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[[Protein Overview - Nutrition|Protein]] digestion '''begins in the [[Monogastric Stomach - Anatomy & Physiology|stomach]] of both dogs and cats''' with the secretion of hydrochloric acid and pepsinogen in response to the presence of protein in the stomach<ref>National Research Council (NRC). Comparative Digestive Physiology of Dogs and Cats. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.5-9.</ref>. Pepsinogen is activated to pepsin in the presence of hydrochloric acid and begins the enzymatic breakdown of protein into polypeptides. In the [[Duodenum - Anatomy & Physiology|duodenal lumen]] polypeptides (especially those containing [[Phenylalanine and Tyrosine - Nutrition|phenylalanine]]) stimulate the secretion of cholecystokinin (CCK) from the intestinal mucosa. CCK in turn stimulates [[Pancreas - Anatomy & Physiology|pancreatic]] contraction and release of pancreatic juices into the duodenum. This secretion contains chymotrypsinogen and (trypsinogen) both of which are activated in the intestinal lumen which further cleaves polypeptides into tripeptides, dipeptides and single amino acids. Specific carrier proteins are present on the mucosal surface of the duodenum and [[Jejunum - Anatomy & Physiology|jejunum]] (to a lesser extent in the [[Ileum - Anatomy & Physiology|ileum]]) to transport specific amino acid families (e.g. acidic-, neutral- or [[Nutrition Glossary#Dibasic Amino Acids|dibasic-amino acid]] transporters). Proteins or polypeptides that resist denaturation in the stomach or enzymatic cleavage in the upper small intestine can be fermented by colonic bacteria. Depending on the amino acid utilized by the intestinal microflora, malodorous by-products of bacterial fermentation can be created, such as cadaverine<ref name="NRC">National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.111-114.</ref>.

[[Protein Overview - Nutrition|Protein]] digestion '''begins in the [[Monogastric Stomach - Anatomy & Physiology|stomach]] of both dogs and cats''' with the secretion of hydrochloric acid and pepsinogen in response to the presence of protein in the stomach<ref>National Research Council (NRC). Comparative Digestive Physiology of Dogs and Cats. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.5-9.</ref>. Pepsinogen is activated to pepsin in the presence of hydrochloric acid and begins the enzymatic breakdown of protein into polypeptides. In the [[Duodenum - Anatomy & Physiology|duodenal lumen]] polypeptides (especially those containing [[Phenylalanine and Tyrosine - Nutrition|phenylalanine]]) stimulate the secretion of cholecystokinin (CCK) from the intestinal mucosa. CCK in turn stimulates [[Pancreas - Anatomy & Physiology|pancreatic]] contraction and release of pancreatic juices into the duodenum. This secretion contains chymotrypsinogen and (trypsinogen) both of which are activated in the intestinal lumen which further cleaves polypeptides into tripeptides, dipeptides and single amino acids. Specific carrier proteins are present on the mucosal surface of the duodenum and [[Jejunum - Anatomy & Physiology|jejunum]] (to a lesser extent in the [[Ileum - Anatomy & Physiology|ileum]]) to transport specific [[Amino Acids Overview - Nutrition|amino acid]] families (e.g. acidic-, neutral- or [[Nutrition Glossary#Dibasic Amino Acids|dibasic-amino acid]] transporters). Proteins or polypeptides that resist denaturation in the stomach or enzymatic cleavage in the upper small intestine can be fermented by colonic bacteria. Depending on the amino acid utilized by the intestinal microflora, malodorous by-products of bacterial fermentation can be created, such as cadaverine<ref name="NRC">National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.111-114.</ref>.

The term “protein quality” is used to describe the ability of a given plant or animal protein to provide adequate levels of essential amino acid(s) in order to support biological functions, such as growth or reproduction. Protein quality is determined by the composition of amino acids, and the [[Nutrition Glossary#Digestibility|digestibility]] and [[Nutrition Glossary#Bioavailability|bioavailability]] of amino acids within a given protein type. Protein digestibility can be adversely affected by dysfunction in any of the organs responsible for protein digestion and absorption (e.g., disorders of the stomach, pancreas, small intestine). In healthy animals protein digestibility is influenced by size in dogs<ref>Hannah SS, et al. Digestibility of diet in small and large breed dogs. Vet Clin Nutr 1995;2:145.</ref><ref>Nery J, et al. Influence of dietary protein content and source on fecal quality, electrolyte concentrations, and [[Nutrition Glossary#Osmolarity|osmolarity]], and digestibility in dogs differing in body size. J Anim Sci 2010;88:159-169.</ref> and decreases with age<ref>Teshima E, et al. Nutrient digestibility, but not mineral absorption, is age-dependent in cats. JAPAN (Berl) 2010;94:e251-258.</ref>. Additionally, cats protein digestibility of certain protein sources is lower compared to dogs<ref>de-Oliveira DL, et al. Digestibility for dogs and cats of meat and bone meal processed at two different temperature and pressure levels. JAPAN(Berl) 2012;96:1136-1146.</ref>. Dietary factors known to influence protein digestibility include:  

The term “protein quality” is used to describe the ability of a given plant or animal protein to provide adequate levels of [[Nutrition Glossary#Essential Amino Acids|essential amino acid(s)]] in order to support biological functions, such as growth or reproduction. Protein quality is determined by the composition of amino acids, and the [[Nutrition Glossary#Digestibility|digestibility]] and [[Nutrition Glossary#Bioavailability|bioavailability]] of amino acids within a given protein type. Protein digestibility can be adversely affected by dysfunction in any of the organs responsible for protein digestion and absorption (e.g. disorders of the stomach, [[Pancreas = Anatomy & Physiology|pancreas]], small intestine). In healthy animals protein digestibility is influenced by size in dogs<ref>Hannah SS, et al. Digestibility of diet in small and large breed dogs. Vet Clin Nutr 1995;2:145.</ref><ref>Nery J, et al. Influence of dietary protein content and source on fecal quality, electrolyte concentrations, and [[Nutrition Glossary#Osmolarity|osmolarity]], and digestibility in dogs differing in body size. J Anim Sci 2010;88:159-169.</ref> and decreases with age<ref>Teshima E, et al. Nutrient digestibility, but not mineral absorption, is age-dependent in cats. JAPAN (Berl) 2010;94:e251-258.</ref>. Additionally, cats protein digestibility of certain protein sources is lower compared to dogs<ref>de-Oliveira DL, et al. Digestibility for dogs and cats of meat and bone meal processed at two different temperature and pressure levels. JAPAN(Berl) 2012;96:1136-1146.</ref>. Dietary factors known to influence protein digestibility include:  

#The presence of anti-nutritive properties within certain ingredients (e.g., trypsin inhibitors found in uncooked legumes)<ref>Gilani GS, et al. Impact of antinutritional factors in food proteins on the digestibility of protein and the bioavailability of amino acids and on protein quality. B J Nutr 2012;108:S315-S332.</ref>;  

#The presence of anti-nutritive properties within certain ingredients (e.g., trypsin inhibitors found in uncooked legumes)<ref>Gilani GS, et al. Impact of antinutritional factors in food proteins on the digestibility of protein and the bioavailability of amino acids and on protein quality. B J Nutr 2012;108:S315-S332.</ref>;  

#The formation of [[Nutrition Glossary#Maillard Reaction|Maillard reaction]] products (cross-linkages between sugars and amino acids)<ref>van Rooijen C, et al. The Maillard reaction and pet food processing: effects on nutritive value and pet health. Nutr Res Rev 2013;26:130-148.</ref>; and  

#The formation of [[Nutrition Glossary#Maillard Reaction|Maillard reaction]] products (cross-linkages between sugars and amino acids)<ref>van Rooijen C, et al. The Maillard reaction and pet food processing: effects on nutritive value and pet health. Nutr Res Rev 2013;26:130-148.</ref>; and