteins are complex molecules which are built from building blocks called amino-acids. Some amino-acids can be manufactured by an animal's own body, but many (in the case of a cat, eleven) can not. These amino-acids are very important in keeping the body healthy, and because it is necessary to have them in the diet, they are named "essential amino-acids". Some amino-acids are essential amino-acids for every creature in the whole animal kingdom but not all. For example, arginine is an essential amino-acid for cats but a non-essential amino-acid for humans, since human bodies can synthesize their own arginine.
The amino-acid content of proteins varies considerably, as does the ability of different proteins to be broken back down into amino acids within the body. The usability of a particular protein by the body is termed its 'biological value'. In simple terms, this means how much of a protein can be fully digested and broken down into amino-acids. Eggs have the highest biological value -100. Other animal proteins such as fish and meat fall in the 70-90 range. Vegetable proteins are relatively difficult to digest. Corn, for example, has a biological value of 45. In a cat's body the requirement for protein includes 20% for growth and metabolic function and 12% for maintenance. This is much higher than a dog's requirements, and is why cats need more protein in their diet than other animals, including dogs.
The eleven essential amino-acids for cats are: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, taurine, threonine, tryptophan and valine.
Arginine is important in cell division, the healing of wounds, and removing ammonia from the body. It is also needed for immune function, and the release of hormones. Cats are extremely sensitive to arginine deficiency which can develop if even a single meal is low in arginine. One of the early symptoms of arginine deficiency is hyperammonemia which manifests itself by salivation, neurological abnormalities, over-vocalization, increased sensitivity to touch, pain, or other sensory stimuli, vomiting and, in severe cases, coma. Good food sources of arginine include red meat, fish, and chicken.
Histidine is involved in many metabolic functions including the production of blood cells and tissue formation and repair. Histidine also binds to copper and zinc to improve their absorption into the body. Earlier studies have shown that histidine deficient kittens tended to develop cataracts in their adult life. Pork, poultry and cheese are all a good sources of arginine.
Isolucine is used by the body to produce a number of compounds which aid energy production. It is present in high levels in muscle tissue. Cats deficient in isolucine have an increased discharge from their eyes due to staphyococci (a type of bacteria) - probably due to the weakening of the immune system. The condition is reversible with an increased level of isolucine in the diet. Isoleucine is found in most foods and is particularly high in meats, fish, and cheeses.
Leucine stimulates protein synthesis in muscle tissue, and a deficiency of this amino-acid in kittens will result in reduced growth. Leucine is readily available from poultry, dairy products and red meat.
Lysine is concentrated in muscle tissue. It promotes bone growth and the formation of collagen (which makes up bone cartilage and connective tissues). It is particularly importent for kittens during their first few months of growth. Fish, meat and dairy products are all good sourse of lysine.
Methionine is involved in many bodily functions. It is one of the amino-acids which contain sulphur which helps to maintain healthy fur and skin. Neither young nor old cats need very much methionine and kittens with a methionine deficient diet do not seem to suffer from the lack. Methionine is present in small quantities in most foods.
Phenylalanine is an aromatic amino-acid which cats need to produce another amino-acid called tyrosine. In turn, tyrosine is needed to make melanin - the major pigment in cats' skin and hair. Black cats deficient in phenylalanine do not produce a sufficient amount of tyrosine and their hair colour changes from black to reddish-brown. Interestingly albino cats lack enzymes which convert phenylalanine to tyrosine. Therefore they do not produce melanin and this is why they have white hair and pale skin. Phenylalanine is found in most foods including meats and milk products.
Taurine is very important for cats. It is a major constituent of bile which is responsible for the digestion of fats and the absorption of fat-soluble vitamins. It is also important for eye function, and a severe deficiency in taurine can cause blindness in cats through eye lesions - a condition known as Central Retinal Degeneration. Taurine is also involved in regulating many minerals and prevents the loss of potassium from heart muscle. Again, a lack of taurine in a cats diet may cause thinning of the heart walls - a condition known as Dilated Cardiomyopathy. Taurine can be only found in sufficient levels in meat, and in high levels in beef, lamb and chicken. This is why cats are obligatory carnivores. One important thing to remember is that taurine is unstable in heat, so cooking will reduce the taurine level by approximately 75%.
Threonine is an importantamino-acid for building many body proteins, and it is necessary for the formation of collagen, elastin, and tooth enamel. Threonine also enhances the immune system. Kittens fed with a threonine reduced diet showed reduced growth and presented some neurological problems such as ataxia (jerky, uncoordinated movements.), discoordination and inbalance. They also showed stiffening in the joints. The need for threonine is less in adult cats. Most meats contain good amounts of threonine.
Tryptophan is very important in the body. In pets, tryptophan is used during food digestion, and a lack of tryptophan increases the level of urea in the blood which, in severe cases, reduces kidney function. Another important function of tryptophan is in the central nervous system, because this amino-acid is a precursor of serotonin, a neurotransmitter which effects an animal's behaviour. A low level of serotonin may make cats lethargic and depressed. So it is not helpful that there are only small amounts of tryptophan in food, although meat, eggs and dairy products contain some.
Valine is an essential building block of most proteins. Its functions are similar to these of isoleucine and leucine. Valine is metabolized to produce energy and can be good for building muscle. It can be found in most foods.
The information supplied here is intended as a guideline only.
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