Digestibility of Protein

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Protein digestion begins in the stomach of both dogs and cats with the secretion of hydrochloric acid and pepsinogen in response to the presence of protein in the stomach[1]. Pepsinogen is activated to pepsin in the presence of hydrochloric acid and begins the enzymatic breakdown of protein into polypeptides. In the duodenal lumen polypeptides (especially those containing phenylalanine) stimulate the secretion of cholecystokinin (CCK) from the intestinal mucosa. CCK in turn stimulates 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 (to a lesser extent in the ileum) to transport specific amino acid families (e.g. acidic-, neutral- or 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[2].

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 digestibility and 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[3][4] and decreases with age[5]. Additionally, cats protein digestibility of certain protein sources is lower compared to dogs[6]. Dietary factors known to influence protein digestibility include:

  1. The presence of anti-nutritive properties within certain ingredients (e.g. trypsin inhibitors found in uncooked legumes)[7];
  2. The formation of Maillard reaction products (cross-linkages between sugars and amino acids)[8]; and
  3. High temperature and pressure effects on protein structures[9][10][11]. The digestibility of protein is typically lower in plant-compared to animal-derived proteins[12]. Feeding diets with a high soluble fiber content[13][14][15] or with larger volumes of poorly digestible carbohydrate[2] will also result in a lower apparent protein digestibility in both dogs and cats.


  1. 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.
  2. 2.0 2.1 National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.111-114.
  3. Hannah SS, et al. Digestibility of diet in small and large breed dogs. Vet Clin Nutr 1995;2:145.
  4. Nery J, et al. Influence of dietary protein content and source on fecal quality, electrolyte concentrations, and osmolarity, and digestibility in dogs differing in body size. J Anim Sci 2010;88:159-169.
  5. Teshima E, et al. Nutrient digestibility, but not mineral absorption, is age-dependent in cats. JAPAN (Berl) 2010;94:e251-258.
  6. 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.
  7. 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.
  8. 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.
  9. Johnson ML, et al. Effects of species raw material source, ash content, and processing temperature on amino acid digestibility of animal by-product meals by cecectomized roosters and ileally cannulated dogs. J Anim Sci 1998;76:1112-1122.
  10. Larsen JA, et al. 2010 Bioavailability of lysine for kittens in overheated casein is underestimated by the rat growth assay method. JAPAN (Berl) 2010;94:e102-108.
  11. Kerr KR, et al. Apparent total tract energy and macronutrient digestibility and fecal fermentative end-product concentrations of domestic cats fed extruded, raw beef-based, and cooked beef-based diets. J Anim Sci 2012;90:515-522.
  12. Neirinck K, et al. Amino acid composition and digestibility of four protein sources for dogs. J Nutr 1991;121:S64-S65.
  13. Muir HE, et al. Nutrient digestion by ileal cannulated dogs as affected by dietary fibers with various fermentation characteristics. J Anim Sci 1996;74:1641-1648.
  14. Silvio J, et al. Influences of fiber fermentation on nutrient digestion in the dog. Nutr 2000;16:289-295.
  15. Harper EJ. The effect of fiber on nutrient availability in cats of different ages. Vet Clin Nutr 1995;3:114.

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