Nutrition 4020 (4)

Page 68: Role of Fiber in Digestion

Types of Fiber

• Soluble Fiber: Includes pectin, gums, and mucilages.

  • Found in fruits, legumes, oats, seeds, corn bran, and certain vegetables.

• Insoluble Fiber: Found in cellulose, hemicellulose, lignin(cereals, grains, nuts, and some vegetables and fruits.)

Effects of Fiber on Nutrient Digestión and Absorption

• Delays stomach emptying and changes the transit time in the small intestine.

• Influences the mixing of gastrointestinal contents with digestive enzymes and the function of these enzymes, facilitating nutrient diffusion and absorption of lipids.

• Reduces glycemic response to food

• Affects bile acid excretion, leading to reduced cholesterolemia (cholesterol levels in the blood).

• Soluble fibers are fermented by beneficial bacteria in the colon, which yields energy and substances (like nitrogen) that support bacterial growth. Fermentation produces SCFAs(acetic, butyric and propionic acids) that can be used by host, occurs mostly in cecum and ascending colon.

• Short-Chain Fatty Acids (SCFAs) produced during fermentation (e.g., acetic, butyric, and propionic acids) have beneficial effects:

  • Stimulate absorption of water and sodium ions in the colonic mucosa.

  • Encourage differentiation and growth of colonic mucosal cells.

  • Lower the pH, enhancing calcium binding to bile and fatty acids, and possibly reducing risks of colon cancer.

  • Provide energy for colonic cells, inhibit liver cholesterol synthesis, and enhance blood flow and immune functions in the colon(Which prevent growth of potentially harmful cells)

Page 69: The Gut Microbiota

Definition and Composition

• Microbiota: The collection of microbes inhabiting a specific environment, in this case, the gut.

• Human body hosts ~39 trillion bacteria, comprising 1-3% of body mass, with bacteria outnumbering other microorganisms (ratio of ~1.3 bacterial cells to 1 human cell).

Role of Colonic Bacteria

• Breakdown and fermentation of dietary fibers, resistant starches, and non-carbohydrate substrates in the colon.

• Production of SCFAs(Short chain fatty acids: acetate, propionate, butyrate), which provide energy to colonic cells and contribute to approximately 10% of daily caloric intake. 95% of SCFAs used by colonic cells as source of energy. SCFAs serve as messengers between microbiota and immune system(play a role in development and function of intestinal epithelial cells and leukocytes)

• Bacteria synthesize essential B vitamins (e.g., thiamine, riboflavin, niacin, biotin, pantothenic acid, and folate) and vitamin K.

• Vitamins from diet are absorbed in the small intestine, while those produced by microbiota are absorbed in the colon.

Dietary Fiber

• Definition:

  • Dietary Fiber: Non-digestible carbohydrates and lignin intrinsic in plants.

  • Functional Fiber: Isolated non-digestible carbohydrates with beneficial health effects.

• Total fiber intake includes both dietary and functional fiber combined.

Page 70: Sugar and Its Dietary Recommendations

Recommended Sugar Intake

• US Dietary Guidelines (2015): Less than 10% of total daily energy intake from added sugars, prioritizing sources like fruits and whole grains.

• National Obesity Forum (UK, 2015): Advocates for zero sugar consumption for optimal health.

Glycemic Index (GI)

• Measures how much a food increases blood glucose levels after ingestion, comparing it to a standard (white bread).

• GI is classified as High (>70), Medium (55-70), and Low (<55).

Glycemic Load (GL)

• A modified Glycemic Index that estimates the overall glycemic impact of specific food portions, relevant for obesity prevention and diabetes management.

Page 71: The Brain and Glucose Metabolism

Blood-Brain Barrier and Energy Supply

• The Blood-Brain Barrier (BBB) isolates the brain from lipid soluble molecules and relies primarily on glucose for energy.

• Glucose is oxidized for energy, with minimal lactate released. In famine conditions, the brain can utilize ketones.

• Transporters: GLUT3 (in neurons) and GLUT1 (in BBB) do not require insulin for glucose transport but become saturated during normal glycemia. Mental function is compromised when glucose levels drop below ~2 mmol/L.

Sources of Sugars in Diet

• Categories of sugar sources include fruits, vegetables, sweetened beverages, dairy, and grains, with significant contributions from processed foods and snacks.

Page 72: Glucose Transport and Brain Metabolism

Glucose Transport Mechanism

• Glucose and its derivatives (galactose and fructose) enter enterocytes via sodium-dependent co-transport (SGLT1) and facilitated diffusion (GLUT2 and GLUT5). Once inside the enterocytes, glucose is then transported into the bloodstream through the basolateral membrane by the glucose transporter GLUT2, ensuring a steady supply of glucose for energy production in various tissues, including the brain.

• Liver needs insulin to say what to do with glucose, but not to take it in, as hepatocytes can uptake glucose through GLUT2 without the need for insulin. Insulin primarily regulates the conversion of glucose into glycogen for storage, as well as its utilization for energy production.

• The brain, however, relies on GLUT1 for glucose uptake from the bloodstream, which is crucial for maintaining its energy demands, especially under conditions of low glucose availability. GLUT4 inside of the cell in vesicle, waiting for signal to come from insulin.

• Brain requires approx. 120g of glucose/day, constituting ~20% of the body's energy requirements.

Page 73: Carbohydrate Digestion

Cellulose Structure and Enzymatic Resistance

• Cellulose is a glucose polysaccharide vital in plant cell walls, resistant to human digestion due to its (β1-4) glycosidic bonds, unlike (α1-4) in starch.

• Digestion primarily occurs in the small intestine with the help of enzymes (glycosidases or carbohydrases).

Enzymatic Digestion Process

• Enzymes named glycosidases or carbohydrases hydrolyse CHO

• Begins in the small intestine with pancreatic amylase, primarily affecting polysaccharides like amylopectin, still leaving difficult-to-hydrolyze bonds at branch points. In the duodenum, pH is elevated by bicarbonate to a level that optimizes alpha-amylase activity, allowing polysaccharides in the chyme to be digested in steps.

• Enzymes like glucoamylase and disaccharidases (e.g., lactase, sucrase) facilitate the breakdown of oligosaccharides and disaccharides for absorption.

• Amylopectin —> Oligosaccharides(about 8 glucose units, dextrins), Maltose(Disaccharide), Maltotriose(Trisaccharide)—> alpha-amylase(can’t hydrolyze alpha1-4 and 1-6 bonds next to branch points)

• Alpha-limit dextrinase(glucoamylase): Removes 1 glucose unit from ends of linear alpha1-4-glucosyl chains of dextrins and produce isomaltose(disaccharide with 1 unit attached by alpha1-6 glycolytic bond)

• Alpha1-6-glucosidase: hydrolyzes alpha1-4 bonds

• Disaccharidases: hydrolyzes disaccharides

Page 74: Structure of Carbohydrates

Types of Carbohydrates

• Glucose, Fructose, Galactose(Same as glucose, except HO is upwards which makes it so that different enzymes deal with it) first enter through liver(Converts fructose and galactose into glucose)

• Amylose: Linear chain of glucose (α1-4 glycosidic bonds).(15-20% starch content)

• Amylopectin: Branched structure with both (α1-4 and α1-6) linkages.(80-85% starch content)

• Glycogen: Highly branched compared to amylopectin, crucial for energy storage.

Starch Composition in Foods

• Typically consists of 80-85% amylopectin and 15-20% amylose, found in sources like cereal grains, potatoes, and legumes.

Page 75: Carbohydrate Classifications

Major Types of Carbohydrates

• Carbohydrates are categorized based on their molecular structures:

  • Monosaccharides: Simple sugars (e.g., glucose, fructose).

  • Disaccharides: Composed of two monosaccharides (e.g., sucrose, lactose).

  • Oligosaccharides: 3-10 monosaccharides (e.g., stachyose, raffinose).

  • Polysaccharides: >10 monosaccharides (e.g., starch, glycogen, fiber and resistant starch(cellulose, pectin, hemicellulose), glycosaminoglycans).

Inaccessibility to Human Enzymes

• Certain oligosaccharides (e.g., stachyose, verbacose and raffinose found in beans, peas, bran and whole grains) are not hydrolyzed by human digestive enzymes, often resulting in fermentation by gut bacteria, causing flatulence.

Page 76: Large Intestine Functions

Composition and Role of the Large Intestine

• The large intestine has no villi and primarily absorbs water and electrolytes, processing about 5-7 liters of fluid daily.

• Feces composition: 30% bacteria, 10-20% fat, 10% inorganic matter, and 30% undigested fibers.

• Dries and stores undigested material present in the chyme. Additionally, it plays a crucial role in the fermentation of undigested carbohydrates by gut bacteria, producing short-chain fatty acids that provide energy to the colonic cells.

Microbiota Functions

• Houses over 400 bacterial species capable of producing vitamin K, biotin, and SCFAs beneficial for colonic health.

• Probiotics: Beneficial bacteria in foods (e.g., yogurt).

• Prebiotics: Non-digestible food components (e.g., fiber) that enhance beneficial bacterial growth.

Page 77: Pancreatic Function

Pancreatic Secretions

• The pancreas secretes juices rich in bicarbonate, electrolytes, and digestive enzymes, regulated by hormones such as secretin and cholecystokinin (CCK).

• Key enzymes include:

  • Proteases (e.g., trypsinogen, chymotrypsinogen) for protein and CHO digestion(About 50% of the amount ingested).

  • Alpha-Amylase for carbohydrate digestion.

  • Pancreatic lipase for fat digestion, breaking down ~80-90% of dietary fats.

  • Turns enzyme secretion on/off based on the presence of specific substrates in the digestive tract, ensuring efficient digestion and nutrient absorption.

Page 78: Bile Functions

Role and Composition of Bile

• Bile acids/salts emulsify large fat droplets, aiding fat digestion and removing excretory substances (e.g., bilirubin).

• Gallbladder: Concentrates and releases bile in the presence of CCK when food enters the small intestine.

• Gallstones form from excessive cholesterol, leading to potential complications in bile flow.

Page 79: Nutrient Absorption

Overview of Nutrient Pathways

• Nutrients pass through enterocytes into the portal vein and lymphatics for distribution.

• The liver intricate role: produces bile and plays a central role in metabolism, detoxification, and nutrient absorption.

Bile Acid Composition

• Major components include bile acids (chenodeoxycholic and cholic acids), with conjugation occurring with amino acids to form bile salts, aiding in digestion.

Page 80: Vitamins

Definition and Classification

• Vitamins: Organic compounds necessary for physiological functions, existing as water-soluble or fat-soluble types.

• Essential for various chemical reactions and cannot be synthesized in adequate amounts by the body.

• Exceptions: Vitamin D, K, and biotin can be partially sourced from body synthesis (e.g., via gut bacteria).

Importance of Water

• Water is a critical nutrient, essential for survival, significantly more than any other nutrient. Intracellular volume(60%), Extracellular volume(40%), Interstitial volume(28% of 40), Plasma volume(8% of 40), Transcellular volume(4% of 40), and Total body water (about 60% of body weight) play vital roles in maintaining homeostasis, regulating temperature, and facilitating various biochemical reactions.

Page 81: Gastric Secretion Phases

Stages of Gastric Acid Production

• Gastric acid secretion occurs in three stages:

  • Cephalic Phase: Triggered by food anticipation (30% of gastric acid).

  • Gastric Phase: Induced by stomach distension and protein presence (60%).

  • Intestinal Phase: Initiated by small intestine stretch and food materials (10%).

• Gastric function is crucial for nutrient breakdown, particularly for protein and fat digestion.

Page 82: Structure of Small Intestine

Anatomy of the Small Intestine

• Key functions: digestion and absorption.

• Surface area is enlarged through folds, villi, and microvilli, enhancing absorption capabilities significantly.

Enterocyte Functionality

• Enterocytes renew every 3-5 days, supporting nutrient absorption and maintaining gut barrier integrity.

Page 83: Gastric Acid Secretion Mechanisms

Active Regulation of HCI Production

• Secreted through three primary signals: acetylcholine, histamine, and gastrin for maximal acid production.

• Critical for activating digestive enzymes and aiding in iron absorption, while intrinsic factor seats in stomach for vitamin B12 absorption.

Page 84: Stomach Functionality

Overview of Gastric Actions

• Stomach mechanisms include:

  • Grinding food and releasing gastric juice.

  • Stomach protects itself via alkaline mucus against acidic content.

  • Chyme released in small quantities into the duodenum, facilitated by peristaltic waves.

Page 85: Body Fluid Compartments

Estimates for a 73kg Man

• Breakdown of total body water: intracellular (60%), extracellular (40%), with specific volumes for interstitial and plasma.

Digestive System Overview

• Major components include oral cavity, esophagus, stomach, small and large intestines, and accessory organs, each contributing to digestion, absorption, and metabolism.

Page 87: Lipids and Proteins

Lipids

• Definition: Soluble in organic solvents; includes triglycerides (largest component) and others like cholesterol.

• Functions of lipids:

  • Energy storage, insulation, cell signaling, and vitamin absorption.

  • Essential fatty acids include linoleic and linolenic acids due to inability to produce them adequately.

Proteins

• Definition: Composed of amino acids, critical for cellular structure, functions, enzymes, transport, antibodies, and more.

Page 88: Nutrient Classification

Macronutrients vs. Micronutrients

• Macronutrients: Carbohydrates, proteins, and fats (needed in large amounts).

• Micronutrients: Vitamins and minerals (needed in smaller quantities).

Carbohydrate Functions

• Sourced from grains, legumes, fruits, and dairy, primarily as glucose for energy, vital for brain functioning.

• Dietary fiber is crucial in maintaining GI health without providing substantial energy.

Page 89: Nutrition Science Overview

Nutrition as a Field of Study

• Focused on nutrient interactions within the body for energy usage, growth, tissue repair, and regulation of bodily functions.

Functions of Nutrition

• Supporting growth, energy production, and contributing to homeostatic regulation and immune functions, fundamental to human health.