Page 93 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 93

Amino Acids of Special Importance There is no evidence that taurine is an essential amino acid
VetBooks.ir Several amino acids have important roles in the nutrition and for dogs; however, research indicates that it may be condition-
ally essential. In one study, investigators showed that feeding
health of dogs and cats. Among these are taurine, arginine and
a high-fat food (24% DM) significantly reduced plasma tau-
glutamine/glutamate.
rine concentrations. Taurine values for some dogs were mar-
Taurine ginally deficient (Sanderson et al, 1996). Dilated cardiomy-
Taurine is a sulfur-containing β-amino acid. The amino opathy in American cocker spaniels and golden retrievers has
group resides on the second (β) carbon rather than the first also been associated with plasma taurine deficiency and low
(α) carbon as with other amino acids. Taurine also has a sul- myocardial taurine concentrations (Kramer et al, 1995)
fonic acid (SOOH) rather than a carboxylic acid group. (Chapter 36).
Taurine is not incorporated into proteins synthesized by the
body because of its structure. Rather, taurine is found as a free Arginine
amino acid in many tissues, including brain, retina, myocardi- Arginine is a basic amino acid that is essential for dogs and cats
um, skeletal muscle, liver, platelets, leukocytes, and in fluids of all ages.Cats develop signs of deficiency rapidly.Within three
such as milk and in complexes with bile salts (Zelikovic and hours after consuming a meal devoid of arginine, cats develop
Chesney, 1989). hyperammonemia, vomiting, ataxia, vocalization (moaning) and
Taurine is conjugated to bile acids to form water-soluble bile hyperactivity; death may occur. Similar signs of ammonia toxic-
salts that assist in absorption of dietary fats. Taurine also serves ity such as tremors, vomiting, profuse salivation and hyper-
as a neurotransmitter and neuromodulator in the central nerv- glycemia appear quickly in dogs following a meal that lacks argi-
ous system and is involved with body temperature regulation, nine. Arginine is a key intermediate in the urea cycle, which is
brain development, maintenance of normal retinal structure the major metabolic pathway that detoxifies nitrogenous wastes,
and normal heart function (Zelikovic and Chesney, 1989). such as ammonia (Milner, 1989). Ornithine and citrulline can
Taurine is also thought to conjugate toxic compounds, serve as substitute for arginine and prevent hyperammonemia because
an antioxidant, stabilize cell membranes and regulate cell vol- they are also urea cycle intermediates. However, they cannot
ume and osmolarity (Zelikovic and Chesney, 1989). restore growth rates (Morris et al, 1979). Ornithine and cit-
Taurine is an essential amino acid because cats have mini- rulline are not present in high enough quantities to substitute for
mal ability to synthesize it and have obligatory losses due to arginine in typical commercial foods for dogs and cats.
the necessity for conjugating bile acids to taurine. Unlike Most protein sources provide adequate arginine; therefore,
other animals, cats conjugate bile acids only to taurine and not most commercial pet foods are not supplemented with argi-
to glycine. Furthermore, cats have an obligatory loss of taurine nine. Supplements, such as milk replacers, or products intend-
in the feces due to bacterial degradation in the intestine and ed for other species, such as some human enteral products,
intestinal losses of taurine through enterohepatic circulation. should be evaluated carefully for their arginine content before
This obligatory loss coupled with a minimal capacity for cats being administered to dogs and cats. In studies, the minimum
to synthesize taurine makes it an essential amino acid for this arginine content of food that maximized growth in kittens was
species. 0.83% (DM) (Anderson et al, 1979; Morris and Rogers, 1978;
Documented signs of taurine deficiency include reproduc- Costello et al, 1980). AAFCO recommends that foods for
tive failure in queens, developmental abnormalities in kittens, growing kittens and adult cats contain at least 1.25 and 1.04%
central retinal degeneration and dilated cardiomyopathy DM arginine, respectively (2007).
(Pion et al, 1987; Morris et al, 1994). Taurine deficiency is For growing puppies, a dietary arginine content of 0.4 to
more likely to occur in cats that are fed dog foods, homemade 0.56% (DM) supported maximum weight gain (Czarnecki and
foods and vegetarian foods that are not supplemented with Baker, 1984); however, 0.56% DM supported optimal growth
taurine. and nitrogen balance (Ha et al, 1978). AAFCO recommends
Taurine requirements in cats are highly dependent on that foods for growing puppies and adult dogs contain at least
ingredient sources and processing. The requirement increases 0.62 and 0.51% DM arginine, respectively (2007).
slightly with increased dietary protein. Certain proteins (e.g., The role of arginine in the formation of nitric oxide in the
isolated soya protein) and the canning process increase the body has been investigated. Nitric oxide is classified as a hor-
dietary taurine allowance compared with freezing or using mone that helps regulate flow through blood vessels and is
casein as the protein source. Processing neither destroys nor thought to be involved with blood pressure regulation. Nitric
binds appreciable quantities of taurine; however, processing oxide is synthesized by the endothelial cells lining blood vessels
apparently alters food so that enhanced numbers of intestinal and by macrophages to assist in infection control.
bacteria degrade taurine. The specific biochemical alteration
of food responsible for this change has not been described. Glutamine/Glutamate
Current recommendations are to include at least 1 g of puri- Glutamine and glutamate are five-carbon amino acids that are
fied crystalline taurine/kg (0.1% DM) in dry foods and at structurally similar and play key metabolic roles in the citric acid
least 2 g of purified crystalline taurine/kg for moist foods cycle, transamination reactions, generation of NADPH, γ-
(0.2% DM) (AAFCO, 2007). aminobutyric acid (GABA), the antioxidant glutathione and as