Digestibility of Carbohydrates

Unlike humans, dogs and cats lack salivary amylase and enzymatic digestion of carbohydrate begins in the small intestine. 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. Dietary monosaccharide can be absorbed directly via facilitated diffusion and Na²⁺-dependent glucose transporters, while disaccharide and absorbable polysaccharide carbohydrates must first be broken down by mammalian enzymes into their monosaccharide subunits. 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 α-1,4 glycosidal linkages in starch. 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 and amino acids, not carbohydrates, in the duodenal lumen.

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 Na2+-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.6 Absorbed glucose directly contribute to circulating blood glucose concentrations, while galactose and fructose are first metabolized by hepatic fructokinase.7 Cats have lower concentrations of pancreatic amylase8 as well as lower levels of hepatic glucokinase9 relative to dogs, but are still able to digest and absorb dietary carbohydrates.1,10 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 muscle11, or used for lipid synthesis.12 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