Cats and dogs versus pigs and poultry : a nutritional perspective.

Abstract

107 Cats and dogs versus pigs and poultry: a nutritional perspective W.H. Hendriks Institute of Food Nutrition and Human Health, Massey University, Private Bag 11_222, Palmerston North, New Zealand Summary Cats and dogs are the most popular companion animals worldwide. The dog is classified as an omnivorous carnivore while cats are true carnivores. Due to their dietary habits throughout evolution both species have developed a specific nutrient metabolism with the cat showing a unique nutrient metabolism compared to omnivorous production animals. The essentiality of nutrients and requirements for protein of both species are discussed. Criteria for the formulation of diets for cats and dogs involve palatability of the product, marketing strategies and a complete and balanced diet. Commercially produced diets for cats and dogs are among the most intensively heattreated animal diets with extrusion, baking, pasteurisation and sterilisation being the most widely used processes. The effect of heat processing on the nutritive value of pet foods is discussed. Currently premium dry dog foods and moist canned cat foods are overformulated with respect to essential amino acids. Costsavings are possible by substitution in the protein fraction with less expensive ingredients. Domestication of the dog is believed to have occurred around 9000 BC, and that of the cat around 3000 BC (R�hrs 1987). Since that time, many new breeds of dogs and cats have appeared and currently there are approximately 150 different breeds of dogs and more than 100 different breeds of cats. Nowadays, domestic cats and dogs are often kept in an environment where they cannot obtain their food naturally and it is the responsibility of the owner to provide the animal with the nutrients it requires. It becomes imperative, therefore, that the food supplied is nutritionally balanced and meets the requirements of the animal. Failure to meet the nutritional needs of the animal for one or more essential nutrients may result in deficiency symptoms, disease or death. Past presentations at this conference have seen an excellent review on nutrition of dogs in Australia (Costa 1997), research articles on dogs (Rowe et al. 1997) and cats (Harper and SieverKelly 1997), and an update on companion animal nutrition (Divola 1993). This contribution focuses on the nutrition of cats and dogs and some fundamental differences that exist between these species and the nutrition of the pigs and poultry. Introduction Domestic dogs (Canis familiaris) and cats (Felis catus) are the most popular companion animals worldwide. Owning a pet has been recognised to have health and psychological benefits such as decreased loneliness, increased selfesteem, increased interaction with others and development of assertiveness. As a result pets are often used in therapy for functionally disturbed children, criminally insane prisoners and elderly people in nursing homes. Although the cat is a true companion, the dog serves as much more than merely a companion in our society. Dogs are actively used to aid blind and deaf people as well as those confined to wheelchairs. They play a vital role in search and rescue work and are frequently used to detect drugs and bombs at international airports. Furthermore, dogs are indispensable in the farming of sheep and assist police with law enforcement. Recent Advances in Animal Nutrition in Australia, Volume 12 (1999) Commercial pet foods: a short historical perspective Commercially pet foods have increased in popularity ever since the first commercial dog cake was developed in 1860, in England by James Spratt. This dog cake consisted of meat, vegetables, beetroot and wheat meals. In 1908 the first dry biscuit type dog food was developed and marketed in the United States. Not until 1922 did the first canned dog food that consisted largely of horse and mule meat, become available (Corbin 1995). The development of extruder technology in the 1950s resulted in the introduction of expanded dry dog foods in 1957 and ultimately led to the disappearance of the true meal and kibble type dry dog food (Kallfelz 1985). It was not until 100 years after the first commercial dog 108 Hendriks, W.H. food was produced that the first commercial cat food became available. Around 1960 one company began marketing tuna fish as cat food in a 6_ ounce can (Kallfelz 1985), something cat owners apparently had been waiting for. In the 1940s, a new development occurred in the pet food industry, namely the production of commercial therapeutic diets. One of the first diets developed was a low protein ration for the feeding of dogs with chronic renal failure. Many other diets have since been developed to aid in the management of diseases/ conditions such as idiopathic chronic colitis, obesity, pancreatitis, hypertension, feline lower urinary tract disease, etc. Global and Australian pet food market The dollar value of the world pet food market has increased since 1993 by 22% to A$ 42 billion in 1997. In terms of volume sales, the world market was 14.6 million tonnes in 1997 with dog food accounting for 63% and cat food for 37% of total sales (Irwin 1998). Four large companies (Mars, Nestl�, Ralston Purina and Heinz) dominate the world pet food market, and between them account for around 60% of sales. From 1997 to 2002, the world pet food market is expected to grow by around 19% in volume terms and 42% in terms of current value. The market for cat food is expected to grow more rapidly as a result of the growing cat population in developing markets (Irwin 1998). In 1997, 261,000 tonnes of dog food were sold in Australia, up 7.4% since 1993. Cat food sales were 118,000 tonnes, down 6.8% since 1993. Total value of the Australian pet food market is around A$ 700 million per annum with dog food accounting for 60% and cat food accounting for 40%. Australia was, furthermore, a major exporter of pet foods between 1991 and 1995 with the principal customer being Japan (Heyhoe 1997). Criteria for diet formulation in companion animals Humans have access to a wide variety of foods and will consume a number of food types to meet their nutrient requirements. Farm animals are fed compound diets formulated for efficient production and rarely live out their natural lives. Cats and dogs, on the other hand, are nowadays expected to live a long, happy and relatively disease free life similar to humans. However, unlike humans, cats and dogs are often kept in an environment whereby they cannot obtain their natural food and, as a result, rely on their owners to provide them with the nutrients they require. Commercial pet food manufacturers focus much of their effort and a significant proportion of their research budget on diet formulation for palatability. A diet can be complete and balanced nutrition, but if the diet is not ingested it does not provide nutrients to the animal and, therefore, is of no value. The palatability of pet foods is influenced by many factors including food texture, food composition, ingredients, smell, taste, temperature, past experience of the animal, heat treatment, etc. Feeding their companion animal is one of the most rewarding services an owner can perform (Booles 1993). Seeing the enjoyment of their pet in consuming its diet gives the owner a strong sense of fulfilment and often determines whether that particular diet (or brand) will be purchased again. Little information is present in the literature on palatability testing for cats and dogs. Griffin (1996) and Tarttelin (1997) discussed the most commonly used palatability test in the pet food industry, the twobowl, free choice method. Much of the data generated with these tests, however, is kept inhouse and is not published by manufacturers, for obvious reasons. The owner s perception of a diet is another important criterion in diet formulation as it also strongly determines the repurchasing of diets. The food purchased should be perceived by the pet owner as something that is good enough for their companion to eat. Attributes such as consistency, colour, smell, can head space and appearance of the product are important variables in formulating diets. A dark looking paste with an unpleasant smell will be perceived by the owner as being unnutritious food, although the diet might actually be highly palatable and provide complete and balanced nutrition. Marketing strategy is another important criterion in formulating diets for companion animals. Different varieties of pet food are often produced by manufacturers, such as fish, lamb, beef, garlic and rabbit flavour. The variety and interest concept supposes that a cat or dog needs variety in their diet. Tarttelin (1997) noted that the concept of variety and interest is aimed solely at the purchaser of the product and not the consumer. OMalley (1995) argued that the desire for variety is naturally present in cats and dogs to allow development of knowledge of available foods and increase the chances of survival in the wild. Nowadays, there are positive aspects in providing a pet with a variety of diets, as it is unlikely that nutrient deficiencies will occur on a variety of diets rather than the feeding of one highly palatable preferred product. Vitamin E deficiency has been noted in cats due to the sole feeding of highly palatable fish or fishbased diets (Koutinas et al. 1993). Additionally, marketing strategies influence diet formulation because commercial pet food manufacturers often produce different brands in different price categories. Super Premium foods are generally formulated using more expensive ingredients than Premium or Budget foods. Super Premium foods are sold in smaller size containers that are easier to open and have highly attractive labels. Budget foods on the other hand are produced using less expensive raw materials and contain less attractive labels. As well as Cats and dogs versus pigs and poultry: a nutritional perspective 109 the difference in price between these three categories, differences in the average nutrient composition of Super Premium, Premium and Budget foods can be found (Table 1). One criterion, often not as apparent as other criteria used in diet formulation for companion animals but extremely important, is the formulation of complete and balanced diets. Nowadays a significant proportion of the research budget of commercial pet food manufacturers is spent on nutritional testing of produced diets. Morris and Rogers (1994) and Costa (1997) discussed the most widely used testing procedure for cat and dog foods, that of the American Association of Feed Control Officials (AAFCO). The reader is referred to these publications for a summary and a critical discussion of the AAFCO procedures. Protein and amino acid metabolism The ideal amino acid patterns for growing dogs, cats, pigs and chickens are presented in Table 2. The patterns are similar although the cat seems to require more sulphurcontaining amino acids and leucine, while the requirement for tryptophan seems to be high in dogs. There are, however, several metabolic idiosyncrasies in the metabolism of amino acids in the cat and the dog not commonly seen in pigs and poultry. Nine amino acids are dietary essential for most mammals. Arginine, however, is an essential nutrient for adult as well as growing cats and dogs (NRC 1985; 1986). Consumption of a diet devoid of arginine results in severe signs of ammonia toxicity in both species. Cats have been reported to die within three hours after the ingestion of a diet devoid of arginine (Morris and Rogers 1978). The cause of the essentiality of arginine in cats is the low activity of the enzymes ornithine amino transferase and pyrroline5carboxylase involved in its synthesis. When cats and dogs are fed an arginine free diet in which all other amino acids are present at normal levels, insufficient arginine is synthesised in vivo to maintain the urea cycle and subsequently causes hyperammonaemia. Table 1 Average gross composition (g/kg dry matter) of budget, premium and super premium moist canned cat foods. Budget 382 360 Essentiality and requirements of nutrients for cats and dogs The domestic dog and cat both belong to the order Carnivora. Many species within the order Carnivora, however, are far from meat eaters. The giant panda (Ailuropoda melanoleuca) for example eats almost solely bamboo, while brown bears (Ursus arctos) ingest an omnivorous diet of plant and animal material. Many scientists regard the domestic dog as an omnivorous carnivore while the domestic cat is often referred to as a true carnivore. This terminology is largely based on the metabolism of nutrients as the dogs metabolism resembles more that of the rat and pig, while the cats metabolism shows specialisation towards a meat diet similar to other true carnivores such as the fox, mink, and ferret. The following section will discuss some of the metabolic peculiarities of the cat and the dog as these differ from the metabolism of typical omnivores such as the pig, rat, and chicken. Fraction Crude Protein Crude Fat Carbohydrate Ash Premium Super premium 506 276 102 115 550 223 139 88 172 86 From Hendriks and Tarttelin (1997). Estimated by difference as (100_ crude protein_crude fat_ash). Table 2 Ideal essential amino acid pattern for growth in cats, dogs, pigs, and chickens relative to lysine = 100. Ideal essential amino acid pattern Amino acid Lysine Methionine + cystine Tryptophan Threonine Arginine Isoleucine Valine Leucine Histidine Phenylalanine + tyrosine � Cat Dog 100 76 29 91 98 70 70 114 35 139 Pig� 100 60 18 65 42 60 68 100 38 95 � Chick 100 72 18 74 110 73 82 109 32 122 � 100 94 19 88 125 63 75 150 38 106 From NRC (1986), NRC (1985), Baker and Czarnecki_Maulden (1991) and NRC (1994) 110 Hendriks, W.H. Taurine, 2amino ethanesulphonic acid, is another amino acid regarded as an essential nutrient for the cat. The essentiality of this amino acid results from a combination of insufficient synthesis and a high metabolic demand. The metabolic basis for the lack of taurine synthesis lies in the transamination of cysteinesulfinate, an intermediate in the synthesis, rather then the decarboxylation leading to taurine (Edgar et al. 1998). Besides the low rate of synthesis, the obligatory use of taurine to conjugate bile acids results in a continuous loss of taurine from the body. Taurine is not an essential nutrient for the dog as this animal can synthesise sufficient to meet its requirements. Cats, unlike dogs, excrete several unusual sulphur containing amino acids in their urine: felinine, isovalthine and isobuteine. Although little is known of the latter two amino acids, rates of felinine excretion of 95 and 19 mg/day have been recorded for entire male and entire female cats, respectively (Hendriks et al. 1995a). The biological significance of felinine or the other sulphur containing amino acids to the animal is still a matter for speculation but a function as a precursor to a pheromone seems likely (Hendriks et al. 1995b). The high excretion rates of felinine in entire male cats may have a significant effect on the daily sulphur amino acid requirement. Another idiosyncrasy in the metabolism of amino acids is the high protein requirement of cats which is approximately 4060 % higher than the requirements of the growing and adult dog and rat (Table 3). This is not caused by a higher requirement for essential amino acids, but rather a higher requirement for nonessential amino acids, a result of nonadaptive nitrogen metabolising enzymes in the liver of the cat which are permanently set to handle a high protein diet (Rogers et al. 1977). insufficient synthesis of arachidonic acid. Sinclair (1979) proposed an alternative pathway for synthesis involving D5 and D8desaturase enzymes but it is still unclear whether this allows sufficient synthesis of arachidonic acid to meet the needs of the cat. Vitamin metabolism For pigs, chickens, rats and dogs vitamin A is a semi essential nutrient because dietary �carotene can be converted by these animals to vitamin A. The cat, however, either lacks the enzyme dioxygenase to convert �carotene to retinol, or the activity of this enzyme is grossly deficient, making vitamin A an essential nutrient for the cat. Most mammals derive their niacin (nicotinic acid) requirements from metabolism of the essential amino acid tryptophan. The cat, unlike the dog, rat and pig, is again unusual in this respect in that tryptophan cannot replace nicotinic acid in the diet and cats die within 20 days when fed diets high in tryptophan but lacking in nicotinic acid (Da Silva et al. 1952). The inability of the cat to utilise tryptophan for synthesis of nicotinic acid is not due to a deficiency in one of the enzymes involved in the synthesis of nicotinic acid but rather is due to a highly active alternative metabolic pathway. The latter results in insufficient tryptophan being available for the synthesis of niacin. Recently, How et al. (1994) showed that cats and dogs are unable to synthesise sufficient vitamin D to meet requirements. Low concentrations of 7dehydrocholesterol in the skin of cats and dogs results in insufficient vitamin D synthesis. Morris (1996) subsequently showed in cats that this inability is due to a high activity of the enzyme 7dehydrocholesterol D 7 reductase which catalyses the conversion of 7dehydrocholesterol to cholesterol. Essential fatty acids Arachidonic acid is an essential fatty acid for cats but not for the dog, rat, pig or chicken. Again the requirement for this nutrient originates from the lack of the activity of an enzyme in the metabolic pathway leading to its synthesis. In the cat, the activity of the D6desaturase enzyme is low or absent resulting in Mineral metabolism There do not appear to be any idiosyncrasies in the dietary amounts of minerals required by cats and dogs (NRC 1985; 1986). However, minerals have long been recognised to be involved in the most important disorder Table 3 Minimum dietary crude protein and total essential amino acid requirements (g/kg cat, dog and rat. Growing 0.75 /day) for the growing and adult Adult Rat 8.41 Cat 2.77 Dog 1.98 Rat 1.40 Nutrient Crude protein Essential amino acids Free base Protein_bound Difference Cat 12.00 Dog 7.57 3.43 2.98 9.02 4.11 3.59 3.98 3.09 2.69 5.72 (0.56) (0.49) (2.28) 0.60 0.52 1.46 0.44 0.38 1.02 From Hendriks (1996). Values between brackets are extrapolated. Crude protein minus total protein_bound essential amino acids. Cats and dogs versus pigs and poultry: a nutritional perspective 111 affecting the lower part of the urinary system of cats and one of the most common conditions to afflict cats, urolithiasis or feline urological syndrome (FUS). Naturally occurring feline uroliths (urinary stones) may be composed of magnesium ammonium phosphate hexahydrate (struvite), calcium oxalate, ammonium phosphate, calcium phosphate, cystine and xanthine, with struvite being the most predominant form (Osborne et al. 1989). Strategies for preventing struvite formation in cats through dietary manipulation either target urinary pH or the excretion of magnesium in the urine. Lowering urinary pH to 6.06.4 by the addition of urinary acidifiers to diets is one of the most effective measures to prevent the formation of struvite crystals, and has become normal practice in commercial diets. Dry diets especially, which are more liable to cause formation of struvite in the cat, are formulated to include urinary acidifiers such as phosphoric acid, ammonium chloride, calcium chloride and methionine. In conjunction with urinary acidifiers, diets are often formulated to contain low magnesium levels, which is the primary mineral found in the struvite crystal. Buffington et al. (1985), however, showed that previous studies claiming that excessive dietary magnesium causes struvite urolithiasis were the result of an error in the experimental design because the magnesium salt was added in a form which increased urinary pH. The importance of urinary magnesium concentration in the prevention of struvite formation in the cat, therefore, may be overemphasised. Recently, the importance of water in the prevention of FUS was noted by Markwell et al . (1998). These authors noted that more then half of cats classified as having idiopathic lower urinary tract disease may show no recurrence of this condition if maintained on a diet high in moisture content. It is evident from the above that the dog more closely resembles an omnivore than the cat. However, the dog has still adapted its metabolism to the carnivorous component in its diet and for this reason the dog is characterised as an omnivorouscarnivore. The cat on the other hand has fully adapted to a carnivorous diet. The constant composition and high nutrient content of the cats diet throughout evolution has resulted in several metabolic adaptations. All the nutrients which the cat cannot synthesise (taurine, arginine, arachidonic acid, vitamin A, vitamin D, niacin) can be found in a diet consisting of mammalian tissues. Prepared cat and dog food Dry, semimoist and moist pet foods are manufactured from ingredients such as meats/offals (beef, fish, poultry, deer, lamb, etc.), cereal grains, meat byproducts, fats/ oils, vegetable protein concentrates, sugar, water, humectants, gelling agents, emulsifiers, colourants, vitamins, and minerals. The large numbers of ingredients contributes to the observed variability in the nutritional content of these types of diets (Hendriks and Tarttelin 1997). To increase shelf life, achieve a desired physical form, and(or) increase palatability, the unprocessed mixture is either extruded, baked, pasteurised or sterilised depending on the type of pet food that is manufactured. There is a plethora of information concerning the influence of various heat treatments on the protein quality of feeds for pigs and poultry (e.g. Van Barneveld 1993; Van der Poel et al. 1993; Voragen et al. 1995). Little information is available on the effects of heat treatments used in the manufacturing of pet foods which are among the most intensively processed of all animal foods. Heat processing generally is believed to have a negative impact on the nutritive value of pet foods (NRC 1986; Lewis et al. 1987; Heinicke 1995). Loss of vitamins during the production of pet foods has been extensively documented (Roche 1981). Thiamine is particularly sensitive to heat and, as a precaution, compensatory amounts are added to pet foods to obtain adequate postprocessing levels of thiamine in the product. Heat processing of a canned cat food, furthermore, has been noted to change the taurine status of cats because moist diets increase the microbial deconjugation of bile acids in the small intestine (Kim et al. 1996). Besides the negative effect on nutritive value, heat processing has been noted to decrease the palatability of moist diets for cats (Heinicke 1995; Hendriks et al. 1999). Heat processing of pet foods can be expected to affect the protein fraction of the diet. Backus et al. (1995) found that heat sterilisation of a commercial canned moist cat food increased the apparent digestibility of the crude protein. It is possible that there was a decrease in digestibility in the small intestines but microbial deamination in the caecum and colon of the increased flow of amino acids, diffusion of ammonia across the mucosa reducing faecal N loss and thus resulting in increased apparent digestibility. In diets that have undergone processing or prolonged storage the eamino group of lysine can react with compounds present to produce nutritionally unavailable derivatives. Determination of unmodified lysine (reactive, available, lysine) before and after heat processing, therefore, may present a useful in vitro method to assess heatdamage to protein. Rutherfurd and Moughan (1997) found that the extrusion process used in the production of a dry cat food did not alter the digestibility of total and available lysine. Production of a moist cat food was associated with a 10% reduction in both digestibility values, indicating that heat damage to lysine may have occurred. Recently Hendriks et al. (1999) measured the effect of heat sterilisation on the protein quality of a canned cat food mixture using in vitro and in vivo assays. A standard recipe cat food was heat treated for different times and analysed for crude protein, amino acids, and reactive lysine, and fed to rats to determine the true ileal digestibility of amino acids. There were no changes in the gross composition of nutrients (crude protein, amino acids and reactive lysine) due to the heat treatment. However, the true ileal digestibility of amino acids generally decreased with increasing duration of heating (Figure 1). Amino acid nitrogen and proline 112 Hendriks, W.H. digestibility increased with mild heat treatment but decreased with more severe heat processing. The reactive lysine content in the unheated and heattreated diets was found to be about 10% lower than the total lysine content, a similar value to that found by Rutherfurd and Moughan (1997). The lower value was attributed to the relatively high proportion in the diet of collagen (from connective tissue) that naturally contains covalent crosslinks involving lysine to maintain the native threedimensional structure of the protein. Hendriks et al. (1999) concluded that crosslinking involving cystine is the likely mechanism for the decrease in the true digestibility of amino acids with increasing heat treatment. Over_formulation of commercial diets for cats and dogs Feeds for production animals are formulated using data on the digestibility of nutrients of potential ingredients, and leastcost formulation to obtain the most cost effective compound feed. Diets for cats and dogs, however, are formulated using criteria such as high palatability, appearance and nutritional completeness and balance. Formulation of diets based on these criteria will, in most cases, result in a nutrient content that is well in excess of the requirements of the animal. This fact is illustrated in Table 4 where the total and true ileal digestible content of essential amino acids for a dry dog food and moist canned cat food are compared to the nutrient requirements of adult dogs and cats. The essential amino acid requirements of adult cats were calculated from a computerbased factorial model developed from data presented by Hendriks (1996) and relate to a 4 kg adult male cat consuming a moist canned cat food. It must be noted that the latter estimates of requirements are not minimum values, but rather are optimum values for maintenance. As can be seen from Table 4, the premium dry dog food contains all amino acids in amounts well in excess of the dietary requirements. If the protein content were to be reduced in the diet, the first limiting amino acid is leucine as the true ileal digestible leucine content is only 50% above the dietary requirement. Arginine is the most abundant essential amino acid present in the dry dog food with the true ileal digestible arginine content more than 400% above requirement. The moist canned cat food also contains more than sufficient amounts of essential amino acids to meet the requirement of the animal. The true ileal digestible amino acid content of the moist cat diet is approximately 3.2 times the dietary amino acid requirement. 90 True ileal digestibility (% ) 80 70 60 50 40 0 5 10 15 20 25 Lethality value (min) Figure 1 True ileal digestibility of lysine, threonine, glycine and amino acid nitrogen in a moist cat food heated to different lethality values (time equivalent of a heating process to destroy micro-organisms at the reference temperature of 121.1�C). lys ine thre onine glyc ine amino ac id N Table 4 Dietary amino acid contents of a dry dog food and a moist cat food and the dietary amino acid requirements of cats and dogs; all values are g/kg dry matter. Adult dog Dry dog food Dietary requirement 5.1 4.7 5.0 3.6 3.9 5.8 1.8 Total Adult cat Moist cat food� True ileal digestible 29.4 17.1 28.5 14.2 21.3 35.1 9.7 Dietary requirement 7.6 7.2 8.2 4.7 6.4 9.2 3.8 � Amino acid Total True ileal digestible 9.7 7.3 25.6 7.3 8.5 15.4 4.7 Lysine Threonine Arginine Isoleucine Valine Leucine Histdine Phenylalanine + tyrosine 11.2 8.5 28.1 8.3 9.9 17.4 5.5 36.2 24.2 35.1 18.6 28.3 44.0 14.5 9.3 14.6 � 7.2 � 42.8 32.7 9.4 From Sritharan (1998), NRC (1985), Hendriks et al. (1999) and Hendriks unpublished. Cats and dogs versus pigs and poultry: a nutritional perspective 113 As the two diets in Table 4 are typical examples of premium dry dog foods and moist canned cat foods (Table 1), it can be concluded that the majority of companion animal diets are overformulated with respect to essential amino acids. Potential cost savings, therefore, are possible if less expensive ingredients can be incorporated in the protein fraction without affecting the formulation criterion of palatability. Overformulation of pet foods with respect to minerals has also been noted. Johnson et al. (1992) measured the iodine content of moist and dry cat foods in New Zealand and found a variation of more than a hundredfold ranging from 47 to 5304 mg/kg as sold. This large variation in iodine content, as well as other minerals, has also been observed by Mumma et al. (1986) in dog and cat foods sold in America. The latter authors hypothesised that the variation in iodine content of commercial pet foods may be attributed to varying amounts of thyroids included. In a followup study, Tarttelin et al. (1992) fed adult cats diets containing high, medium or low iodine concentrations for a short time (2 wks) and measured urinary iodine excretion and serum freethyroxine. The cats were found to closely regulate serum freethyroxine levels and the latter authors found an inverse relationship between serum free thyroxine and urinary iodine concentration. Further evidence that the cat is able to maintain normal levels of thyroid hormone when maintained on high and low iodine levels for longer periods (5 mo) was presented by Kyle et al. (1994). Da Silva, A.C., Fried, R. and de Angelis, R.C. (1952). The domestic cat as a laboratory animal for experimental nutrition studies. III. Niacin requirements and tryptophan metabolism. Journal of Nutrition 46, 399409. Divola, C. (1993). Nutrition of companion animals: recent advances. In: Recent Advances in Animal Nutrition in Australia 1993, pp. 163169 (ed. D.J. Farrell). University of New England, Armidale. Edgar, S.E., Kirk, C.A., Rogers, Q.R. and Morris, J.G. (1998). Taurine status in cats is not maintained by dietary cysteinesulfinic acid. Journal of Nutrition 128, 751757. Griffin, R.W. (1996). 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