The Basics of Digestion

Turnover Rate of GI Lining Cells

The cells lining the gastrointestinal (GI) tract are replaced rapidly to maintain digestive health and barrier function.

• Turnover Rate: GI epithelial cells are replaced every 3–5 days.

• Significance: Rapid turnover helps repair damage from digestive processes and exposure to foodborne pathogens.

Definition of Digestion

Digestion is the process by which food is broken down into absorbable components through mechanical and chemical means.

• Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning).

• Chemical Digestion: Enzymatic breakdown of macronutrients into smaller molecules.

Accessory vs Primary Organs

• Primary Organs: Directly form the GI tract (mouth, esophagus, stomach, small intestine, large intestine).

• Accessory Organs: Aid digestion but are not part of the GI tract (salivary glands, liver, gallbladder, pancreas).

Functions of Digestive Organs

• Mouth: Chewing (mechanical), saliva (chemical, contains amylase for starch breakdown).

• Esophagus: Transports food via peristalsis.

• Stomach: Churns food, secretes acid and pepsin for protein digestion.

• Gallbladder: Stores and releases bile for fat emulsification.

• Liver: Produces bile, processes absorbed nutrients.

• Pancreas: Secretes digestive enzymes and bicarbonate into small intestine.

• Small Intestine: Main site for digestion and absorption; enzymes break down all macronutrients.

• Large Intestine: Absorbs water, forms feces, houses gut bacteria.

Order of Digestive Organs

1. Mouth

2. Esophagus

3. Stomach

4. Small Intestine

5. Large Intestine

Five Sphincters: Locations and Functions

Mechanical vs Chemical Digestion

• Mechanical: Chewing, mixing, and churning food.

• Chemical: Enzymatic breakdown (e.g., amylase, pepsin, lipase).

Peristalsis

Peristalsis is the coordinated, rhythmic contractions of smooth muscle that propel food through the GI tract.

Factors Affecting Stomach Emptying

• Liquids empty faster than solids.

• Fat-rich meals slow gastric emptying.

• Carbohydrates empty faster than proteins or fats.

Majority of Digestion and Absorption

• Occurs in the small intestine.

Mechanisms Increasing Small Intestine Surface Area

• Folds of Kerckring (circular folds)

• Villi

• Microvilli (brush border)

• Benefit: Maximizes nutrient absorption by increasing surface area.

Absorption Location and Transport Mechanism of Macronutrients

Organ Receiving Nutrient-Rich Blood After Absorption

• Liver receives blood via the hepatic portal vein.

Chemical Digestion in Large Intestine

• Primarily bacterial fermentation of undigested carbohydrates and fibers.

Products of Large Intestine Digestion

• Short-chain fatty acids

• Gases (e.g., methane, hydrogen, carbon dioxide)

Diarrhea vs Constipation: Causes and Management

• Diarrhea: Causes include infection, food intolerance, or rapid transit; managed by hydration and addressing cause.

• Constipation: Causes include low fiber, dehydration, inactivity; managed by increasing fiber, fluids, and activity.

Heartburn/GERD: Causes and Management

• Causes: Weak lower esophageal sphincter, overeating, obesity, certain foods.

• Management: Eat smaller meals, avoid trigger foods, elevate head during sleep, weight management.

Carbohydrates: Sugars, Starches, and Fibers

Carbohydrate Creation in Plants

• Via photosynthesis: plants convert CO2 and H2O into glucose using sunlight.

Monosaccharides

• Glucose

• Fructose

• Galactose

Disaccharides

• Sucrose (glucose + fructose)

• Lactose (glucose + galactose)

• Maltose (glucose + glucose)

Polysaccharides

• Long chains of glucose units.

• Common types: Starch (plants), Glycogen (animals), Fiber (plants, indigestible).

Most Commonly Used Monosaccharide in the Body

• Glucose is the primary energy source for cells.

Organs/Systems Dependent on Glucose

• Brain

• Red blood cells

• Nervous system

DRI/AMDR for Carbohydrate

• DRI (Dietary Reference Intake): 130 g/day minimum for adults.

• AMDR (Acceptable Macronutrient Distribution Range): 45–65% of total calories from carbohydrates.

Nutrient Dense Food Sources of Carbohydrate

• Whole grains, fruits, vegetables, legumes, dairy.

Whole Wheat vs Enriched Flour Nutrient Content

• Whole wheat: Contains bran, germ, and endosperm; higher in fiber, vitamins, minerals.

• Enriched flour: Only endosperm; some nutrients added back, but less fiber and micronutrients.

Carbohydrate Digestion Steps

1. Mouth: Salivary amylase begins starch breakdown.

2. Stomach: Acid inactivates amylase; little digestion.

3. Small intestine: Pancreatic amylase and brush border enzymes complete digestion to monosaccharides.

4. Absorption: Monosaccharides absorbed into blood.

Soluble vs Insoluble Fiber

Type I Diabetes: Causes and Treatment

• Cause: Autoimmune destruction of pancreatic beta cells (no insulin production).

• Treatment: Insulin therapy, carbohydrate counting.

Type II Diabetes: Cause and Treatment

• Cause: Insulin resistance, often associated with obesity and genetics.

• Treatment: Diet, exercise, oral medications, sometimes insulin.

Term for Low Blood Sugar

• Hypoglycemia

Hormones Controlling Blood Sugar

• Insulin: Released when blood glucose is high; lowers blood sugar.

• Glucagon: Released when blood glucose is low; raises blood sugar.

• Other hormones (e.g., epinephrine): Used during stress or fasting.

Glycogenesis, Glycogenolysis, Gluconeogenesis

• Glycogenesis: Formation of glycogen from glucose (after meals).

• Glycogenolysis: Breakdown of glycogen to glucose (fasting, exercise).

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (prolonged fasting).

Diverticula/Diverticulitis: Causes and Prevention

• Diverticula: Small pouches in colon wall; caused by low fiber diet.

• Diverticulitis: Inflammation/infection of diverticula.

• Prevention: High fiber diet, adequate fluids.

Top Sources of Added Sugars in American Diet

• Soft drinks, desserts, candy, sweetened dairy products.

• Additional info: Refer to bar graph in slides for specific percentages.

Lactose Intolerance: Cause and Management

• Cause: Low lactase enzyme activity in small intestine.

• Management: Lactose-free products, lactase supplements, small amounts of dairy with meals.

DRIs for Fiber

• 14 g/1000 kcal

• 25 g/day for women

• 38 g/day for men

Dietary Guidelines for Added Sugars

• Less than 10% of total daily calories from added sugars.

Fats, Oils, and Other Lipids

Three Classes of Lipids: Name, Structure, and Function

Purpose/Benefits of Fats in Foods

• Flavor, texture, satiety, absorption of fat-soluble vitamins (A, D, E, K).

Purpose/Benefits of Fats in the Body

• Energy storage, insulation, protection of organs, cell membrane structure, precursor for hormones.

Fat Digestion and Absorption

• Bile: Made in liver, stored/released from gallbladder; emulsifies fats in small intestine.

• Pancreatic lipase digests fats to fatty acids and monoglycerides.

• Absorbed into intestinal cells, reassembled into triglycerides, packaged into chylomicrons, and transported via lymph.

Controllable vs Non-Controllable Risk Factors for Heart Disease

Saturated vs Unsaturated Fats: Structure, Health Effects, and Food Sources

Benefits of Omega-3 Fatty Acids and Food Sources

• Reduce inflammation, lower heart disease risk, support brain health.

• Sources: Fatty fish (salmon, sardines), flaxseed, walnuts.

Trans Fats

• Formed by hydrogenation of unsaturated fats; raise LDL and lower HDL cholesterol.

• Found in processed foods, baked goods, margarine.

Dietary Fat Recommended Intake

• Total fat: 20–35% of total calories.

• Saturated fat: Less than 10% of calories.

• Trans fat: As low as possible.

Chylomicron, VLDL, LDL, HDL: Definition and Function

Improving High Cholesterol Through Diet and Lifestyle

• Increase soluble fiber, physical activity, omega-3 intake.

• Reduce saturated/trans fats, maintain healthy weight.

Seed Oils: Health Effects

• Seed oils (e.g., soybean, canola) are high in unsaturated fats; evidence suggests they are not harmful when consumed in moderation.

Mediterranean Diet Principles and Health Effects

• Emphasizes fruits, vegetables, whole grains, olive oil, fish, moderate wine.

• Associated with lower risk of heart disease, improved metabolic health.