Digestive System Functions

• 5 challenges the human body faces during digestion and nutrition include:

  • Breathing vs eating
  • Passageway for food travel
  • Moving food from one point to another
  • Breaking down food
  • Eliminating waste

• Course of digestion through anatomical structures:

  • Mouth (mechanical chewing & saliva)
  • Pharynx (epiglottis prevents choking)
  • Esophagus (two sphincters: upper opens during swallowing, lower prevents stomach content backflow)
  • Stomach (gastric glands release acid and enzymes, pyloric sphincter controls food passage to small intestine)
  • Small Intestine (duodenum, jejunum, ileum); 10 feet long, major site of nutrient digestion and absorption
  • Large Intestine (colon: cecum, ascending, transverse, descending, sigmoid) – water removal
  • Rectum & Anus

• Chyme is a semi-liquid mixture of partially digested food, stomach acid, and digestive enzymes.

  • It forms in the stomach and is slowly released into the small intestine.
  • It allows for further digestion and nutrient absorption.

• Two types of muscle contractions that aid digestion and transport of food:

  • Segmentation: inner circular muscle contraction to churn chyme
  • Peristalsis: outer longitudinal muscles contract rhythmically to move chyme forward

• The pyloric sphincter senses acidic chyme and immediately shuts to prevent further release from the stomach.

• Five structures/organs/glands that release substances aiding in digestion:

  • Salivary Glands: release salivary amylase for carbohydrate digestion in the mouth
  • Stomach: releases HCl and enzymes (pepsin) to break down proteins and kill bacteria
  • Pancreas: releases pancreatic amylase, lipase, and proteases into the small intestine for digesting carbs, fats, and proteins
  • Liver: produces bile to help digest and absorb fats
  • Small Intestine (Brush Border Enzymes): releases maltase, sucrase, and lactase to break down sugars for absorption

• An enzyme is a protein that speeds up chemical reactions, like food breakdown during digestion.

• The stomach maintains a pH of 1.5 - 1.7 with a mucus coating to protect the stomach wall

• The stomach's gastric enzymes function well in an acidic environment, while salivary enzymes stop functioning.

• A low pH (hydrochloric acid) is needed in the stomach to:

  • Break down protein so digestive enzymes can work
  • Kill harmful bacteria, preventing infections

• The pancreas releases bicarbonate to neutralize acid from the stomach contents

  • This protects the small intestine and allows digestive enzymes to function properly.

• Bile:

• Is made by the liver and stored in the gallbladder

• Breaks down (disperses) fat in water, allowing digestive enzymes to access and digest fats more easily

• The small intestine is responsible for most of the nutrient digestion and absorption.

• Fiber is digested and water is reabsorbed in the large intestine.

  • Gut bacteria ferments some fiber.
  • Water reabsorption helps form solid waste before elimination.

Small Intestine unique lining components:

• Villi: finger-like projections that absorb nutrients in the blood or lymph

• Microvilli: tiny hair-like structures on villi with enzymes and protein pumps for nutrient absorption

• Crypts: tubular glands that secrete intestinal juices.

• Goblet Cells: secrete mucus to protect and lubricate the intestine

• Small water-soluble molecules are absorbed into the bloodstream and transported to the liver.

• Larger fats and fat-soluble vitamins are absorbed into the lymphatic system.

• Liver function:

  • Production of bile to break down and digest fats
  • Processes and distributes nutrients after absorption; nutrients go to the liver (hepatic portal vein)
  • Detoxifies blood

• Probiotics are bacteria in foods while prebiotics are food for bacteria in the gut microbiome.

• The two systems that help maintain homeostasis/feedback loops in the body are the endocrine (hormones) and neural (nerves) systems.

• Gastrin increases stomach acid for digestion and stops when pH is low enough.

• Stomach Stretch Receptors detect food and trigger digestive juices/muscle contractions.

• The Pyloric Sphincter controls chyme release and stops too much acid from entering the small intestine.

• Secretin signals the pancreas to release bicarbonate to neutralize acid.

• Cholecystokinin (CCK) signals the gallbladder to release bile for fat digestion and slows stomach emptying.

• Carbohydrate sources in the diet:

  • Grains (wheat, rice, oats, corn)
  • Vegetables (potatoes, yams, legumes)
  • Fruits (grapes, apples, oranges)
  • Milk & Dairy (milk, yogurt)
  • Processed Foods High in Added Sugars and Low in Nutrients (soft drinks, candy, baked goods, cereals with added sugars, processed snacks)

• Carbohydrates are divided into:

  • Simple Sugars: monosaccharides (single sugar), disaccharides (pairs of monosaccharides)
  • Complex Carbohydrates: polysaccharides (chains of monosaccharides)

• Carbohydrates contain the atoms carbon (C), hydrogen (H), and oxygen (O).

• The three monosaccharides are fructose, glucose, and galactose; fructose is the sweetest.

• Glucose, commonly known as blood sugar, is the monosaccharide most used by the body.

• Glucose is present in all disaccharides.

• The three disaccharides:

  • Maltose = Glucose + Glucose (from starch breakdown)
  • Sucrose = Glucose + Fructose (table sugar)
  • Lactose = Glucose + Galactose (milk sugar)

• A polysaccharide is a complex carbohydrate of many monosaccharides (glucose units) linked.

• Glycogen is a polysaccharide naturally found in humans.

• The two polysaccharides found in plants are starch (stores energy) and fiber (provides structure).

• We can digest starch into glucose, but we cannot break down all plant polysaccharides (fiber), because our enzymes can't break the bonds.

Soluble vs Insoluble Fiber:

• Soluble Fiber: Forms a gel-like substance in water and helps with heart health, diabetes, weight loss, and acts as a prebiotic

• Insoluble Fiber: Does not dissolve in water, supports GI health and digestion.

• Carbohydrate digestion begins in the mouth, with the enzyme salivary amylase.

• It breaks down starch into smaller polysaccharides and maltose.

• Hydrolysis is a chemical reaction that breaks down molecules by adding water.

• It splits disaccharides into monosaccharides (e.g., Sucrose → Glucose + Fructose + water).

• Carbohydrate digestion stops in the stomach because stomach acid inactivates salivary amylase.

• Carbohydrate digestion resumes in the small intestine via:

  • Pancreatic Amylase (from the pancreas): breaks down starch into maltose and glucose chains
  • Maltase (from the small intestine): breaks maltose into glucose
  • Sucrase (from the small intestine): breaks sucrose into glucose and fructose Lactase (from the small intestine): breaks lactose into glucose and galactose

• Fiber is not digested in the small intestine.

• A food bolus/chyme takes about 1 to 4 hours after a meal to reach the large intestine.

• Digestive processes in the large intestine include:

  • Absorbing water to form solid waste
  • Bacteria fermenting fiber for gut health
  • Forming and storing stool for elimination

• Fructose and galactose do not immediately circulate in the blood after absorption, but first go to the liver, where they convert into glucose.

• Lactose intolerance occurs when the body doesn't have enough lactase to break down lactose in milk: Without lactase, lactose stays in the intestine, causing bloating, gas, and diarrhea.

• An estimated 35% of the adult population has adequate levels of lactase.

• Side effects of lactose intolerance include bloating, gas, abdominal discomfort, water attraction, and diarrhea.

• The primary purpose of glucose in the body is to provide energy for cells.

  • It fuels the brain, muscles, and red blood cells.
  • Cells use glucose to create ATP.

• Yes, the body can make glucose from protein through gluconeogenesis when blood sugar is low.

• Gluconeogenesis converts amino acids to carbohydrates.

• An intake of 50-100g of carbohydrate in the diet per day (200-400kcal) will prevent the body from going into ketosis.

• Yes, glucose can turn into fat if there is extra glucose after energy needs and glycogen stores are full

  • This process is called lipogenesis.

• Terms for high/low blood sugar:

  • Hypoglycemia: Low Blood Sugar
  • Hyperglycemia: High Blood Sugar

• The two main hormones for regular glucose homeostasis are insulin and glucagon.

  • Insulin (from the pancreas) lowers blood sugar by helping cells take in glucose after eating.
  • Glucagon (from the pancreas) raises blood sugar by releasing stored glucose from the liver when needed.

• Glycemic response measures how fast glucose is absorbed, how high blood sugar spikes, and how quickly it returns to normal after eating

• High Response: Fast absorption, big spike, then quick drop.

• Low Response (Ideal): Slow absorption, steady rise, smooth return to normal.

• The ideal goal is a low glycemic response with slow glucose absorption, a moderate spike in blood sugar and smooth return to normal.

• Glycemic Index Ranges:

  • Low GI: 55 or less
  • Medium GI: Between 56 and 69
  • High GI: 70 or more

• More than 50% of food intake in the United States comes from ultra-processed foods.

• High fructose corn syrup is the most commonly added sugar in foods

• Manufacturers use this to improve the characteristics of food an lower costs:

  • Cheaper than regular sugar
  • Enhances flavor
  • Improves texture and moisture
  • Prevents crystallization
  • Helps preserve color and freshness

• Nutrient density decreases as added sugar increases.

• Foods high in added sugar provide energy (calories) but lack important nutrients like protein, fiber, vitamins, and minerals.

• A can of soda contains about 10 teaspoons of sugar, which is 30-50 grams of sugar.

• Dietary Guidelines for Americans recommend limiting added sugar intake to 10% of total daily kcal

• 5% has more health benefits.

• The recommended daily carbohydrate intake is 5 to 10 grams per kilogram (g/kg) of body weight, depending on activity level.

• Fiber Content per Serving:

  • Grains: 1 to 5 grams
  • Fruits: 2 grams
  • Vegetables: 2 to 3 grams
  • Legumes: 5 to 8 grams

• Health conditions which fiber helps:

  • Heart disease
  • Diabetes
  • Cancer
  • Weight management
  • Gut health benefits

• The average American consumes about 16g of fiber daily, which is lower than the recommended amount.

• The RDA for fiber on a 2000 kcal diet is 28 grams per day.

• Carbohydrate Content per Serving:

  • Grains: 15 grams
  • Fruits: 15 grams
  • Starchy Vegetables: 15 grams
  • Non-Starchy Vegetables: 5 grams

• One cup of milk has 12 grams of carbohydrates.

• Protein typically has no carbohydrate content, with two exceptions

• Nuts contain a little starch and fiber along with fat.

• Legumes (beans, lentils, peas) are high in both starch and fiber (about 20g of carbohydrates per ½ cup, with 1/3 of that as fiber).