View source for Digestibility of Carbohydrates

Unlike humans, '''dogs and cats lack salivary amylase''' and enzymatic digestion of [[Carbohydrates Overview - Nutrition|carbohydrate]] begins in the [[Small Intestine - Anatomy & Physiology|small intestine]].<ref name="Morris">Morris JG, ''et al''. (1997) '''Carbohydrate digestion by the domestic cat (''Felis catus). '''Br J Nutr'' 1997;37:365-373.</ref><ref name="Hilton">Hilton J. (2006) '''Carbohydrates in the nutrition of dog.''''' Can Vet J ''1990;46A:359-369.</ref> The sugar alcohols mannitol, sorbitol, and xylitol are found as straight chain carbons instead of hexose (glucose and galactose) or pentose (fructose) carbon ring structures and sugar alcohols are absorbed by diffusion across the intestinal mucosa without hydrolysis.<ref name="NRC">National Research Council (NRC). (2006) '''Carbohydrates and Fiber. In Nutrient Requirements for Dogs and Cats. '''2006 ''Washington, DC: National Academies Press'' p.51-54.</ref> Dietary monosaccharide can be absorbed directly via facilitated diffusion and Na<sup>2+</sup>-dependent glucose transporters, while disaccharide and absorbable polysaccharide carbohydrates must first be broken down by mammalian enzymes into their monosaccharide subunits.<ref name="NRC" /> Disaccharides are hydrolysed by small intestinal enzymes (maltase, sucrase and lactase) while longer chain polysaccharides (i.e. absorbable starches) must first be hydrolysed by pancreatic α-amylase. Pancreatic α-amylase breaks the <font color="red">α-1,4 glycosidal linkages in starch (IMAGE)</font><ref name="Colonna">Colonna P, ''et al.'' (1992) '''Limiting factors of starch hydrolysis.''''' Eur J Clin Nutr ''1992;46:S17-S32.</ref>. Secretion of pancreatic α-amylase, with lipase, colipase and trypsin, is under the influence of cholecystokinin (CCK), though CCK release itself is stimulated by the presence of free [[Fatty Acids Overview - Nutrition|fatty acids]] and [[Amino Acids Overview - Nutrition|amino acids]], not carbohydrates, in the duodenal lumen.<ref name="Backus">Backus RC, ''et al.'' (1995)''' Elevation of plasma cholecystokinin (CCK) immunoreactivity by fat, protein, and amino acids in the cat, a carnivore. '''''Regul Pept ''1995;57:123-131.</ref>

Glucose, galactose and fructose, whether initially consumed as monosaccharides, disaccharides or part of a polysaccharide, are readily absorbed across the small intestinal mucosa and enter the portal circulation after meal consumption. The Na<sup>2+</sup>-dependant GLUT-1 transporter is found on small intestinal cells and facilitates transport of both glucose and galactose into the cells; fructose absorption is less well understood but is thought to involve a separate GLUT-5 transporter.<ref name = "Levin">Levin RJ. (1994) '''Digestion and absorption of carbohydrates: From molecules and membranes to humans.''''' Am J Clin Nutr'' 1994;59:690S-698S.</ref> Absorbed glucose directly contributes to circulating blood glucose concentrations, while galactose and fructose are first metabolized by hepatic <font color="red">fructokinase.<ref>Ref 7 missing</ref></font> Cats have lower concentrations of pancreatic amylase<ref name="McGeachin">McGeachin RL and Akin JR. (1979) '''Amylase levels in the tissues and body fluids of the domestic cat ''(Felis catus). '''Comp Biochem Physiol B ''1979;63:437-439.</ref> as well as lower levels of hepatic glucokinase<ref name="Washizu">Washizu T, ''et al.'' (1999)''' Comparison of the activities of enzymes related to glycolysis and gluconeogenesis in the liver of dogs and cats.''''' Res Vet Sci ''1999;67:205-206.</ref> relative to dogs, but are still able to digest and absorb dietary carbohydrates.<ref name="Morris" /><ref name="Kienzle">Kienzle E. (1993) '''Carbohydrate metabolism in the cat. 2. Digestion of starch.''''' JAPAN ''1993;69:102-114.</ref> In both species, absorbed glucose can be transported directly into cells for further metabolism and oxidation to form ATP, can be used to form glycogen (the storage form of carbohydrates within animal tissues) in liver or muscle<ref name="Ebiner">Ebiner JR, ''et al.'' (1979)''' Comparison of carbohydrate utilization in man using indirect calorimetry and mass spectrometry after oral load of 100 g naturally-labelled (13C) glucose. '''''Br J Nutr ''1979;41:419-429.</ref>, or used for lipid synthesis.<ref name="Flatt">Flatt JP, ''et al. '' (1985) '''Effects of dietary fat on postprandial substrate oxidation and on carbohydrate and fat balances. '''''J Clin Invest'' 1985;76:1019-1024.</ref>

A number of animal factors  impact carbohydrate digestion. These include age related changes in enzyme activities as well as inherent species differences in metabolic pathways. Lactase activity is highest in puppies and kittens and decreases with age,13,14  . In contrast pancreatic amylase activity increases with age.  Low fructokinse activity in cats means they can develop galactosuria or fructosuria if given these monosaccharides.15 Overall carbohydrate digestibility  decreases with age in otherwise healthy dogs and cats.16,17
In both dogs and cats starch digestibility is also affected by the source and type of carbohydrate present18 as well as the degree of processing of the carbohydrate.19,20 The type and amount of non-absorbable carbohydrate (i.e. fibre) present in the diet will also influence the post-prandial glycaemic response in both dog and cats. The presence of high soluble, fermentable fibre content in the diet will slow carbohydrate digestion and absorption resulting in dampened post-prandial blood glucose in both healthy21,22 and diabetic animals.23,24 Ground, cooked and extruded starches are almost 100% digestible in both dogs and cats,1,2,25,26 while digestibility of raw (uncooked) starches varies  from 0-65% depending on type of starch. Resistant starches are formed when solubilised dietary starch recrystallize upon cooling forming a structure that is resistant topancreatic amylase.27 Undigested starches and resistant starch can then be fermented by intestinal bacteria, 28-30 which may contribute to clinical signs of bacterial overgrowth. Maldigestion and malabsorption of dietary starch are believed to be a feature of inflammatory bowel disease.31 

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