Structure of Major Dietary Carbohydrates
Simple carbohydrates, such as glucose and fructose, can be absorbed in their free form—for instance, they are abundant in fruits and honey. However, most dietary carbohydrates are disaccharides and complex polysaccharides that require prior breakdown.
Key dietary disaccharides differ in their monomer composition and the type of glycosidic bond:
- Maltose: consists of two glucose residues joined by an α-1,4-glycosidic bond.
- Isomaltose: also consists of two glucose molecules, but joined by an α-1,6-glycosidic bond.
- Sucrose: includes glucose and fructose linked by an α-1,2 bond.
- Lactose (milk sugar): composed of galactose and glucose via a β-1,4 bond.
A special place is occupied by cellulose (dietary fiber), a plant polysaccharide whose molecules are connected by β-1,4-glycosidic bonds.
Luminal Digestion in the Stomach and Intestine
The initial stages of digestion are carried out by enzymes functioning within the lumen of the digestive tract.
Digestion in the Stomach Although gastric juice is highly acidic (pH around 2), carbohydrate breakdown continues here thanks to salivary α-amylase. The optimal pH for this enzyme is 6.7. Acid rapidly inactivates amylase; however, inside the food bolus where gastric juice does not penetrate immediately, the enzyme manages to function for some time and break down polysaccharides.
Duodenum When the acidic gastric contents enter the small intestine, they are neutralized by bicarbonates present in pancreatic juice. The environment becomes weakly alkaline (pH 7.5–8), creating ideal conditions for pancreatic α-amylase. This enzyme is localized in the upper small intestine and hydrolyzes starch and dextrins.
The main feature of pancreatic α-amylase is that it can cleave only α-1,4-glycosidic bonds (releasing disaccharide fragments), but has no effect on α-1,6-bonds. As a result of its action, maltose and isomaltose are formed.
Brush Border (Membrane) Digestion
Final hydrolysis of disaccharides occurs directly on the surface of the small intestine cells—at the apical membrane of enterocytes, which forms the so-called brush border.
The enzymes responsible for this process (glycosidases) are synthesized by the enterocytes themselves. They are not secreted into the intestinal lumen, but are firmly anchored to the cell membrane, assembling into large enzymatic complexes:
- Sucrase-isomaltase complex — exhibits broad specificity, hydrolyzing bonds in sucrose, isomaltose, and maltose, yielding glucose and fructose.
- Glucoamylase complex — exhibits exoamylase activity, cleaving monomers from oligosaccharides, and also breaks down bonds in maltose into two glucose molecules.
- β-glycosidase complex (lactase) — specifically cleaves lactose into glucose and galactose.
The monosaccharides resulting from membrane hydrolysis are immediately ready for cellular uptake.
Role of Indigestible Carbohydrates Cellulose passes through the small intestine in transit because the human body completely lacks specific enzymes capable of cleaving β-1,4-glycosidic bonds. Fiber reaches the large intestine, where it is partially fermented by resident bacteria. The main function of undigested cellulose is the mechanical stimulation of the intestinal walls, ensuring normal peristalsis.