🥗Intro to Nutrition Unit 4 – Digestion, Absorption & Metabolism

Key Concepts

• Digestion breaks down food into smaller components (nutrients) that the body can absorb and utilize
• Absorption transports nutrients from the digestive tract into the bloodstream for distribution throughout the body
• Metabolism refers to all chemical reactions involved in maintaining the living state of cells and organisms
• Catabolism breaks down complex molecules into simpler ones, releasing energy in the process
• Anabolism builds complex molecules from simpler ones, requiring an input of energy
• Basal Metabolic Rate (BMR) represents the minimum energy required to maintain vital functions at rest
• Thermic Effect of Food (TEF) is the energy expended above the BMR due to food consumption and processing
• Energy balance occurs when energy intake equals energy expenditure, maintaining body weight

Digestive System Overview

• The digestive system consists of the gastrointestinal tract (mouth, esophagus, stomach, small intestine, large intestine) and accessory organs (salivary glands, liver, gallbladder, pancreas)
• The mouth initiates mechanical digestion through chewing (mastication) and chemical digestion with salivary amylase
• The esophagus transports food from the mouth to the stomach via peristalsis (rhythmic muscle contractions)
• The stomach stores food, mixes it with digestive enzymes (pepsin), and releases it slowly into the small intestine
  • Hydrochloric acid in the stomach creates an acidic environment, activating pepsin and killing bacteria
• The small intestine is the primary site of digestion and absorption, with three segments: duodenum, jejunum, and ileum
  • Bile from the liver and gallbladder emulsifies fats, increasing surface area for enzymatic action
  • Pancreatic enzymes (lipase, amylase, trypsin) break down fats, carbohydrates, and proteins
• The large intestine absorbs water and electrolytes, forming and storing feces until elimination

Breakdown of Nutrients

• Carbohydrates are broken down into monosaccharides (glucose, fructose, galactose) by salivary and pancreatic amylase and brush border enzymes
• Proteins are denatured by stomach acid and broken down into amino acids by pepsin (stomach) and trypsin, chymotrypsin, and carboxypeptidase (small intestine)
• Fats are emulsified by bile and broken down into fatty acids and monoglycerides by pancreatic lipase
• Nucleic acids (DNA and RNA) are broken down into nucleotides by pancreatic nucleases
• Fiber (non-digestible carbohydrates) aids in digestion by promoting regular bowel movements and feeding beneficial gut bacteria
• Vitamins and minerals are not chemically altered during digestion but are released from food components for absorption
• Digestive enzymes are specific to their substrates, working optimally at certain pH levels and temperatures

Absorption Process

• Absorption primarily occurs in the small intestine, with some absorption in the stomach and large intestine
• Villi and microvilli in the small intestine increase surface area for efficient absorption
• Passive diffusion allows small, non-polar molecules (fatty acids, glycerol) to move down their concentration gradient across the intestinal wall
• Facilitated diffusion uses carrier proteins to transport specific molecules (glucose, amino acids) down their concentration gradient
• Active transport requires energy (ATP) to move molecules (calcium, iron) against their concentration gradient
• Endocytosis involves the engulfing of larger particles or molecules (vitamin B12) by the cell membrane
• Fat-soluble vitamins (A, D, E, K) are absorbed along with dietary fats and packaged into chylomicrons for transport
• Water-soluble vitamins (B-complex, C) are absorbed directly into the bloodstream
• Minerals are absorbed through specific transport mechanisms (iron via heme carrier protein, calcium via vitamin D-dependent pathways)

Metabolism Basics

• Metabolism is divided into two processes: catabolism (breakdown) and anabolism (synthesis)
• Adenosine Triphosphate (ATP) is the primary energy currency of the cell, produced during catabolic reactions and consumed during anabolic reactions
• Glucose is the body's preferred energy source, with glycolysis being the first step in its catabolic pathway
  • Glycolysis occurs in the cytoplasm and produces pyruvate and a net gain of 2 ATP
• Pyruvate enters the mitochondria and is converted to acetyl-CoA, which enters the Krebs cycle (citric acid cycle)
  • The Krebs cycle produces high-energy molecules (NADH, FADH2) and a small amount of ATP
• Oxidative phosphorylation (electron transport chain) uses NADH and FADH2 to create a proton gradient, driving ATP synthase to produce large amounts of ATP
• Fatty acids undergo beta-oxidation in the mitochondria, producing acetyl-CoA for the Krebs cycle and ATP production
• Amino acids can be converted into glucose (glucogenic) or ketone bodies (ketogenic) when needed for energy

Energy Balance

• Energy balance is the relationship between energy intake (calories consumed) and energy expenditure (calories burned)
• Positive energy balance occurs when intake exceeds expenditure, leading to weight gain and energy storage (adipose tissue)
• Negative energy balance occurs when expenditure exceeds intake, leading to weight loss and the breakdown of stored energy (fat, muscle)
• Basal Metabolic Rate (BMR) is the minimum energy required to maintain vital functions at rest and accounts for 60-70% of total daily energy expenditure
  • Factors affecting BMR include age, sex, body composition, genetics, and hormones (thyroid)
• Physical Activity Level (PAL) represents the energy expended through physical activity and exercise, varying based on intensity and duration
• Thermic Effect of Food (TEF) is the energy required to digest, absorb, and metabolize nutrients, accounting for ~10% of total daily energy expenditure
• Adaptive thermogenesis is the body's adjustment of energy expenditure in response to changes in energy intake, aiming to maintain energy balance

Digestive Disorders

• Gastroesophageal Reflux Disease (GERD) occurs when stomach acid frequently flows back into the esophagus, causing heartburn and potential damage to the esophageal lining
• Peptic ulcers are open sores in the lining of the stomach or duodenum, often caused by Helicobacter pylori bacteria or excessive use of NSAIDs
• Inflammatory Bowel Disease (IBD) includes Crohn's disease and ulcerative colitis, characterized by chronic inflammation of the digestive tract
• Celiac disease is an autoimmune disorder triggered by gluten consumption, leading to damage of the small intestine's lining and nutrient malabsorption
• Lactose intolerance is the inability to digest lactose (milk sugar) due to a deficiency in the enzyme lactase, causing bloating, diarrhea, and abdominal pain
• Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder characterized by abdominal pain, bloating, and changes in bowel habits (diarrhea, constipation)
• Diverticulosis is the presence of small pouches (diverticula) in the colon wall, which can become inflamed (diverticulitis) and cause pain, fever, and digestive issues

Nutrition's Impact on Digestion

• Fiber promotes digestive health by adding bulk to stools, promoting regular bowel movements, and feeding beneficial gut bacteria (prebiotics)
  • Soluble fiber (fruits, oats, legumes) dissolves in water and forms a gel-like substance, slowing digestion and increasing feelings of fullness
  • Insoluble fiber (whole grains, vegetables, nuts) does not dissolve in water and helps to prevent constipation by promoting the movement of material through the digestive tract
• Probiotics are live microorganisms (bacteria, yeasts) that, when consumed in adequate amounts, confer a health benefit to the host by supporting the balance of gut microbiota
  • Fermented foods (yogurt, kefir, sauerkraut, kimchi) are natural sources of probiotics
• Adequate hydration is essential for optimal digestive function, as water helps to dissolve nutrients, soften stools, and prevent constipation
• Mindful eating practices (eating slowly, chewing thoroughly, avoiding distractions) can improve digestion by promoting proper mechanical and chemical breakdown of food
• Certain foods may trigger digestive issues in sensitive individuals, such as spicy foods, high-fat foods, or foods containing FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols)
• A balanced diet rich in fruits, vegetables, whole grains, lean proteins, and healthy fats provides the necessary nutrients for maintaining a healthy digestive system