Written by: Dr. Jacquie Jacob, University of Kentucky
Starch is the major source of energy in mature cereal grains. Starch is a polysaccharide made up of linked sugar (glucose) molecules (referred to as monosaccharides). The sugar molecules are linked together by α-glycosidic bonds, which are easily broken down in the digestive tract of birds and mammals.
Polysaccharides are identified by the carbon atoms of each sugar involved in the bond and the orientation of the hemiacetal oxygen atom (α or ß). In starch, the glucose molecules are connected by α-(1→4) linkages, with a few α-(1→6) linkages. The α-(1→4) and α-(1→6) linkages in starch, as well as the α-(1→2) linkages between glucose and fructose in sucrose, the ß-(1→4) linkage between glucose and galactose in lactose, and the α-(1→1) linkages between glucose molecules, are easily broken by the endogenous enzymes in birds and mammals. All other glycosidic bonds are resistant to the endogenous digestive enzymes of animals, although they can be digested by microbe-derived enzymes.
Beta-glucans bind with water in the intestines, resulting in the formation of gels that increase the viscosity of the intestinal contents. Viscosity is dependent on several factors, including the size of the molecule, whether the molecule is branched or linear, and the concentration present. At low concentrations, polysaccharides interact directly with water molecules. As the concentration increases, the polysaccharide molecules begin to interact with each other and become tangled. This increases viscosity and is dependent on the formation of linkages between the molecules. As the interaction between the molecules increases, gel formation begins.
There is a negative correlation between intestinal viscosity and nutrient availability. The increase in viscosity of the digesta associated with increased gel formation adversely affects digestion and absorption of nutrients. The increased viscosity reduces the mixing of intestinal contents and/or alters the transport properties of the nutrients at the mucosal surface. The release of bile and pancreatic enzymes occurs at localized points in the duodenum and necessitates the mixing of the intestinal contents for the bile and enzymes to reach the targeted dietary components. A viscous intestinal content requires more time to reach a complete mixing, which is not possible with the flow rate of the digesta. The viscous nature of the digesta can also result in sticky droppings, which increases the moisture content of litter, thereby reducing the air quality in the poultry house.
High intestinal viscosity has also been shown to be associated with digestive and health problems. The decreased digesta passage rate makes colonization with potentially pathogenic bacteria easier. Necrotic enteritis is a breakdown of the intestinal wall. It is typically caused by Clostridium perfringens, an anaerobic bacterium often found in small numbers in the intestinal tract of poultry. At low levels, C. perfringens do not typically cause problems to the host. At higher levels, however, necrotic enteritis can be a serious disease problem. Chickens fed barley-based diets have been shown to have an increased incidence of necrotic enteritis associated with increased levels of C. perfringens. The presence of ß-glucans in a feedstuff reduces its feeding value for use in poultry diets. There are, however, several enzymes (ß-glucanases) that can be added to feed to decrease intestinal viscosity and thereby increase nutrient availability.