Page 69 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 69

starch was digested to the same degree as cooked rice starch based on their effect on blood glucose. Carbohydrates that
VetBooks.ir (>94% digested); however, uncooked potato (0%) and tapioca result in a low postprandial blood glucose response have a lower
glycemic index and vice versa. Animals with impaired glucose
starches (<70%) were poorly digested in the small intestine.
tolerance should consume foods that have a relatively low
The uncooked tapioca starch subsequently resulted in increased
bacterial fermentation rates as evidenced by high volatile fatty glycemic index. Altering carbohydrate sources or adding fiber
acid concentrations in the feces. Large amounts of easily fer- can modulate the glycemic index.
mentable carbohydrates (e.g., tapioca starch) in the colon The central nervous system and erythrocytes require glucose
increase the risk of excessive fermentation causing gas and flat- for their energy needs, whereas other tissues can use other sub-
ulence and upsetting the balance of microflora. strates (e.g., muscle uses fatty acids). Glucose is metabolized via
In feeding trials with cats, investigators demonstrated that glycolysis followed by the TCA cycle (Figure 5-7). Complete
cooked cornstarch was nearly 100% digested when consumed oxidation of glucose to carbon dioxide, water, ATP and heat
by cats at 4.7 g/kg body weight per day (Meyer and Kienzle, requires oxygen and is termed aerobic metabolism. The final
1991; Kienzle, 1993, 1993a). Raw cornstarch was only 60 to transfer of energy from carbohydrate to ATP occurs via the
70% digested and raw potato starch was 40% or less digested electron transport chain (Figure 5-4). If there is a shortage of
when consumed at 8.8 and 8.9 g/kg body weight per day. Most oxygen in tissues, such as occurs with intense exercise, some
commercial cat foods contain approximately 30 to 35% DM ATP can be derived from glucose via anaerobic metabolism in
carbohydrate. This level provides approximately 5 to 8 g of which glucose is partially metabolized to pyruvate (via glycoly-
starch/kg body weight per day, which should pose no digestive sis) and then converted to lactic acid.
or metabolic problems for cats (Meyer and Kienzle, 1991). Glucose consumed in excess of immediate needs may be
stored as glycogen. The enzyme glycogen synthetase synthe-
ABSORPTION sizes glycogen from glucose units. This enzyme is particularly
Glucose and galactose are absorbed through an active trans- active in liver and muscles. Endurance athletes have used car-
port mechanism using specific carrier proteins and a sodium bohydrate loading (i.e., eating large amounts of carbohydrate
gradient. Fructose is absorbed by a separate carrier system that several days before competition) to maximize muscle glycogen
appears not to be sodium dependent. Absorption occurs across stores. Carbohydrate loading in canine athletes has been prac-
the small intestinal mucosa through villi. The enterocytes cov- ticed, but has not been widely researched (Chapter 18). After
ering the villi contain the carbohydrate-digesting enzymes, glycogen stores are filled, additional dietary carbohydrates are
transport proteins and other enzymes used to synthesize converted to long-chain fatty acids and stored as adipose tissue.
triglycerides and chylomicrons. Enterocytes can use absorbed In the hours following digestion and absorption of a carbo-
sugars for energy or the sugars can be released into portal blood hydrate-containing meal, the liver and other body tissues
for transport to the liver and beyond. switch from glycogen storage to glycogenolysis under the influ-
Deficiency of digestive enzymes or failure of the energy- ence of an increased glucagon to insulin ratio. This ratio also
dependent transport system may cause carbohydrate intoler- stimulates lipolysis, thus overall body metabolism switches
ance or malabsorption. Many disaccharidase deficiencies result toward lipid use for energy. Glucose is synthesized from carbon
from intestinal mucosal damage induced by infections and skeletons of amino acids, glycerol and lactic acid (gluconeoge-
other diseases. The resulting colonization of the lower small nesis) to maintain plasma glucose concentration. This function
intestine by colonic bacteria may result in bacterial proteolysis is critical to provide an adequate supply of glucose to the brain
of carbohydrate-digesting enzymes. (Figure 5-7). The liver and kidneys, but not muscles, are the
Unabsorbed carbohydrates in the intestinal lumen create sites of gluconeogenesis; therefore, muscle cannot supply glu-
high osmotic pressure, reduce water and mineral absorption cose to the bloodstream.
and may result in small bowel diarrhea. In addition, excessive
fermentation of unabsorbed carbohydrates leads to bacterial STORAGE
overgrowth, production of gas (carbon dioxide, hydrogen and The body stores sugar as glycogen, a glucose polysaccharide.
methane) and short-chain fatty acids. Excessive carbohydrate Its highest concentration is in the liver, but muscle tissue,
fermentation can lead to flatulence, abdominal distention and because of its greater mass, stores the most glycogen.
diarrhea. Carbohydrate intolerance may be diagnosed by find- Ribose, although not a true carbohydrate store readily avail-
ing increased concentrations of hydrogen in the breath (breath able for oxidation, is found as part of nucleic acids, ATP and
hydrogen analysis) as a result of bacterial fermentation (Bissett guanosine triphosphate (GTP). The body also has stores of
et al, 1997). sugar-protein complexes (glycoproteins, mucus and proteogly-
cans) and sugar-lipid complexes (glycolipids).
USAGE
Glucose and other sugars derived from food arrive at the liver EXCRETION
via the portal blood. The liver plays a central role in synthesiz- Excreted products resulting from normal carbohydrate
ing, storing, converting and releasing glucose for use by other metabolism include carbon dioxide in the breath, heat radiating
organs. Insulin and glucagon finely control the concentration of from the body and water. In cases of malabsorption of simple
blood glucose. The glycemic index ranks dietary carbohydrates sugars and starches, increased intestinal fermentation may lead