PROTEIN

Protein

Proteins are made up of amino acids, in predefined chains that determine their roles within the body. Amino acids, which are produced by the breakdown of dietary proteins in the digestive tract, play a role in synthesis of proteins needed to build and repair organs and tissues, transport molecules, send messages from one organ to another (hormones) and help combat disease (antibodies). Good sources of proteins are from animal products (meat, offal and fish) and some vegetable products (cereal gluten, rice and soy). Some physiological conditions require more protein than others; for example growth, gestation, lactation or physical activity are demanding in terms of protein utilisation. Dogs and cats use 30-35% of dietary protein to maintain skin and coat health.

Amino acids

Building blocks of proteins

Background

Amino acids are the building blocks of proteins and their derivatives. Proteins include a total of 20 different amino acids, only 11 (cats) or 10 (dogs) of which are essential; that means they cannot be produced by the body so must be provided in the diet.

Role in the body

Amino acids are required by the body to ensure healthy physiological function. Without essential amino acids, the growth of kittens and puppies will be slow and health may be compromised. Processes such as nitrogenous waste elimination and haemoglobin synthesis will be disrupted in deficient adult animals.

Common sources

All dietary proteins of animal or plant origin are composed of a series of chemically bound interlinked amino acids. Dietary proteins of ‘high biological value’ are those that combine good digestibility and a high content of essential amino acids, such as egg, meat (including organ meats such as heart, kidney, liver and lung), fish proteins and cereal glutens.

Deficiency and excess

The absence of any of the essential amino acids from the diet stops the synthesis of essential proteins. Under these conditions, the animal then breaks down body tissue to provide the required amino acids, seriously compromising health.

Methionine and cysteine

Key ingredients in the hair protein, keratin

Background

Methionine and cysteine are sulphur amino acids that are important for the synthesis of the hair protein, keratin. The keratin synthesis needed to maintain skin and hair can account for up to 30% of an adult dog’s daily protein requirement.

Role in the body

Methionine is essential within the diet. Cysteine can be synthesized from methionine. However, if cysteine is provided in sufficient quantities, it helps free up methionine for other functions. The metabolism of sulphur amino acids produces sulphuric acid, which is eliminated through the urine. A carnivore’s natural diet, rich in sulphur amino acids, therefore tends to produce acidic urine.

Common sources

Methionine and cysteine are particularly abundant in fish and egg proteins, as well as casein. Wheat and maize glutens are also very rich sources.

Deficiency and excess

A deficiency of methionine and cysteine can result in hair loss, slow hair growth and a generally dry and brittle appearance of hair.

Taurine

Healthy eyesight, healthy heart, natural antioxidant

Background

Taurine is only essential for cats as, unlike dogs, they cannot synthesise it themselves. Taurine was discovered in 1826 in the bile of cattle (Bos taurus), hence its name. It is a sulphur amino acid found in most animal tissue. Unlike other essential amino acids, it does not have any role in protein synthesis. For reasons yet to be fully understood, wet cat food requires twice the level of taurine supplementation of dry food, to allow the cat to absorb adequate levels of the nutrient.

Role in the body

Taurine enables the liver to synthesise bile salts. It also works by regulating calcium flow into and out of the cells and has a role in healthy cardiac function. Taurine is required for healthy reproduction, healthy eyesight and hearing. It is an important antioxidant, as well as playing a role as a precursor for the synthesis of complex fats (glycosphingolipids), that support the barrier function of the skin.

Common sources

Animal protein sources, in particular the organs (e.g. heart, kidney, liver), are the main natural sources of taurine.

Deficiency and excess

Taurine deficiency can result in feline central retinal degeneration (FCRD) and subsequent blindness, inadequate immune response, poor growth, and poor reproductive function including decreased live birth rate and congenital birth defects.

Arginine

Essential amino acid for growth and urea production

Background

Arginine is important for the synthesis of urea from ammonia. In the absence of arginine, cats rapidly develop clinical signs of ammonia intoxication (hyperammonaemia), which include vomiting, hypersalivation and nerve problems. This deficiency may prove fatal within hours if left untreated.

Role in the body

In addition to its involvement in the excretion of ammonia, arginine plays a role in blood vessel relaxation and the release of several hormones.

Common sources

Arginine is abundant in meat including organs. Gelatine is a rich source.

Deficiency and excess

Arginine free diets are associated with excessive salivation, muscle tremors, vomiting and death. In the long term, marginal arginine deficiency can lead to cataract development. Signs of deficiency tend to be more severe in cats than dogs. As ammonia is produced from the breakdown of protein, the higher the protein content of the diet the higher the arginine requirement.

Lysine

Essential amino acid for the synthesis of all proteins

Background

Lysine is often the first limiting amino acid in the diet, which means it has the greatest risk of being deficient if a pet food is not carefully formulated. Lysine is sensitive to heat, and during pet food processing it undergoes a chemical reaction with sugar (Maillard reaction), thought to be important for generating flavours and aromas.

Role in the body

Lysine is an essential amino acid that is used for the synthesis of proteins.

Common sources

Lysine is abundant in animal sources especially muscle tissue. Soy proteins are also a good source.

Deficiency and excess

Lysine is an essential amino acid that must be provided in the diet. Deficiency can lead to reduced food intake and weight loss. Too much lysine in puppies can cause signs of arginine deficiency.

Phenylalanine and tyrosine

Thyroid and adrenal gland function, hair pigmentation

Background

Phenylalanine and tyrosine are aromatic amino acids (so named because of their ring structure) that are vital for the production of pheomelanins (yellow to red pigments) and eumelanin (brown to black pigments) that define the colour of an animal’s coat. Of the aromatic amino acids, only phenylalanine is considered to be essential. Tyrosine can be synthesised from phenylalanine. However, if tyrosine is provided in sufficient quantities, it helps free up phenylalanine for other functions.

Role in the body

Phenylalanine is essential for the production of thyroid hormones and other key metabolites as well as being vital for tyrosine synthesis. Besides its role in hair and iris pigmentation, tyrosine is also a dopamine, noradrenalin and adrenalin precursor. These molecules are required for the proper functioning of the brain and in reproduction.

Common sources

Phenylalanine is found in most animal protein sources such as beef,pork, poultry and fish. Tyrosine is either provided directly in the diet or synthesised from phenylalanine, an essential amino acid. Rice is the only vegetable source to contain useful quantities of tyrosine.

Deficiency and excess

Signs of deficiency include neurological dysfunction, uncoordinated gait and hyperactivity in cats. In dogs, signs include weight loss and reduced food intake, and reddening of black coats.

With phenylalanine and tyrosine

Without phenylalanine and tyrosine

Branched chain amino acids (BCAA)

Essential amino acids for synthesis of muscle proteins

Background

Leucine, isoleucine and valine constitute the class of branched chain amino acids (BCAA)within the essential amino acid family. The body is unable to make them sufficiently quickly, and therefore a dietary source is required.

Role in the body

Leucine, isoleucine and valine stimulate the synthesis of proteins and slow their breakdown in the muscles. The effectiveness of BCAA in both these actions appears to diminish with age. BCAA help increase lean mass and help prevent muscle wasting.

Common sources

Leucine, isoleucine and valine are commonly found in muscle meats including beef, lamb and poultry.

Deficiency and excess

Deficiency of any of the BCAA can result in weight loss and lethargy. Deficiency of isoleucine in particular can result in rough coat, lesions on the paws and an uncoordinated gait.

Histidine

A structural protein

Background

Histidine was first isolated in 1896. Histidine acts as a precursor for a number of important compounds.

Role in the body

Besides its structural function in proteins, histidine is a precursor for a number of neurological compounds such as histamine.

Common sources

Meat is a good source of histidine. It is present in particularly high concentrations in blood.

Deficiency and excess

Histidine deficiency can result in weight loss and refusal to eat. In cats, even a marginal deficiency when fed over a long period of time can result in cataracts.

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Threonine

Essential for energy production

Background

Threonine is an alpha amino acid and is the only essential amino acid to contain an alcohol group within its structure.

Role in the body

Threonine acts as a precursor for a number of metabolically active molecules including pyruvate, involved in the production of energy.

Common sources

Poultry, fish, lamb, pork and beef are all good dietary sources of threonine.

Deficiency and excess

Threonine deficiency can result in weight loss and refusal to eat in both cats and dogs. In cats, even a marginal deficiency can result in nervous system problems.

Tryptophan

Required for hormone production

Background

Tryptophan was first isolated in 1901 and is a precursor of many important metabolic molecules.

Role in the body

Tryptophan acts as the precursor of niacin synthesis in dogs. Although cats do have the ability to synthesise niacin from tryptophan, the activity of the enzyme picolinic carboxylase diverts tryptophan away from this function. Tryptophan also acts as the precursor of serotonin and melatonin.

Common sources

Poultry, fish and soy are excellent sources of tryptophan.

Deficiency and excess

Tryptophan deficiency can result in refusal to eat and weight loss.