Carbohydrate Types and Utilization
Carbohydrates are composed of simple sugars, or monosaccharides, such as glucose (dextrose), fructose, galactose, and xylose. Many glucose molecules attached together by alpha bonds form the polysaccharides starch, present in plants, and glycogen, present in animals. These are called nonstructural, soluble, or nonfiber carbohydrates; they are also called nitrogen-free-extract (NFE) because of the way in which their amount in a feed is determined. Nonfiber carbohydrates are readily utilized and provide much of the horse’s dietary energy. However, other forms of carbohydrates in feeds also provide a substantial amount of dietary energy for the horse.
If glucose molecules are attached together by beta, instead of alpha, bonds, they form the structural polysaccharide, or insoluble fiber, cellulose. Cellulose is a hollow fibril that is analogous to the reinforcing rod in the concrete of the plant’s cell wall, the concrete being the structural polysaccharide or insoluble fiber hemicellulose, which is many molecules of the monosaccharide xylose attached together by beta bonds. Since monosaccharides are the only form of carbohydrate that is absorbed from the intestinal tract, the alpha and beta bonds between monosaccharides must be broken for carbohydrates to be utilized directly by animals.
The alpha bonds in starch, glycogen, and other soluble carbohydrates are broken by the digestive enzyme amylase. Amylase is secreted by all animals primarily from the pancreas into the small intestine. Amylase digestion breaks starch and glycogen down to the disaccharide maltose. Maltose, like the disaccharides sucrose (table sugar) and lactose (milk sugar), are split into their two monosaccharide units by the disaccharidase enzymes maltase, sucrase, and lactase, which are a part of the intestinal brush border. If this brush border is damaged or missing, such as with enteritis, carbohydrate utilization is impaired and a high amount of it remains in the intestinal tract, which may cause diarrhea. Animals past nursing age lose lactase and, therefore, the ability to digest and absorb the milk sugar lactose. Horses over 3 years old have little lactase. As a consequence, sudden introduction of lactose-containing milk products into a mature horse’s diet may induce diarrhea.
Absorbed monosaccharides are used for energy. If dietary energy isn’t needed at that time, they are stored as glycogen. If glycogen storage depots in the liver, kidney, and muscle are full, the monosaccharides are converted to and stored as fat. Glycogen and fat can be utilized for energy when needed.
This article is from "Feeding and Care of the Horse", second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.
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