IV. Carbohydrates, Digestion and Absorption

This chapter provides an introduction to the different processes that are involved in the digestion or fermentation of carbohydrates in monogastric and ruminant animals.

New Terms
Amylase
Dextrins
Disaccharidase
Maltase
Nonstarch polysaccharides
Sucrase
Volatile fatty acids

Chapter Objectives

  • To discuss the digestion and/or fermentation carbohydrates in food-producing animals
  • To discuss carbohydrate fermentation-related disorders in ruminant animals

The primary site of carbohydrate digestion is in the lumen of the small intestine, where pancreatic amylase begins the digestion of starch granules (amylose and amylopectin). In some birds, there is some salivary amylase action in the mouth, but not in farm animals.

There are two forms of amylase, one that cleaves α 1,4 bonds in a random fashion, while the other removes disaccharides units (maltose) from the polysaccharide chain. Pancreatic amylase does not act on α 1,6 bonds that form the branch points in the structure of amylopectin. The end products of amylase digestion include a mixture of glucose, maltose, and dextrins (residues containing α 1,6 branch points). Dextrins are acted upon by α 1,6 glucosidase.

 

The small intestine is the site of the digestion of carbohydrates in farm animals.

Dietary simple sugars, such as glucose and fructose, do not need to be digested, as they can be absorbed through the intestinal epithelium directly. The end products of starch digestion diffuse into the brush border, where the final digestive processes occur. Disaccharides such as maltase and isomaltase on the intestinal brush border then complete the degradation and are hydrolyzed to their constituent monosaccharides by enzymes on the brush border, and the monosaccharides released are absorbed into the enterocyte. Sucrose is acted upon by sucrase to yield glucose and fructose for absorption. In young animals kept on milk (preweaning), lactose is acted upon by lactase to yield glucose and galactose. Amylase activity is very low in young animals consuming milk and is stimulated by solid food consumption.

 

The end product of carbohydrate digestion in monogastric animals is mainly glucose.

Carbohydrate-Digesting Enzymes

  • Amylase
  • Disaccharidase
  • Maltase
  • Sucrase
  • Lactase

Monosaccharides are absorbed both by simple diffusion and adenosine triphosphate (ATP)-dependent active transport. A sodium-dependent glucose transport protein binds glucose and Na+ and transports them through the enterocyte and releases them in the cytosol.

 

Table 4.1. Overview of carbohydrate digestion: Site, enzymes, and end products produced in monogastric animals
Site Enzyme Product
Mouth Salivary Amylase Not too significant
Small Intestine Pancreatic Amylase

Di/oligosaccharidases

Monosaccharides
Large Intestine None Some volatile fatty acid

Monogastric animals do not secrete enzymes that digest the complex carbohydrates (β 1,4 linkages; e.g., nonstarch polysaccharides [NSP], glucans, cellulose) that are components of plant fiber (e.g., wheat, barley) and are acted upon by hindgut microbes to yield volatile fatty acids (VFAs). High levels of NSP and glucans in a monogastric diet can cause viscous digesta and can interfere with digestion processes leading to malabsorption. In poultry, high-NSP-containing diets (e.g., barley, rye) can produce wet litter, dirty eggs, and diarrhea.

Carbohydrate Digestion in Ruminants

Carbohydrate digestion in ruminant animals is through microbial fermentation in the rumen. Dietary carbohydrates are degraded (fermented) by rumen microbes (bacteria, fungi, protozoa). The purpose of rumen fermentation is to produce energy as ATP for the bacteria to use for protein synthesis and their own growth. VFAs, also known as short-chain fatty acids (shown below), are produced as a product of rumen fermentation and are absorbed through the rumen wall and are utilized by the animal as an energy source.

Major Volatile Fatty Acids Produced in the Rumen

  • Acetic acid
  • Propionic acid
  • Butyric acid

The three major VFAs are acetic (C2), propionic (C3), and butyric acid (C4; shown below).



The end products of digestion in ruminants are volatile fatty acids and some monosaccharides.

In young ruminants, rumen and the reticulum are not fully developed and are relatively small. The reticular/esophageal groove reflex, a tube-like fold of tissue, channels milk or water that is sucked from a nipple directly through the omasum to the abomasum. This is a reflex, stimulated by sucking. When the animal is weaned, it normally loses this reflex. Solid food, such as creep feed, passes into the small rumen and fermentation starts. The neonatal ruminant animal has no ruminal bacterial population but from birth, it starts to pick up bacteria from the mother and environment, particularly through contact. Solid food is then fermented forming VFAs, which stimulate the growth and development of the rumen, particularly the growth of the papillae for absorption.

All the digested and absorbed monosaccharides and volatile fatty acids enter into the liver.

The end products of rumen fermentation are microbial cell masses, or microbial protein-synthesized VFA, and gases such as carbon dioxide, methane, hydrogen, and hydrogen sulfide. The products of fermentation will vary with the relative composition of the rumen microflora. The microbial population also depends on the diet, since this changes the substrates for fermentation and subsequently the products of fermentation. For example, starch is the major dietary constituent in concentrate-fed ruminants (e.g., feedlot cattle). The rumen of such animals will have higher amylolytic bacteria than cellulolytic bacteria present in the rumen of roughage- and pasture-fed animals. Factors such as the forage:concentrate ratio, the physical form of the diet (ground vs. pelleted), feed additives, and animal species can affect the rumen fermentation process and VFA production.
Molar ratios of VFAs are dependent on the forage:concentrate ratio of the diet. Cellulolytic bacteria tend to produce more acetate, while amylolytic bacteria produce more propionic acid. Typically three major VFA molar ratios are 65:25:10 with a roughage diet and 50:40:10 with a concentrate-rich diet. Changes in VFA concentration can lead to several disorders of carbohydrate digestion in ruminants. Rumen acidosis occurs when animals are fed high-grain-rich diets or when animals are suddenly changed from pasture- or range-fed to feedlot conditions.

 

 

 

Key Points

  1. Very little digestion occurs in the mouth in farm animals.
  2. The small intestine is the site of carbohydrate digestion in monogastrics.
  3. Pancreatic amylase acts on alpha 1,4 links, and other disaccharidases and remove disaccharide units.
  4. The end product (mainly glucose) diffuses into the brush-border using ATP-dependent glucose transporters.
  5. Undigested (fiber, nonstarch polysaccharides [NSP]) in the hindgut can serve as an energy source for hindgut microbes in monogastrics.
  6. Ruminant carbohydrate digestion is very different from monogastrics. First, there is no amylase secreted in the saliva and then most carbs are fermented in the rumen by microbial enzymes.
  7. Carbohydrates are fermented to volatile fatty acids (VFAs) in the rumen. These include acetic acid, propionic acid, and butyric acid.
  8. VFAs are absorbed through the rumen wall into the portal vein and are carried to the liver.
  9. Ratios of the VFAs change with the type of diet. Roughage diets favor microbes that produce more acetic acid, whereas concentrate diets favor microbes that produce more propionic acid.
  10. Carbohydrate fermentation disorders in ruminants include rumen acidosis (grain overload), when cattle are fed high-starch-based cereal or grain-rich diets or when there is a sudden change from pasture to feedlot.

Review Questions

    1. List the enzymes involved in carbohydrate digestion in monogastric animals.
    2. What are the end products of carbohydrate digestion in monogastrics and in ruminants?
    3. What are the major volatile fatty acids (VFAs) produced by rumen fermentation?
    4. What causes rumen acidosis?
    5. Feeding too much barley to broiler chickens can cause sticky feces and digestibility problems. Why?
    6. In ruminants fed concentrate-rich diets, the major VFA produced in the rumen is?
    7. In ruminants fed roughage-rich diets, the major VFA produced in the rumen is?
    8. Fill in the monosaccharides that compose each disaccharide listed below and the enzyme required to break the bond between them.
Sugar Enzyme Monosaccharide 1 Monosaccharide 2
Sucrose
Maltose

 

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