Organic compounds comprising Carbon. Hydrogen and Oxygen joined by chemical bonds. Carbohydrates are formed by green plants from carbon dioxide and water in the process of photosynthesis. Essential as energy stores, they also form part of the DNA molecule. The basic structural unit is that of a simple sugar or monosaccharide like glucose. These may be joined in pairs to form disaccharides such as sucrose, or chains of three to six called oligosaccharides. In polysaccharides the chains may contain up to thousands of simple sugar units. Examples include starch, cellulose and lignin in plants and glycogen, the principal carbohydrate storage compound in animals. Once ingested by the horse, to release the energy from these complex molecules they must first be broken down by enzyme digestion to yield simple sugars, primarily glucose, or by bacteria to yield volatile fatty acids.
Simple sugars have a molecular size small enough to be absorbed intact through the gut wall into the blood. More complex carbohydrates like starch require digestion by enzymes like Amylase in the small intestine to split the large molecule into simple sugars for absorbtion. Larger polysaccharides like cellulose resist enzyme attack and can only be digested by bacterial fermentation in the hind gut. Those yield mainly volatile fatty acids, not glucose as the energy bearing product.