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dogs have a relatively short gastrointestinal tract, ingesta transit time is relatively rapid even when a low-fiber diet is fed (Banta et al., 1979). As a result of their short gastrointestinal tract, dogs have a low colonic-rectal surface area per unit body weight. For example, Beagle dogs have only 17 cm2 colonic-rectal mucosal area per kilogram of body weight, whereas the comparable value for pigs is 200 cm2 per kilogram of body weight (Herschel et al., 1981). Consequently, dogs are unlikely to absorb significant amounts of energy from the fermentation of fiber that might occur during its relatively rapid transit through the lower gastrointestinal tract. Finally, although data are unavailable for the dog, it should be recognized that inclusion of large amounts of fiber in the diet may adversely affect nutrient availability.

Protein And Amino Acids

Dietary protein is required to supply specific essential amino acids that cannot be synthesized in sufficient quantities by tissues to allow for optimum performance. Additionally, dietary nitrogen is required to allow for optimal biosynthesis of the dispensable amino acids and other nitrogenous compounds. Recent studies by Milner (1979 a,b) using purified diets containing 4.1 kcal ME/g have established that the following amino acids are required for optimum growth and nitrogen balance in the immature Beagle:

Studies of Rose and Rice (1939) established that all of the above amino acids except for arginine were also required to maintain nitrogen equilibrium in adult female dogs. However, recent studies of Burns et al. (1981) have shown that dietary arginine is required by the mature dog to maintain body weight and to prevent emesis and other signs associated with hyperammonemia.

Numerous factors may modify the percentage of protein required in the diet. In establishing this requirement, factors such as digestibility, amino acid composition, availability of the protein source, caloric density of the diet, and physiological state of the dog must be considered. The quantity, including excesses and deficiencies, of essential (indispensable) and dispensable amino acids, plus other nonspecific nitrogen sources are factors that may influence the minimal percentage of dietary protein required for optimum growth and health. Estimates of the protein requirement of the dog can also vary depending on the methods and criteria used in their derivation.

Signs of Deficiencies

Protein deficiency in the dog results in depressed food intake, severe growth retardation or weight loss, hypoproteinemia, depletion of protein reserves, muscular wasting, emaciation, and, ultimately, death (Chow et al., 1945; Allison et al., 1946; Allison and Wannemacher, 1965; Burns et al., 1982). Edema sometimes accompanies the hypoproteinemia. Generally, during limited access to protein the hair coat becomes rough and dull in appearance, antibody formation is impaired, and milk production is depressed. Although the signs of protein deficiency are nonspecific and can be created by other dietary deficiencies, including caloric restriction, these signs do indicate the severity of dietary limitations on the dog's health and performance.

Removal of a single essential amino acid results in a prompt reduction in food consumption leading to a negative nitrogen balance. Generally there is a return to normal within a few days after replacing the limiting amino acid. Prolonged deficiency of any of the essential amino acids leads to a syndrome similar to that occurring during protein deficiency. Limitation in a dietary essential amino acid tends to be reflected by lowered concentrations of the specific amino acid in the blood plasma (Longnecker and Hause, 1959). Specific signs characteristic of a deficiency of individual amino acids in the dog have not been adequately documented.

Amino acid imbalances or antagonisms are known to increase the requirements for individual amino acids (Harper and Rogers, 1965; Harper et al., 1970). Some adaptation to minor imbalances and antagonisms appears to occur. Data obtained in other species indicate that the effects of imbalances or antagonisms are greater when suboptimal dietary nitrogen is offered but of lesser importance if all amino acids are in excess in the diet.

Amino Acid Requirements

Indispensable

The dietary requirement of a particular protein or a mixture of proteins is determined by the ability of the protein(s) to meet the dog's metabolic requirements for amino acids and nitrogen. The closer the supply of the complement of amino acids to the requirement, the lower the percentage of protein required in the dog's diet (Allison et al., 1947; Kade et al., 1948; Arnold and Schad, 1954). Amino acid requirements, as a percentage of the diet, decline from birth to maturity. However, Wannemacher and McCoy (1966) have suggested that