4.2 Digestion and Absorption of Nutrients

Your digestive system is a complex machine that breaks down food into usable nutrients. From mechanical grinding to chemical breakdown, each step is crucial. Carbs, proteins, and fats are processed differently, with specific enzymes and organs playing key roles.

Once nutrients are broken down, your body absorbs them through various methods. Some use special transporters, while others diffuse directly into cells. Your large intestine plays a vital role in fluid balance and waste management, ensuring you stay hydrated and healthy.

Digestive Processes

Process of macronutrient digestion

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• Mechanical digestion breaks down food physically
  • Chewing in the mouth grinds food into smaller particles
  • Peristalsis in the esophagus and intestines propels food along the digestive tract
  • Churning in the stomach mixes food with gastric juices
• Chemical digestion of carbohydrates begins in the mouth and continues in the small intestine
  • Salivary amylase in the mouth initiates starch breakdown
  • Pancreatic amylase in the small intestine further breaks down complex carbohydrates
  • Brush border enzymes (maltase, sucrase, lactase) complete carbohydrate digestion
• Chemical digestion of proteins starts in the stomach and finishes in the small intestine
  • Pepsin in the stomach begins protein breakdown in acidic environment
  • Trypsin, chymotrypsin, and carboxypeptidase in the small intestine further break down proteins into smaller peptides
• Chemical digestion of lipids occurs primarily in the small intestine
  • Lingual lipase in the mouth initiates fat digestion
  • Gastric lipase in the stomach continues fat breakdown
  • Pancreatic lipase in the small intestine completes lipid digestion
  • Bile salts aid in emulsification, increasing surface area for lipase action

Nutrient absorption and transport

• Carbohydrate absorption occurs through specific transporters
  • Glucose and galactose enter cells via SGLT1 transporter
  • Fructose enters cells through GLUT5 transporter
• Protein absorption involves multiple mechanisms
  • Di- and tripeptides enter cells via PEPT1 transporter
  • Free amino acids use various amino acid transporters for absorption
• Lipid absorption requires special processing
  • Micelles form to transport lipids to enterocyte surface
  • Lipids diffuse into enterocytes
  • Triglycerides reform and package into chylomicrons for transport
• Micronutrient absorption varies based on their properties
  • Fat-soluble vitamins (A, D, E, K) absorb alongside lipids
  • Water-soluble vitamins and minerals use specific transporters
• Nutrient transport to cells occurs through two main routes
  • Bloodstream (portal vein) carries most nutrients to the liver
  • Lymphatic system transports chylomicrons containing lipids

Intestinal Functions

Large intestine in fluid balance

• Water absorption crucial for maintaining hydration
  • Approximately 1.5-1.8 liters absorbed daily
  • Sodium-dependent water reabsorption prevents dehydration
• Electrolyte absorption and secretion maintain balance
  • Sodium and chloride reabsorption helps regulate blood pressure
  • Potassium secretion aids in maintaining proper muscle and nerve function
• Bacterial fermentation produces beneficial compounds
  • Short-chain fatty acids provide energy for colon cells
  • Additional water and electrolyte absorption occurs during fermentation
• Fecal formation concentrates waste
  • Undigested matter compacts as water is removed
  • Waste storage and elimination regulated by nervous system signals

Factors affecting nutrient processing

• Dietary factors influence digestion and absorption efficiency
  • Fiber content affects transit time through the digestive tract
  • Nutrient interactions can enhance or inhibit absorption (calcium and iron)
  • Antinutrients (phytates, oxalates) can bind to minerals, reducing their absorption
• Physiological factors impact digestive processes
  • Age-related changes in digestive enzymes can affect nutrient breakdown
  • Hormones (gastrin, cholecystokinin) regulate digestive secretions and motility
  • Stress can alter digestive secretions and gut motility
• Health conditions may impair nutrient utilization
  • Celiac disease damages small intestine, reducing nutrient absorption
  • Inflammatory bowel diseases can interfere with normal digestive processes
  • Pancreatic insufficiency reduces enzyme production, affecting macronutrient digestion
• Medications can alter digestive environment
  • Antacids change stomach pH, affecting protein digestion
  • Antibiotics disrupt gut microbiota, potentially impacting nutrient absorption
• Lifestyle choices affect digestive health
  • Alcohol and tobacco use can impair nutrient absorption and utilization
  • Physical activity influences gastrointestinal blood flow and motility, affecting digestion and absorption