• The principal energy sources in human food which are simple sugars (monosaccharides) or chains of repeated sugar units (disaccharides, trisaccharides and polysaccharides). The polysaccharides have many repeated sugar units and are starches or various types of cellulose. They are all with the exception of cellulose and a few starches broken down in the gut into absorbable monosaccharides either by acids and enzymes in the gastric juices or by the action of microorganisms.