Comprehensive Guide to Digestion and Absorption of Macronutrients and Micronutrients

Overview of Digestion and Absorption Processes

• The digestive system involves distinct processes for the breakdown and uptake of carbohydrates, proteins, and fats.
• The primary goal is to ensure the body acquires the energy and building blocks necessary for cellular function.
• The majority of digestion and absorption occurs in the small intestine, where nutrients move from the lumen through epithelial cells to the blood or lymph.

Digestion and Absorption of Carbohydrates

• Macronutrient Components: Carbohydrates primarily enter the diet as starch (polysaccharides), sucrose, and lactose (disaccharides).
• Initial Breakdown in the Mouth:
  • Digestion begins with salivary amylase, which metabolizes starch into smaller polysaccharides and maltose.
  • Salivary amylase is later inactivated by gastric acid within the stomach.
• Major Digestion in the Small Intestine:
  • Most carbohydrate digestion occurs in the small intestine, specifically within the first $20\,\%$ of its length.
  • Pancreatic amylase is secreted into the small intestine to continue breaking starch into disaccharides like maltose and short glucose chains.
  • Brush border enzymes located on the apical membrane of epithelial cells include:
    • Maltase: Converts maltose into glucose.
    • Sucrase: Splits sucrose into glucose and fructose.
    • Lactase: Splits lactose into glucose and galactose.
• Absorption of Monosaccharides:
  • Transport Across the Apical Membrane:
    • Glucose and galactose are absorbed via secondary active transport through the sodium-glucose cotransporter ($\text{SGLT1}$).
    • This process is powered by the sodium gradient maintained by the $\text{Na}^+/\text{K}^+\text{-ATPase}$ pumps.
    • Fructose is absorbed through facilitated diffusion via the $\text{GLUT5}$ transporter.
  • Transport Across the Basolateral Membrane:
    • All three monosaccharides (glucose, galactose, and fructose) exit the epithelial cell into the interstitial fluid via facilitated diffusion through $\text{GLUT2}$ transporters.
  • Entry into the Bloodstream:
    • Monosaccharides diffuse into capillaries and are transported directly to the liver through the hepatic portal vein for metabolism.

Digestion and Absorption of Proteins

• Initial Breakdown in the Stomach:
  • Digestion begins with pepsin, which is secreted as an inactive zymogen ($\text{pepsinogen}$) and activated by hydrochloric acid ($\text{HCl}$).
  • The acidic environment ($\text{HCl}$) denatures proteins, increasing their accessibility to enzymes.
  • Pepsin breaks proteins down into smaller polypeptides and peptides.
• Further Digestion in the Small Intestine:
  • Pancreatic zymogens are secreted: trypsinogen, chymotrypsinogen, and procarboxypeptidases.
  • Enteropeptidase: This brush border enzyme activates trypsinogen into trypsin.
  • Trypsin: Once active, trypsin activates other zymogens including chymotrypsin and carboxypeptidases.
  • Enzymatic Specificity:
    • Trypsin and chymotrypsin cleave specific peptide bonds within the polypeptide chain.
    • Carboxypeptidases remove amino acids from the carboxyl end of the peptide.
    • Aminopeptidases (on the brush border) remove amino acids from the amino end.
• Final Breakdown and Absorption:
  • Intracellular Peptidases: Dipeptides and tripeptides are hydrolyzed into individual amino acids within the epithelial cells.
  • Apical Transport:
    • Free amino acids use secondary active transport coupled to $\text{Na}^+$.
    • Dipeptides and tripeptides are absorbed through $\text{H}^+\text{-dependent}$ cotransporters such as $\text{PepT1}$.
  • Basolateral Transport: Free amino acids exit into the interstitial fluid via facilitated diffusion through specific transporters classified by the type of amino acid (neutral, acidic, or basic).
  • Circulation: Amino acids enter capillaries and travel via the hepatic portal vein to the liver for energy production or protein synthesis.
• Special Consideration for Infants: Newborns can absorb small amounts of intact proteins via endocytosis and exocytosis, allowing the transfer of maternal antibodies from breast milk for passive immunity.

Digestion and Absorption of Fats (Lipids)

• Composition: Most dietary fats are triglycerides, consisting of a glycerol backbone and three fatty acids.
• Initial Processes: Minor digestion occurs in the mouth and stomach via lingual and gastric lipases.
• Digestion in the Small Intestine:
  • Emulsification: Large fat droplets are mechanically disrupted (GI motility) and coated by bile salts and phospholipids (emulsifying agents) to prevent reaggregation and increase surface area.
  • Pancreatic Lipase: The primary enzyme that breaks triglycerides into two free fatty acids and one monoglyceride.
• Role of Micelles:
  • Micelles are small aggregates of bile salts, fatty acids, monoglycerides, and other lipids.
  • They keep lipid digestion products soluble and transport them to the intestinal brush border, where individual lipid molecules diffuse into the cells.
• Resynthesis and Packaging:
  • Inside epithelial cells, fatty acids and monoglycerides are resynthesized into triglycerides in the smooth endoplasmic reticulum.
  • Triglycerides are packaged into chylomicrons (lipid droplets coated with amphipathic proteins).
• Transport via Chylomicrons:
  • Chylomicrons are too large to enter blood capillaries.
  • They are released into the interstitial fluid and enter lacteals (lymphatic vessels) within the intestinal villi.
  • They travel through the lymphatic system before entering the bloodstream for distribution to tissues.

Absorption of Vitamins, Water, and Minerals

• Fat-Soluble Vitamins (A, D, E, K):
  • Absorbed following the fat pathway (micelles to chylomicrons to lymph).
  • Malabsorption syndromes or bile secretion issues can lead to deficiencies (e.g., Vitamin D deficiency affects calcium homeostasis).
• Water-Soluble Vitamins (C, B-complex):
  • Absorbed via diffusion or carrier-mediated transport (some $\text{Na}^+\text{-dependent}$).
  • Vitamin $\text{B}_{12}$ (Cobalamin): Requires intrinsic factor (secreted by stomach parietal cells). The complex is absorbed in the ileum via endocytosis. Deficiency causes pernicious anemia.
• Water: Most water in chyme is absorbed in the small intestine via osmotic gradients created by solute absorption.
• Sodium ($\text{Na}^+$): Actively transported using the $\text{Na}^+/\text{K}^+\text{-ATPase}$ pump.
• Iron:
  • Absorbed in small daily amounts.
  • Stored as ferritin in intestinal cells; transported in blood bound to transferrin.
  • Excess iron causes hemochromatosis (toxic), managed via blood withdrawal.
  • Iron from liver sources is more readily absorbed than from egg yolks.

Function of Antioxidants

• Definitions:
  • Free Radical: An unstable molecule/atom with an unpaired electron in its outermost orbital that damages cells, proteins, and DNA.
  • Antioxidant: A substance that inhibits the oxidation of other molecules.
• Mechanisms of Action:
  • Neutralizing Free Radicals: Antioxidants donate electrons to stabilize free radicals, stopping chain reactions of damage.
  • Preventing Oxidative Stress: They maintain the balance between free radicals and cellular defense, preventing chronic diseases (cancer, cardiovascular, neurodegenerative).
  • Supporting Cellular Health: They slow aging processes.
• Examples: Vitamin C, Vitamin E, beta-carotene, and selenium (found in fruits, vegetables, nuts, and grains).

Conditions Related to Abnormal Digestion and Absorption

• Malabsorption Syndromes:
  • Celiac Disease: An autoimmune sensitivity to gluten that damages the intestinal brush border surface area, reducing absorption of calcium, iron, and fat-soluble vitamins.
  • Lactose Intolerance: Caused by a deficiency in the enzyme lactase; leads to bloating, gas, and diarrhea.
  • Pancreatic Insufficiency: Caused by chronic pancreatitis or cystic fibrosis, resulting in poor digestion of all macronutrients due to lack of enzymes.
• Vitamin Deficiencies:
  • B12 Deficiency: Leads to pernicious anemia and neurological symptoms.
  • Fat-Soluble Deficiencies: Result in poor vision (Vit A), bone disorders (Vit D), and clotting issues (Vit K).
• Protein Malnutrition:
  • Kwashiorkor: Severe protein deficiency characterized by edema, muscle wasting, and immune dysfunction.
• Gastrointestinal Disorders:
  • IBS (Irritable Bowel Syndrome): Functional disorder with abdominal pain and altered bowel habits.
  • IBD (Inflammatory Bowel Disease): Includes Crohn’s disease and ulcerative colitis; inflammation causes nutrient malabsorption.
  • Short Bowel Syndrome: Reduced surface area for absorption following surgical removal of intestine.
  • Bacterial Overgrowth: Excess bacteria in the small intestine interfere with nutrient uptake, causing diarrhea and bloating.