Carbohydrates

Glucagon

another hormone secreted from pancreatic α-cell. Glucagon is secreted when blood glucose concentration decreases (during fasting). Glucagon stimulates the breakdown of glycogen in liver to increase blood glucose concentration.

Gluconeogenesis

is the synthesis of glucose from noncarbohydrate sources. In the ruminant, this is a continuous process that is highest after a meal. Sources of carbon or precursors come from absorbed nutrients. The major precursors are propionic acid (one of the VFA), glucogenic amino acids, and lactate.

b. acetic acid, propionic acid, and butyric acid

he main volatile fatty acids produced by microbes in the GI tract are: Select one: a. linoleic acid and alpha linolenic acid b. acetic acid, propionic acid, and butyric acid c. linoleic acid, alpha linolenic acid, and arachidonic acid d. acetic acid and butyric acid

Inulin

is composed of fructoses and is found in potatoes. Inulin is not digestible by endogenous enzymes from animals.

The first stage of carbohydrate digestion in ruminants

is digestion of carbohydrates to simple sugars. This process relies on extracellular microbial enzymes which are secreted into the rumen environment. Some of these are similar to enzymes produced by mammalian cells such as α-amylase and sucrase. However, other enzymes such as cellulase and hemicellulase are not produced by mammalian cells. Production of these enzymes by the microorganisms gives the animal the ability to hydrolyze cellulose and hemicellulose producing simple sugars

a. composed of glucose units and found in the muscle of animals

Choose the correct description of glycogen. Select one: a. composed of glucose units and found in the muscle of animals b. an example of an organic catalyst c. composed of glucose units and found in the seeds of plants d. given to ruminants as a source of nitrogen

stomach

Dietary carbohydrates will pass to stomach through esophagus but no digestion occurs as there is no amylolytic enzyme secreted in the stomach.

Absorption

End products of carbohydrate digestion in the small intestine are monosaccharides. Absorption of monosaccharides occurs mainly in duodenum and jejunum but partly in ileum through mucosal cells on villi. Absorption of monosaccharides is a selective process by gut epithelial cells. Galactose and glucose are absorbed with high efficiency whereas mannose is not efficiently absorbed.

carb function

A main function of carbohydrate in an animal body is to provide energy in the form of glucose. Animal cells obtain energy from oxidation of glucose. When animals obtain enough glucose, extraneous glucose is stored as glycogen in liver and muscle. Once the capacity of glycogen storage is reached, the excess glucoses are converted and stored as fat. Carbohydrates also provide carbons for the synthesis of amino acids, nucleic acids, and fatty acids.

Effect of Diet on Digestion in Ruminants

Diet has a major influence on fermentation. Hydrolysis of the carbohydrates in forages is a slow process. A diet containing long forage particles increases rumination, saliva production, and buffering. Rumen pH is in the range of 6.3 to 6.8 in animals on a high forage diet. The starch in feedstuffs such as grains is rapidly hydrolyzed in the rumen. Increasing grains in the diet decreases rumination, saliva production and buffering. Rumen pH will be in the range of 5.5 to 5.9.

mouth

Dietary carbohydrates will be mechanically broken down to smaller particles by chewing and mastication in the mouth. There is an enzyme secreted in mouth called salivary amylase which will digest amylose to dextrin and maltose. However, carnivores and ruminants do not secrete salivary amylase.

glycogenolysis

Glucose can also be obtained from breakdown of glycogen in liver and muscle. This process is called

Prevention of acidosis

requires careful management such as slow introduction of concentrates. This gives the rumen time to adapt to new feedstuffs. Feeding less grain is also an option; however, grains are added in situations where a higher energy diet is needed. Buffers are often added to diets of dairy cattle to prevent acidotic conditions in the rumen. Two feed additives, rumensin and tylan are included in diets of beef cattle to minimize problems with acidosis.

clinical acidosis

rumen pH is less than 5.0. There is increased lactic acid production by rumen microorganisms and an increase in lactic acid absorbed from rumen leading to metabolic acidosis. This can lead to rumen stasis, damage to the papillae that line the rumen, and death of microbes and release of endotoxins. Associated problems include liver abscesses and laminitis. Laminitis is the inflammation of the laminae in the foot and rupture of peripheral blood vessels.

Small intestine: Intestinal mucosal cells

secrete enzymes to complete carbohydrate digestion. Maltose and isomaltose from digestion of pancreatic amylase will be further digested by maltase and isomaltase which are secreted from intestinal mucosal cells. Maltose and isomaltose will be digested to glucose molecules. Other enzymes secreted from intestinal mucosal cells are lactase and sucrase. Lactase will digest lactose to glucose and galactose and sucrase will digest sucrose to glucose and fructose. Ruminant does not secrete sucrase in small intestine. Oligoglucosidase is another enzyme secreted from intestinal mucosal cells digesting glucose oligosaccharides to glucose molecules.

Small intestine: Pancreas

secretes an amylolytic enzyme called pancreatic amylase which will digest starch and dextrin from dietary carbohydrates to maltose and isomaltose. This is a key process of carbohydrate digestion in non-ruminants. For ruminants, pancreatic amylase is not highly secreted because starch is not major part of dietary carbohydrates compared with non-ruminants. However, pancreatic amylase becomes important for ruminants consuming high-grain diets.

subclinical acidosis

the rumen pH is less than 5.5 but not below 5.0. Animals have decreased intake, decreased gain, poorer feed efficiency, and decreased milk production. They also exhibit cyclic feeding where they will eat a large amount of feed one day leading to a drop in rumen pH, eat less feed the next day, and then increase intake again the following day.

non-starch polysaccharides (NSP)

which include cellulose, hemicelluloses, and pectin; Dietary fiber is also termed

Glycolysis cycle

Pyruvates will enter mitochondria and are converted to acetyl-CoA generating NADH. Acetyl CoA will be further oxidized in the Krebs cycle (also called tri-carboxylic acid cycle, or citric acid cycle). Each acetyl-CoA will produce 3 molecules of NADH, 1 molecule of FADH2, and 1 molecule of ATP. Therefore, 1 molecule of glucose, when oxidized in glycolysis and the Krebs cycle, will generate 4 molecules of ATP, 10 molecules of NADH, and 2 molecules of FADH2. These NADH and FADH2 will be oxidized in the electron transport chain to NAD+ and FAD+, respectively. About 3 ATP can be generated from oxidation of 1 NADH whereas about 2 ATP can be generated from oxidation of 1 FADH2. In summary, 1 molecule of glucose can generate 38 ATP via glycolysis, Krebs cycle, and electron transfer chain (Figure 6).

Glucose Homeostasis in Ruminants

Rumen fermentation converts most of the dietary carbohydrates to VFA. As a result, the ruminant never absorbs enough glucose to meet its needs. There is also less of a true absorptive and post-absorptive period following feeding because of the continual fermentation in the rumen and absorption of end products. As a result, glucose homeostasis revolves around gluconeogenesis.

gluconeogenesis

Some amino acids (glucogenic amino acids) and glycerol can be sources of glucose. This process is called

carb hydrolization in cecum and or colon

Some dietary carbohydrates have beta linkages and cannot be hydrolyzed by mammalian enzymes. These carbohydrates may be hydrolyzed by microbial enzymes. This process takes place in the cecum and/or colon of most animals and in the rumen of ruminants. Following fermentation, VFA are absorbed from the GI tract at the site of production.

a. acetic acid

The VFA that makes the most significant contribution to oxidation in the body is __________. Select one: a. acetic acid b. lactic acid c. butyric acid d. propionic acid

carbohydrate digestion

The major goal of carbohydrate digestion is to break down dietary carbohydrates into monosaccharides for absorption. Only monosaccharides can be absorbed from the gastrointestinal tract. Ingested dietary carbohydrates will pass through gastrointestinal tract including mouth, esophagus, stomach, small intestine, and large intestine in sequence.

The primary carbohydrate digested in adult ruminant

The primary carbohydrate digested in the small intestine of adult ruminants is starch. As the amount of grain in the feed increases, some of the starch in the grain will escape digestion in the rumen and flow to the small intestine as part of the digesta. The starch is digested as previously described. Pancreatic amylase begins starch digestion in the small intestine. Some carbohydrates will not be digested completely by the end of the small intestine. These carbohydrates can be partially digested by microorganisms in the cecum and colon. Anaerobic fermentation will produce VFA.

Oligosaccharides

composed of 3-10 monosaccharides. There are numerous oligosaccharides composed of various monosaccharides. Some oligosaccharides nutritionally important are introduced.

Lactose

composed of galactose and glucose. Lactose is the major carbohydrate in milk and can be hydrolyzed by lactase.

Raffinose

composed of galactose, glucose, and fructose.

Sucrose

composed of glucose and fructose. Cane and beet sugar are great sources of sucrose. Sucrose can be hydrolyzed by sucrase.

Cellulose

composed of glucoses connected by β-1,4 linkage and thus not digestible by endogenous enzymes from animals. Cellulose is one of the cell wall components of plants.

Polysaccharides

composed of more than ten monosaccharides. *starch, dextrin, glycogen, cellulose, Hemicellulose, pectin, inulin

Maltose

composed of two glucoses connected by α-1,4 linkage. Thus the first glucose forming a maltose should be in α-form. Maltose is hydrolyzed to two glucoses by maltase.

Dietary fiber

defined as the edible parts of plants or analogues carbohydrates that are resistant to digestion by endogenous enzymes from host animals which will be partly or completely utilized by microorganisms in the large intestine (cecum or colon) or rumen. Microbial fermentation of dietary fiber in the cecum/colon or in the rumen would promote beneficial effects to the host animals. Dietary fiber can partly or completely be digested by microorganisms in rumen and cecum/colon providing volatile fatty acids that can be used as energy sources to the host animals. Dietary fiber is also termed as non-starch polysaccharides (NSP) which include cellulose, hemicelluloses, and pectin. Lignin is also part of dietary fiber. Lignin is not a carbohydrate but is discussed with carbohydrates because of their association in the cell wall.

Starch

good example of soluble polysaccharides composed of many glucose molecules. Glucoses making starch can be connected by α-1,4 linkage forming a linear structure or by both α-1,4 linkage and α-1,6 linkage forming a branched structure. Amylose and amylopectin are forms of starch.

Glycogen

highly branched polysaccharides composed of glucoses. Glycogen is a storage form of glucose in animal body rapidly providing glucose when needed. Glycogen is found in liver and muscle of animals.

Disaccharides

include maltose, cellobiose, lactose, and sucrose. Disaccharides are formed by a condensation reaction that joins two monosaccharides. Disaccharides are hydrolyzed to form two monosaccharides. Condensation and hydrolysis reactions are important general reactions in digestion and metabolism. *Maltose, Cellobiose, Lactose, Sucrose

The second stage of carbohydrate digestion

is the metabolism of the simple sugars by the microorganisms. These concepts are illustrated by the diagram in Figure 5. Metabolism by the microorganisms produces ATP for microbial growth plus other end products. The end products include: acetic acid, propionic acid, and butyric acid (the three major VFAs), methane, CO2, ATP, H2O and heat. The volatile fatty acids (VFA) produced are absorbed from the rumen through the rumen papillae into capillaries that line the rumen wall. These lead to larger veins and eventually the portal vein which delivers nutrients to the liver.

Pectin

linear polymer mainly composed of galactouronic acids. Pectin is not digestible by endogenous enzymes from animals.

Glucose storage

more glucose absorbed from diets than needed, extraneous glucose will be stored as glycogen in liver and muscle. However, animals have limited capacity of storing glycogen. Once the capacity of glycogen storage is reached, the excess glucoses are converted to fatty acids in adipose tissue of most domestic animals. This process takes place in liver of birds. These fatty acids will be stored as depot fat in adipose tissue.

glycolysis

occurs in cytoplasm and uses glucose producing 2 molecules of pyruvate, 2 molecules of ATP, and 2 molecules of NADH. Glucose obtained from diet, glucogenesis, glycogenolysis, and gluconeogenesis can be used in cells to generate energy via oxidation called

Carbohydrate

one of major nutrients in feed. Carbohydrate is the key energy source for animals. However, carbohydrate makes up less than 1% of the weight of an animal body mostly found as glycogen. Animals obtain carbohydrates from feed and utilize them as energy source, store as glycogen, or transform to fat. Most of non-ruminant animals use soluble forms of carbohydrates such as starch as energy source whereas ruminants and other herbivores use structural carbohydrates such as fiber for fermentation and, as a result, energy. Major source of carbohydrates for animals are plants. Carbohydrates are important parts for plants comprising 70% of dry matter.

Insulin

a hormone synthesized in pancreatic β-cells. Insulin is secreted when blood glucose concentration rises (after eating) stimulating the uptake and use of glucose by cells in various tissues including muscle and liver. Insulin will also stimulate the synthesis of glycogen in liver and muscle. Glycogen will be stored in these two tissues to provide glucose when blood glucose concentration decreases. Glycogen storage is limited. Insulin also stimulates conversion of glucose to fatty acids and triglycerides for storage in adipose tissue.

Fructose

a keto-hexose and is the sweetest tasting of all monosaccharides. Fruits and honey are good sources of fructose.

Amylose

a linear form of starch

Ruminal acidosis

a nutritional and management problem that can occur in animals fed high grain diets or high silage diets. In this condition, the rate of acid production is greater than the ability to absorb or buffer the acids.

Dextrin

a partial hydrolytic product of starch.

Ribose

also a pentose and being parts of ATP, DNA, and RNA.

Cellobiose

also composed of two glucoses, but connected by β-1,4 linkage. Thus the first glucose forming a cellobiose should be in β-form.

Galactose

an aldo-hexose and known as milk sugar.

Glucose

an aldo-hexose and most abundant in nature. Glucose is also considered as nutritionally most important compared with other monosaccharides. Glucose is also named grape sugar, dextrose, and blood sugar. The concentration of glucose in blood is responsible for the insulin and glucagon secretion and balance. Glucose is a major energy source for various tissues including brain.

Carbohydrate structure

an organic compound containing C, O, and H. The primary structure of a carbohydrate is an aldehyde (HC=O) or ketone (C=O) containing multiple carbons with hydroxyl (OH) groups. A carbohydrate composed of an aldehyde containing hydrated carbons is called an aldose whereas a carbohydrate composed of a ketone containing hydrated carbons is called a ketose.

GLUT

Absorption of monosaccharides by mucosal cells is done by glucose transporters (GLUT) which are passive diffusion transporters not requiring energy. These GLUT absorb all types of monosaccharides. In the case of galactose, glucose, and xylose, they can also be absorbed by sodium-dependent transporters (SGLT) which are active transporters requiring energy.

Raffinose, stachyose or verbascose

All three of these oligosaccharides are not digestible by endogenous enzymes secreted from animals, but are extensively used by microorganisms in large intestine generating massive gas from the fermentation. Thus these oligosaccharides are called flatulence-producing compounds.

Glucose Homeostasis in Simple-Stomached Animals

Animals regulate glucose concentration in the blood for a constant supply of glucose to cells. It is especially important for certain cell types such as nerve cells and red blood cells to have a steady supply of glucose as their major energy source. Liver plays a key role in glucose homeostasis controlled by two major hormones, insulin and glucagon.

carb classification (aldose or ketose)

Based on the chemical structure, carbohydrates can be classified as aldoses and ketoses. Also, based on the number of carbons, carbohydrates can be classified as triose (3 carbons), tetrose (4 carbons), pentose (5 carbons), and hexose (6 carbons). As an example, a glucose is an aldose and also a hexose and thus it is called an aldo-hexose. A fructose is a ketose and also a hexose and thus it is called a keto-hexose. Groups of carbohydrates belonging to aldoses and ketoses are shown in Figures 1 and 2.

carb classification (mono or disaccharide)

Based on the number of molecules, carbohydrate can be a monosaccharide (1 sugar), disaccharide (2 sugars), oligosaccharides (3-10 sugars) or polysaccharides (more than 10 sugars). Examples of monosaccharides are glucose, fructose, galactose, and ribose. Disaccharides are maltose, cellobiose, lactose, and sucrose. Oligosaccharides are raffinose, stachyose, verbascose, mannan-oligosaccharides, fructo-oligosaccharides, and galacto-oligasaccharides. Examples of polysaccharides are starch, glycogen, cellulose, hemicelluloses, pectin, and inulin.Some polysaccharides, such as starch and cellulose, have only 1 type of sugar and are termed homoglycans. Other polysaccharides, such as hemicellulose and pectin, are composed of different types of monosaccharides and are termed heteroglycans.

Glucose Conservation

Because the ruminant has to make most of its glucose there are some differences in its use in the body compared to simple stomached animals. The ruminant will use products from fermentation in some cases rather than glucose. These differences allow the ruminant to conserve glucose to use for vital functions. Glucose is used as a source of energy by cells but is not the major energy source. After feeding there is some formation of glycogen in the liver for storage of glucose produced by gluconeogenesis. Glucose is used to produce the glycerol backbone of triglycerides that are formed in adipose tissue but is not used to form the fatty acid portion of the triglyceride. The major energy source for cells and the absorbed nutrient used to form the fatty acid portion of triglycerides is acetic acid. This is one of the major VFA produced during fermentation.

Carb summary

Carbohydrate is the key energy source for animals. Animals obtain carbohydrates mainly from plants. Carbohydrates are comprised of monosaccharides. Nutritionally important monosaccharides include glucose, fructose, galactose, and ribose. Dietary carbohydrates are mainly digested in the small intestine and absorbed as monosaccharides. Non-starch polysaccharides and some oligosaccharides cannot be digested by endogenous enzymes from animals but these carbohydrates can partly be digested by ruminal and hindgut (cecum and colon) microorganisms. The microorganisms use anaerobic fermentation producing volatile fatty acids which can be used as energy sources by the host animals. Once monosaccharides are absorbed, non-glucose monosaccharides need to be converted to glucose through hepatic glucogenesis. Glucose can be used to generate energy or stored as glycogen or converted to fat for storage. When glucose is needed in a body, glycogen is broken to glucose through glycogenolysis. Some amino

Carbohydrate metabolism

Carbohydrate metabolism is the metabolism of glucose. Glucose may be oxidized by cells or converted to a form for storage. Glucose arises from the diet and can also be produced in the animal body. The body balances the amount of glucose absorbed from the diet with that produced by the body to maintain glucose concentration within a relatively narrow range.

Carbohydrates in Animal Feed

Carbohydrates are important energy source for animals. However, not all carbohydrates contained in feed are biologically available to animals. Depending on carbohydrate sources, the amount of energy that animals can obtain varies. Thus when formulating animal feed, the amount of digestible or metabolizable energy should be considered. Energy contents in corn and wheat bran are shown in Table 2. The amounts of digestible and metabolizable energy are different between corn and wheat bran because corn contains carbohydrates that are highly digestible (such as starch) whereas wheat bran contains carbohydrates that are not readily digestible (such as non-starch polysaccharides) to the animals. Selection of carbohydrate sources should also consider the age of animals. Mature animals are much more capable of digesting starch whereas young animals have a limited ability to digest starch. However, young animals are capable of utilizing lactose as a source of energy whereas mature animals have l

carb digestion in ruminants

Carbohydrates comprise the major portion of the feed of ruminants. Forages are usually a large portion of the feed. Structural carbohydrates are present in the cell wall of forages. The cell contents contain sugars. Grains may also be included in the feed. Starch is the primary carbohydrate in grains. Increasing the grain content of the feed increases the starch content. As feed is consumed, it is chewed and mixed with saliva in the mouth. The bolus then moves down the esophagus into the stomach. In the next section we discuss microbial digestion and metabolism of carbohydrates in the rumen.

a. hemicellulose

Identify the substance that is a heteroglycan. Select one: a. hemicellulose b. starch c. glycogen d. cellulose

b. gluconeogenesis

In the simple stomached animal, which of the following is NOT a major use for absorbed glucose? Select one: a. source of energy b. gluconeogenesis c. glycogenesis d. lipogenesis

Monosaccharides

Monosaccharide is a simple sugar. Monosaccharides with five or more carbons dominantly form a ring (a cyclic form rather than a linear configuration). A monosaccharide forming a ring occurs as both α and β isomeric forms. When a hydroxyl group of 1st carbon is drawn below a plane of the ring, it is called α whereas it is called β form when drawn above a plane of the ring (Figure 3). In a case of glucose, starch is assembled from α-glucose units whereas cellulose is assembled from β-glucose units. *Nutritionally important monosaccharides include glucose, fructose, galactose, and ribose.

monosaccharide digestion

Monosaccharides are absorbed in the small intestine. Digestive enzymes hydrolyze carbohydrates from the feed eventually producing monosaccharides that can be absorbed into the animal body. There are several transporters that function in absorption of the monosaccharides.

large intestine

Non-starch polysaccharides and some oligosaccharides cannot be digested by amylolytic enzymes from animals. However, these carbohydrates can partly be digested by ruminal and gut microorganisms through anaerobic fermentation occurring in the rumen, cecum, or colon. Anaerobic fermentation of carbohydrates results in the production of volatile fatty acid (VFA) such as acetic acid, propionic acid, and butyric acid that can be absorbed and used as energy sources by the host animals.

hepatic glucogenesis

Once monosaccharides are absorbed, non-glucose monosaccharides need to be converted to glucose in liver to generate energy or to be stored. This process is called

mannan-oligosaccharides (MOS), galacto-oligosaccharides (GOS), and fructo-oligosaccharides (FOS).

Other oligosaccharides found in animal feed are mannan-oligosaccharides (MOS), galacto-oligosaccharides (GOS), and fructo-oligosaccharides (FOS). Both FOS and GOS are often added to diets to provide food for health-benefiting microorganisms in the gastrointestinal tract. These are termed prebiotic feed additives.

b. alpha

Out of the different linkages of glucose which one can be digested by mammalian enzymes? Select one: a. beta b. alpha c. gamma d. beta and alpha

fiber terms

Total dietary fiber is often used for primates and companion animals (dogs and cats). It is classified by both its solubility and fermentability. Both soluble and insoluble fiber sources are usually incorporated into diets.

b. glycogenolysis

What is the short-term response of a simple stomached animal to hypoglycemia? Select one: a. gluconeogenesis b. glycogenolysis c. lipogenesis d. glycogenesis

c. kidneys store glucose in the form of glycogen

When blood glucose level is high, the pancreas increases insulin production. Which of the following is NOT caused by increased levels of insulin? Select one: a. liver stores glucose in the form of glycogen b. muscle stores glucose in the form of glycogen c. kidneys store glucose in the form of glycogen d. adipose tissue stores glucose in the form of triglycerides

stachyose or verbascose

When one or two additional galactose(s) is (are) added to raffinose, it becomes:

b. propionic acid

Which of the following fatty acids is used in the formation of glucose in the ruminant? Select one: a. butyric acid b. propionic acid c. arachidonic acid d. linoleic acid

d. All of the above

Which of the following is TRUE about gluconeogenesis in ruminants? Select one: a. It is continuous. b. It is the highest after feeding. c. It uses absorbed nutrients - not body tissue. d. All of the above

b. sucrose

Which of the following is a disaccharide? Select one: a. glucose b. sucrose c. cellulose d. fructose

d. pectin

Which of the following needs microbial enzymes for digestion? Select one: a. glycogen b. glucose c. starch d. pectin e. fructose

amylopectin

a branched form of starch.

Hemicellulose

a branched polymer with xylose, arabinose, glucose, mannose, galactose, etc. Hemicellulose is not digestible by endogenous enzymes from animals. It is also part of the plant cell wall.