Carbohydrate Digestion and Metabolism: Key Concepts for Nutrition Students

Carbohydrate Digestion

Most Carbohydrate Digestion Occurs in Our Small Intestine

Carbohydrate digestion is a multi-step process that begins in the mouth and is completed in the small intestine. The goal is to break down complex carbohydrates into monosaccharides for absorption.

• Digestion begins in the mouth with the enzyme salivary amylase, which starts breaking down starch into smaller polysaccharides and maltose.

• Stomach: Carbohydrate digestion temporarily stops due to acidic conditions that inactivate amylase.

• Small intestine: Pancreatic amylase resumes starch breakdown. Enzymes in the intestinal wall (maltase, sucrase, lactase) further digest disaccharides into monosaccharides.

• Large intestine: Any undigested carbohydrates are fermented by bacteria, producing gases and short-chain fatty acids.

Step	Location	Target	Enzyme(s)	Product(s)
1	Mouth	Starch	Salivary amylase	Smaller polysaccharides, maltose
2	Small Intestine	Starch	Pancreatic amylase	Maltose
3	Small Intestine (brush border)	Maltose, sucrose, lactose	Maltase, sucrase, lactase	Glucose, fructose, galactose
4	Large Intestine	Undigested carbohydrates	Bacterial enzymes	Gases, short-chain fatty acids

Example: Starch from bread is first broken down by salivary amylase, then further digested by pancreatic amylase in the small intestine, and finally converted to glucose by maltase.

Carbohydrate Metabolism: Managing Blood Glucose

How the Body Manages & Stores Glucose

After absorption, monosaccharides (glucose, fructose, galactose) are transported to the liver. The liver converts non-glucose monosaccharides into glucose. Glucose can be used immediately for energy or stored as glycogen.

• Glycogenesis: Formation of glycogen from excess glucose when blood glucose is high.

• Glycogenolysis: Breakdown of glycogen to release glucose when blood glucose is low.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (e.g., amino acids) when dietary intake is insufficient.

Key regulatory hormones: Insulin (lowers blood glucose), Glucagon (raises blood glucose).

Example: After a meal, insulin stimulates glycogenesis in the liver and muscle. During fasting, glucagon stimulates glycogenolysis and gluconeogenesis.

Lactose Intolerance

Causes and Management

Lactose intolerance is a digestive condition resulting from reduced activity of the enzyme lactase, which is needed to digest lactose (the sugar in milk).

• Symptoms: Bloating, gas, diarrhea, and discomfort after consuming dairy products.

• Management: Avoiding lactose-containing foods, choosing lactose-free alternatives, or using lactase enzyme supplements.

Example: Individuals with lactose intolerance may drink lactose-free milk or take lactase tablets before consuming dairy.

Key Terms and Definitions

• Monosaccharides: The simplest form of carbohydrates (e.g., glucose, fructose, galactose).

• Disaccharides: Carbohydrates composed of two monosaccharides (e.g., sucrose, lactose, maltose).

• Polysaccharides: Complex carbohydrates made of many monosaccharide units (e.g., starch, glycogen).

• Enzyme: Biological catalyst that speeds up chemical reactions, such as digestion.

Relevant Equations

• Starch breakdown by amylase: $\text{Starch} \xrightarrow{\text{amylase}} \text{Maltose}$

• Maltose breakdown by maltase: $\text{Maltose} \xrightarrow{\text{maltase}} 2\ \text{Glucose}$

• Lactose breakdown by lactase: $\text{Lactose} \xrightarrow{\text{lactase}} \text{Glucose} + \text{Galactose}$

Summary Table: Carbohydrate Digestion Enzymes

Enzyme	Source	Substrate	Product
Salivary amylase	Mouth	Starch	Maltose, small polysaccharides
Pancreatic amylase	Pancreas (small intestine)	Starch	Maltose
Maltase	Small intestine	Maltose	Glucose
Sucrase	Small intestine	Sucrose	Glucose, fructose
Lactase	Small intestine	Lactose	Glucose, galactose

Additional info:

• Carbohydrate digestion is essential for providing energy to cells, especially the brain and muscles.

• Lactose intolerance prevalence varies by population and age.