The Initial Breakdown: The Oral Cavity
Starch digestion starts with mechanical breakdown in the mouth through chewing (mastication), which increases the food's surface area. Saliva, containing salivary amylase (ptyalin), is also released here. Salivary amylase begins chemically breaking down starch by hydrolyzing alpha-1,4 glycosidic bonds into smaller polysaccharides like maltose and dextrin, which is why starchy foods can taste sweet when chewed for a while.
The Inactive Phase: The Stomach
Swallowed food enters the stomach, where the highly acidic environment (pH 1.5-3.5) inactivates salivary amylase, halting significant starch digestion. The stomach's main roles here are mixing food into chyme and starting protein digestion; the acidic conditions, while vital for protein, pause starch breakdown.
Completing Digestion in the Small Intestine
In the small intestine, the acidic chyme is neutralized by pancreatic bicarbonate. The pancreas also secretes pancreatic amylase, which continues hydrolyzing remaining starch and polysaccharides into maltose.
The final stage involves brush border enzymes on the small intestine lining. Disaccharidases like maltase break down maltose into glucose. Alpha-dextrinase cleaves alpha-1,6 bonds in amylopectin, releasing more glucose. Other brush border enzymes like sucrase and lactase break down other disaccharides.
The Role of Brush Border Enzymes
Brush border enzymes finalize carbohydrate breakdown:
- Maltase: Converts maltose into glucose.
- Sucrase: Breaks down sucrose into glucose and fructose.
- Lactase: Hydrolyzes lactose into glucose and galactose.
- Alpha-dextrinase: Acts on starch branch points.
The Final Stage: Absorption of Glucose
Glucose and other monosaccharides are absorbed into the bloodstream through the small intestine's microvilli. Glucose and galactose use a sodium-glucose co-transporter (SGLT1) for active transport. Fructose is absorbed by facilitated diffusion via GLUT5. All three exit into the blood via the GLUT2 transporter. Absorbed glucose goes to the liver and is used for energy.
The End Goal: Energy Production
This process converts starch into glucose, the body's main energy source. Undigested starches (fiber) reach the large intestine and are fermented by bacteria, benefiting gut health.
| Digestive Phase | Location | Enzymes Involved | Key Action | Environmental pH |
|---|---|---|---|---|
| Initial Digestion | Mouth | Salivary Amylase | Starch to maltose/dextrin | Neutral (approx. 7) |
| Paused Digestion | Stomach | None | Churns food, protein digestion | Acidic (approx. 1.5-3.5) |
| Primary Digestion | Small Intestine | Pancreatic Amylase | Starch to maltose | Slightly Alkaline (approx. 8) |
| Final Digestion | Small Intestine (Brush Border) | Maltase, Alpha-dextrinase, etc. | Disaccharides to monosaccharides (glucose) | Slightly Alkaline |
| Absorption | Small Intestine (Villi) | SGLT1, GLUT2, GLUT5 | Monosaccharides into bloodstream | N/A |
Conclusion
Starch digestion is a multi-step process beginning in the mouth and finishing in the small intestine, involving mechanical action and enzymes like amylase to convert starch into glucose for energy. Chewing and enzyme function are vital for nutrient absorption. The digestive system efficiently processes starch, and undigested fiber supports gut health. More information on digestive enzymes is available.
