Chapter 4: Carbohydrates – Structure, Digestion, Functions, and Health

Carbohydrates

Introduction: Carbohydrates

Carbohydrates are one of the three primary macronutrients essential for human health. They serve as the main source of energy for the body and are found in a wide variety of foods, including grains, fruits, vegetables, and dairy products. Understanding carbohydrates is crucial for nutrition students, as their structure, digestion, and functions impact overall health and disease prevention.

4.1: What Are Carbohydrates and How Are They Classified?

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically in the ratio of 1:2:1. They are classified based on their chemical structure and complexity:

• Monosaccharides: The simplest form of carbohydrates, consisting of single sugar units. Examples include glucose, fructose, and galactose.

• Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond. Examples include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose).

• Polysaccharides: Complex carbohydrates made of many monosaccharide units. Examples include starch (plant storage), glycogen (animal storage), and cellulose (plant fiber).

Classification Table:

Type	Structure	Examples
Monosaccharide	Single sugar unit	Glucose, Fructose, Galactose
Disaccharide	Two sugar units	Sucrose, Lactose, Maltose
Polysaccharide	Many sugar units	Starch, Glycogen, Cellulose

4.2: How Do We Digest and Absorb Carbohydrates?

Carbohydrate digestion begins in the mouth and continues in the small intestine. Enzymes break down complex carbohydrates into simple sugars for absorption:

• Salivary amylase: Initiates starch breakdown in the mouth.

• Pancreatic amylase: Continues starch digestion in the small intestine.

• Disaccharidases: Enzymes (such as sucrase, lactase, maltase) located in the intestinal lining break down disaccharides into monosaccharides.

• Absorption: Monosaccharides are absorbed into the bloodstream via the intestinal wall and transported to the liver.

Example: Lactose intolerance occurs when the enzyme lactase is deficient, leading to poor digestion of lactose.

4.3: What Functions Do Carbohydrates Perform in the Body?

Carbohydrates play several vital roles in human physiology:

• Primary energy source: Glucose is the preferred fuel for the brain, muscles, and other tissues.

• Protein sparing: Adequate carbohydrate intake prevents the breakdown of proteins for energy.

• Fat metabolism: Carbohydrates are necessary for the complete oxidation of fats.

• Structural role: Some carbohydrates, like cellulose, provide structural support in plants and dietary fiber in humans.

Example: During intense exercise, muscle glycogen is rapidly broken down to supply energy.

4.4: How Do We Maintain Blood Glucose Levels?

Blood glucose levels are tightly regulated by hormones and metabolic processes:

• Insulin: Secreted by the pancreas, insulin lowers blood glucose by promoting uptake into cells and storage as glycogen.

• Glucagon: Also from the pancreas, glucagon raises blood glucose by stimulating glycogen breakdown and glucose release from the liver.

• Homeostasis: The body maintains blood glucose within a narrow range (typically 70–110 mg/dL).

Equation:

$\text{Glucose} + \text{Insulin} \rightarrow \text{Glycogen (storage)}$

$\text{Glycogen} + \text{Glucagon} \rightarrow \text{Glucose (release)}$

Example: Diabetes mellitus results from impaired insulin production or action, leading to elevated blood glucose.

4.5: Why Is Dietary Fiber So Important for Promoting Health?

Dietary fiber is a type of carbohydrate that is not digested by human enzymes. It is crucial for digestive health and disease prevention:

• Promotes bowel regularity: Fiber adds bulk to stool and prevents constipation.

• Reduces risk of chronic diseases: High fiber intake is associated with lower risk of heart disease, diabetes, and certain cancers.

• Supports healthy gut microbiota: Fiber serves as a substrate for beneficial bacteria in the colon.

• Types of fiber: Soluble fiber (e.g., oats, beans) dissolves in water and can help lower cholesterol; insoluble fiber (e.g., wheat bran, vegetables) adds bulk to stool.

Example: Eating whole grains, fruits, and vegetables increases dietary fiber intake and supports overall health.