Major Dietary Carbohydrates
Various carbohydrates enter the body with food and are classified by their structure:
- Polysaccharides. The primary representative is starch (the main plant storage polysaccharide). It consists of numerous glucose residues linked by $\alpha$-1,4 and $\alpha$-1,6-glycosidic bonds. This group also includes animal glycogen and plant cellulose (dietary fiber). Notably, cellulose is not hydrolyzed by endogenous human enzymes.
- Disaccharides. To be absorbed, they must be broken down into monomers. The most important disaccharides are:
- Lactose (milk sugar) — composed of galactose and glucose.
- Sucrose (found in beets and cane sugar) — composed of glucose and fructose.
- Maltose (formed during starch hydrolysis) — composed of two glucose residues.
- Monosaccharides. The forms ready for absorption: glucose, fructose, and galactose.
Stages of Digestion in the Gastrointestinal Tract
The hydrolysis of poly- and disaccharides occurs sequentially in different parts of the digestive tract.
Oral Cavity The process begins under the action of salivary $\alpha$-amylase. This enzyme cleaves $\alpha$-1,4-glycosidic bonds in starch. Due to the short duration of food in the mouth, hydrolysis is only partial, with dextrins serving as the main products at this stage.
Stomach Carbohydrate digestion halts in the stomach. Gastric juice contains no specific enzymes for sugar hydrolysis, and the acidic environment inactivates salivary amylase.
Small Intestine This is the primary site of digestion. Pancreatic $\alpha$-amylase acts in the intestinal lumen, breaking down dextrins into maltose and isomaltose. Final hydrolysis is carried out by enzyme complexes on the brush border of enterocytes (glycoamylase, sucrase-isomaltase, and $\beta$-glycosidase complexes).
Absorption of Monosaccharides
The absorption of the resulting monosaccharides—glucose, galactose, and fructose—occurs through the intestinal mucosal cells (enterocytes).
The entry of substances from the intestinal lumen into the enterocyte occurs via two pathways:
- Facilitated diffusion — transport involving specific carrier proteins (characteristic of all three monosaccharides).
- Secondary active transport (sodium-dependent) — specific for glucose and galactose. It operates against a concentration gradient and requires energy provided by the $Na^+$ gradient.
An important role is played by $Na^+,K^+$-ATPase, located on the basolateral membrane of the enterocyte. It pumps $Na^+$ out of the cell, maintaining the gradient required for glucose transport. All monosaccharides exit the enterocyte into the blood via facilitated diffusion, after which they travel through the portal blood flow to the liver and subsequently to peripheral tissues.
Pathology of Digestion
Disorders of carbohydrate digestion and absorption are most commonly associated with hereditary or acquired defects of intestinal enzymes, as well as impairments in monosaccharide membrane transport systems.
As a result, carbohydrates are neither digested nor absorbed. The accumulation of osmotically active sugars in the intestinal lumen causes osmotic diarrhea. Furthermore, undigested carbohydrates are fermented by the gut microbiota. The gases released during this process lead to flatulence and abdominal pain.