Page 68 - Small Animal Clinical Nutrition 5th Edition

68 Small Animal Clinical Nutrition

Table 5-4. Classification and digestion of complex carbohydrates.*
VetBooks.ir Complex Function Digestion site Digestion products
carbohydrate type

Starch, glycogen Storage polysaccharide Small intestine Mono- and disaccharides
in plants and animals (enzymatic) (glucose, maltose)
Hemicellulose, cellulose Structural parts of plant cell walls Large intestine Volatile fatty acids (acetate,
(microbial fermentation) propionate, butyrate)
Lignins, cutins, waxes Associated cell wall substances Not digested or fermented Excreted in feces
Gums, mucilages, pectins Naturally occurring polysaccharides Large intestine Carbon dioxide, methane,
in plants (microbial fermentation) hydrogen, volatile fatty acids
*Adapted from the British Nutrition Foundation. Complex Carbohydrates in Foods. New York, NY: Van Nostrand Reinhold, 1990.

Starch is initially cleaved by the enzyme α-amylase, which
IMPORTANCE OF CARBOHYDRATES creates branched oligosaccharides, the disaccharide maltose and
The primary purpose for adding carbohydrates and starches the trisaccharide maltotriose. The brush border enzymes mal-
to pet foods is to supply energy. Generally, assuming an average tase, sucrase and isomaltase cleave the larger glucose chains into
digestibility (84%), carbohydrates supply about 3.5 kcal/g. single glucose molecules that are then absorbed. Sucrase also
Although there is no minimum dietary requirement for simple splits the disaccharide sucrose into glucose and fructose units.
carbohydrates or starches per se, certain organs and tissues (e.g., Lactase, another brush border enzyme, splits lactose, the sugar
brain and red blood cells) require glucose for energy. Glucose found in milk, into glucose and galactose. Lactase activity is
can be obtained from precursor nutrients such as glucogenic usually high in young, suckling animals but often declines in
amino acids or glycerol from fats via gluconeogenic pathways. adults. Conversely, amylase, maltase and isomaltase display a
The body always maintains a glucose supply to key tissues; thus, reverse temporal pattern; concentrations of these enzymes are
if adequate dietary carbohydrates are unavailable, amino acids low in suckling animals and higher in adults (Meyer and
will be shunted away from muscle growth, fetal growth and Kienzle, 1991; Kienzle, 1988). For example, in puppies, amylase
milk production to be used for glucose synthesis. and sucrase activities increase by 21 days after birth and
When energy needs are high and tissue accretion is occurring increase further by Day 63 postpartum. This pattern suggests
(e.g., during growth, gestation and lactation), adequate dietary that growing dogs have an increasing ability to digest carbohy-
carbohydrates or glucose precursors are necessary to maintain drates from foods (Buddington et al, 2003).
metabolic processes (Romsos et al, 1981; Kienzle et al, 1985; Starch is made up of glucose units in straight chains (amy-
Meyer and Kienzle, 1991; Blaza et al, 1989). In these situations, lose) and with branches (amylopectin) linked with α-bonds
carbohydrates become conditionally essential; therefore, foods (Figure 5-11). Starches are contained within granules in plants
fed to growing animals and those with high-energy needs in a highly crystalline formation. As foods containing starches
should contain at least 20% carbohydrates. are heated or cooked with water, the starch crystals are melted
In addition to nutritional reasons for adding carbohydrates to and hydrated, a process called gelatinization (Camire et al,
pet foods, carbohydrates also are important in pet food process- 1990). The extent to which starch granules are disrupted and
ing. Chapter 8 provides detailed information. the extent of gelatinization depend on many factors including
grinding, moisture, cooking time and temperature. For most
Metabolism starches, digestibility increases with the degree of gelatiniza-
DIGESTION tion. Extrusion cooking, a process used in dry pet food produc-
Digestion of simple carbohydrates and starches occurs tion, increases overall digestibility of starches in grains by gelat-
throughout the digestive tract and involves mechanical, enzy- inizing starch.The canning process also results in gelatinization
matic and microbial processes. Mechanical breakdown occurs of starch.
primarily in the oral cavity. Because dogs and cats lack salivary Several reports indicate that dogs and cats readily digest
α-amylase, enzymatic digestion of starch is not initiated in starches in commercial pet foods (Meyer and Kienzle, 1991;
the mouth. In the stomach, food is mixed with gastric juices Gross et al, 1998; Walker et al, 1994; Schunemann et al, 1989).
(i.e., hydrochloric acid and proteolytic enzymes). Although In studies, dogs were fed foods in which 30 to 57% of the food
the stomach plays an important role in protein digestion, lit- came from extruded corn, barley, rice or oats. The starch from
tle carbohydrate digestion occurs here. Simple carbohydrates all grains was nearly 100% digested in the small intestine;
and starches are digested and absorbed in the small intestine. essentially no starch passed into the colon (Walker et al, 1994).
Enzymes secreted from the pancreas digest the majority of Other studies compared the digestibility of isolated raw
starches and sugars in the lumen of the small intestine, where- cornstarch, tapioca and potato starches and cooked rice starch
as enzymes at the small intestinal mucosal brush border are (Meyer and Kienzle, 1991; Schunemann et al, 1989). In these
important in the final stages of carbohydrate digestion and studies, isolated starches contributed 40% of the DM of the
absorption. food. By the time the starch reached the colon, uncooked corn-