Dietary macromolecules must be hydrolyzed to produce components that can be absorbed. This is achieved by mechanical and chemical events that go on together in each compartment of the digestive tract. For example, digestion begins in the mouth with chewing of food (mastication) that breaks it down into particles that can be hydrolyzed by digestive enzymes. At the same time, dietary starches begin to be hydrolyzed by salivary amylase.
Proteases and peptidases hydrolyze proteins into absorbable peptides and amino acids. Lipases hydrolyze triglycerides into fatty acids and monoglycerides. Amylases hydrolyze starches and glycogen into disaccharides that are split into simple sugars by specific disaccharidases. Nucleases hydrolyze nucleic acids into nucleotides. All of these enzymes are secreted into the duodenum to begin the intestinal stage of digestion.
Intestinal glands in the wall of the small intestine also produce digestive enzymes. Brush border enzymes break down disaccharides and small peptides. Maltase digest the disaccharide maltose to glucose; sucrase digests sucrose to glucose and fructose; lactase digests lactose to glucose and galactose. The presence of lactase in adults is genetically determined. Its absence results in lactose intolerance, the inability to digest foods (such as milk products) that contain lactose. Peptidases digest small peptides to amino acids.
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