Polysaccharides are complex carbohydrates, built from long chains of simple sugar molecules (monosaccharides), that serve as primary energy sources in the human diet. These large molecules must be chemically broken down before the body can absorb them for fuel. The speed at which polysaccharides are digested varies greatly, depending entirely on the specific chemical structure of the chain.
How Human Enzymes Break Down Polysaccharides
The chemical breakdown of digestible polysaccharides begins with the digestive enzyme alpha-amylase, initially secreted in the saliva. This enzyme starts the process of hydrolysis—the chemical splitting of the carbohydrate chain by adding water—by cleaving the alpha-1,4-glycosidic bonds that link the glucose units in the mouth.
The enzyme’s action is temporarily halted by the highly acidic environment of the stomach. The majority of polysaccharide digestion resumes when the food enters the small intestine, where the pancreas secretes a much more potent form of the enzyme, pancreatic amylase. This powerful enzyme rapidly breaks the long chains of starch into smaller fragments, primarily the disaccharide maltose and short glucose chains called dextrins. Subsequent enzymes on the intestinal lining complete the process by splitting these smaller units into individual glucose molecules, which are then absorbed into the bloodstream.
Starch Structure Determines Digestion Speed
The speed at which a polysaccharide is broken down is directly determined by its three-dimensional structure, specifically its degree of branching. The fastest-digested polysaccharides have a highly branched architecture, such as amylopectin (75 to 80% of typical plant starch) and glycogen (the animal storage form of glucose). Amylase attacks these molecules at multiple points simultaneously because their numerous branches provide many accessible ends for the enzyme.
This high degree of branching maximizes the surface area available for the enzyme to work. Glycogen, being even more densely branched than amylopectin, offers the quickest release of glucose. This rapid breakdown means the resulting glucose floods the bloodstream quickly, leading to a sharp spike in blood sugar levels.
In contrast, the straight-chain component of starch, known as amylose, is digested much more slowly. Amylose molecules have a linear structure with few branches, causing them to pack tightly together. This compact arrangement limits the number of points where amylase can attach and begin cleavage. Consequently, starches high in amylose are classified as slowly digestible or resistant starches, releasing glucose into the bloodstream at a gradual rate.
The Polysaccharide We Cannot Digest
While starch and glycogen are readily broken down, one major polysaccharide, cellulose, is completely indigestible by human enzymes. Cellulose, the main structural component of plant cell walls, is a long chain of glucose units, just like starch. However, its glucose units are linked by a different chemical configuration known as beta-1,4-glycosidic bonds.
The human digestive system, including all forms of amylase, is only equipped to break the alpha-glycosidic bonds found in starch and glycogen. The beta bond requires a specialized enzyme called cellulase, which humans do not naturally produce. This inability to hydrolyze the beta bonds means that cellulose passes through the entire digestive tract intact.
Undigested cellulose is commonly referred to as dietary fiber and plays a beneficial role despite not providing calories. As it moves through the intestines, it adds bulk to the stool and helps regulate the transit time of food waste. Therefore, cellulose functions not as an energy source but as a structural aid for maintaining digestive health.