02.10.2025 and 02.17.2025_Carbohydrates

Carbohydrates Overview

• Instructor: Margaret Slavin, PhD, RDN, Associate Professor, Nutrition and Food Science, University of Maryland, College Park

• Course: NFSC 100: Elements of Nutrition

Agenda

• Structure

• Digestion & Absorption

  • Lactose intolerance

• Transport, Storage & Metabolism

• Health Effects

  • Diabetes

• Recommended Intakes

Carbohydrate Structure

Monosaccharides

• Definition: Simple sugar units.

• Examples:

  • Glucose

  • Fructose

  • Galactose

Disaccharides

• Definition: Formed by chemical bonding of 2 monosaccharides.

• Examples:

  • Sucrose: Glucose + Fructose (table sugar)

  • Lactose: Glucose + Galactose (milk products)

  • Maltose: Glucose + Glucose (from fermentation & alcohol production)

Simple vs. Complex Carbs

• Simple Carbohydrates:

  • Monosaccharides: single units

  • Disaccharides: two units

• Complex Carbohydrates:

  • Polysaccharides: many units

    • Starch: plant carbohydrate storage

    • Glycogen: animal carbohydrate storage

    • Fiber: undigestible carbohydrate

Forms of Polysaccharides

• Starch: Linked glucose units, can be digested by human enzymes.

• Glycogen: Similar to starch but more highly branched, easily broken down by enzymes.

• Cellulose: Bonds in cellulose are not digestible by human enzymes.

Fiber

• Definition: Indigestible starch.

• Types:

  • Insoluble Fiber: Cellulose, not fermented

  • Soluble Fiber: Gum, pectin, mucilage; some types can be fermented.

Digestion & Absorption of Carbohydrates

• Purpose: Break down starches and disaccharides into monosaccharides for absorption.

• By large intestine, most starches are digested; only fiber remains.

Digestion Process Flow

1. Mouth:

   • Salivary amylase begins starch digestion to maltose.

2. Stomach:

   • Salivary amylase is inactivated; no starch digestion occurs.

3. Pancreas:

   • Secretes amylase to break starch into maltose in the small intestine.

4. Small Intestine:

   • Disaccharides (sucrose, lactose, maltose) are broken down into monosaccharides.

5. Liver:

   • Absorbed monosaccharides travel via portal vein.

6. Large Intestine:

   • Soluble fiber is fermented; insoluble fiber is excreted.

Carbohydrate-Digesting Enzymes

• Amylase: Breaks down starch.

  • Sources: Salivary glands and pancreas.

• Maltase: Digests maltose to glucose.

• Sucrase: Digests sucrose to glucose and fructose.

• Lactase: Digests lactose to glucose and galactose.

Lactose Intolerance

• Overview: Results from lactase deficiency.

• Symptoms: Bloating, gas, discomfort, diarrhea from undigested lactose in the colon.

• Usually not necessary to totally eliminate milk; gradual reintroduction recommended.

Carbohydrate Absorption

• Location: Small intestine.

• Galactose: Active absorption.

• Fructose: Facilitated diffusion.

• Glucose: Both facilitated diffusion and active transport.

Transport, Metabolism & Storage of Carbohydrates

Pathway of Monosaccharides in Body

1. Absorbed into capillaries from small intestine.

2. First destination: liver.

3. Liver processes galactose and fructose into glucose.

4. Blood transports glucose to body cells for energy.

Using Glucose for Energy

• Main role: Supply energy for cells.

• Red blood cells and brain primarily use glucose.

Regulation of Blood Glucose

• After meals: Blood glucose rises; pancreas releases insulin.

• Between meals: Blood glucose drops; pancreas releases glucagon.

Diabetes Overview

• Type 1: Pancreas fails to produce insulin.

• Type 2: Cells resist insulin; can be linked to obesity.

Health Effects of Carbohydrates

Sugars

• Provides energy, enhances flavor, but excess leads to weight gain and health risks like tooth decay and chronic diseases.

Starch

• Generally provides energy; but can also be low in nutrients.

Fiber

• Not broken down, contributes to digestive health.

• Excessive intake can cause gastrointestinal discomfort.

Recommended Intakes

• Total Carbs: 45-65% of total kcal

  • RDA minimum for adults: 130 g

• Fiber: AI is 25 g/day for women, 38 g/day for men.

• Added Sugars: Limit to 10% of total kcal; average U.S. intake is high.

Dietary Sources & Recommendations

• Aim for whole grains, fruits, vegetables, and limit refined grains and added sugars.