Digestion and Absorption

A Comprehensive Guide: Carbohydrates

Carbohydrates are the primary energy source for the human body, contributing to about 50-60% of daily caloric intake. Understanding their digestion and absorption is fundamental for medical students, as it provides insights into both normal physiology and clinical conditions like malabsorption syndromes and diabetes. This post explains carbohydrate digestion and absorption in a stepwise manner with tables, diagrams, and clinical significance.

Step 1: Overview of Carbohydrate Types

Carbohydrates are classified based on their complexity:

Type Examples Sources

Monosaccharides Glucose, Fructose, Galactose Honey, fruits, vegetables

Disaccharides Sucrose, Lactose, Maltose Table sugar, milk, malted grains

Polysaccharides Starch, Glycogen, Cellulose Rice, wheat, potatoes, plant cell walls

Step 2: Digestion of Carbohydrates

Carbohydrate digestion begins in the mouth and is completed in the small intestine. It involves the enzymatic breakdown of polysaccharides into monosaccharides.

A. Mouth

Enzyme: Salivary amylase (ptyalin).

Action: The breakdown of starch into maltose and dextrins. Digestion in the mouth by the salivary alpha-amylase.

Environment: Slightly alkaline (pH ~6.5-7).

Clinical Note: Limited activity as food transit time in the mouth is short.

B. Stomach

No Enzyme Activity: The acidic environment (pH ~1.5-3.5) inactivates salivary amylase.

Mechanical Action: Churning breaks food into smaller particles.

C. Small Intestine

1. Duodenum:

Pancreatic Amylase: Secreted by the pancreas, it acts on polysaccharides to produce maltose, maltotriose, and alpha limit dextrins. In pancreatic juice, alpha-amylase will hydrolyze alpha-1,4 glycosidic linkages randomly, to produce smaller subunits like maltose, isomaltose, dextrins, and branched or unbranched oligosaccharides.

2. Brush Border Enzymes (in the small intestine lining):
These enzymes finalize digestion.

Enzyme Substrate Product

Maltase Maltose 2 Glucose molecules

Sucrase Sucrose Glucose + Fructose

Lactase Lactose Glucose + Galactose

Alpha-Dextrinase Alpha-limit dextrins Glucose

Note: The intestinal juice (succus entericus) and brush border of intestinal cells contain sucrase, maltase, isomaltase and lactase, The monosaccharides are then absorbed.

Step 3: Absorption of Carbohydrates

Carbohydrate absorption occurs primarily in the small intestine (jejunum). Only monosaccharides—glucose, fructose, and galactose—can be absorbed.

A. Mechanisms of Absorption

Monosaccharide Transporter Mechanism

Glucose SGLT-1 (Sodium-Glucose Linked Transporter). Active transport (sodium-dependent).

Galactose SGLT-1 Active transport

Fructose GLUT-5 (Facilitated Diffusion). Passive transport

Once inside the enterocyte, monosaccharides are transported to the bloodstream via GLUT-2 transporters. Only monosaccharides are absorbed by the intestine. Minute quantities of disaccharides that may be absorbed, are immediately eliminated through kidneys.

Absorption rate: Galactose > glucose > fructose

B. Role of Sodium-Potassium Pump

The sodium-potassium pump (Na⁺/K⁺ ATPase) maintains the sodium gradient required for SGLT-1 to function.

Step 4: Transport to the Liver

Monosaccharides absorbed into the bloodstream are transported to the liver via the hepatic portal vein.

Glucose and Galactose: Utilized for energy or stored as glycogen.

Fructose: Metabolized to glucose or other intermediates.

Step 5: Clinical Significance

A. Lactose Intolerance

Cause: Deficiency of lactase enzyme.

Symptoms: Bloating, diarrhea, abdominal cramps after consuming dairy.

Management: Lactose-free diet or lactase supplements.

B. Malabsorption Syndromes

Cause: Conditions like celiac disease or chronic pancreatitis impair carbohydrate digestion/absorption.

Symptoms: Steatorrhea, weight loss, fatigue.

C. Diabetes Mellitus

Relation: Impaired glucose metabolism due to insulin deficiency/resistance.

Relevance: Understanding absorption helps in managing glycemic control.

D. Sucrase-Isomaltase Deficiency

Cause: Genetic defect in sucrase-isomaltase enzyme.

Symptoms: Diarrhea and abdominal pain after sucrose consumption.

References for Further Understanding

1. Guyton and Hall: Textbook of Medical Physiology.

2. Ganong: Review of Medical Physiology.

3. Harper’s Illustrated Biochemistry.

4. Goodman and Gilman’s: The Pharmacological Basis of Therapeutics (for clinical relevance).