Principles of Digestion and Absorption

Introduction

Efficient digestion and absorption depend largely on the small intestine, where most nutrients are processed and taken up. Disruption of this system leads to malabsorption, while coordinated transport mechanisms ensure optimal uptake through mutually supportive pathways.

Physiological Aspects

• Digestion is the process of breaking complex food substances into smaller, absorbable molecules within the gastrointestinal tract. It begins in the mouth and continues predominantly in the small intestine, where most enzymatic activity occurs.
• Multiple enzymes act sequentially to convert carbohydrates, proteins, and fats into their absorbable forms.
• Absorption is the transfer of digested nutrients, water, vitamins, and minerals from the intestinal lumen into blood and lymph.
• The lining cells of the small intestine, called enterocytes, play a key role in this process.
• Enterocytes possess a brush border that contains enzymes and transport proteins.
• Nutrients first cross the intestinal epithelium into interstitial fluid.
• From there, they enter the bloodstream or lymphatic system depending on their nature.
• These coordinated processes ensure efficient nutrient utilization and maintenance of metabolic balance.

Digestion and Absorption of Carbohydrates

Digestion of Carbohydrate

• Dietary carbohydrates include polysaccharides, disaccharides, and monosaccharides, with starch being the major component.
• Digestion begins in the mouth by salivary amylase, which breaks alpha-1,4 glycosidic bonds to form dextrins. This enzyme does not act on alpha-1,6 linkages.
• In the small intestine, pancreatic amylase continues carbohydrate digestion. It produces maltose, maltotriose, and alpha-dextrins.
• Final digestion occurs at the brush border of intestinal enterocytes.
• Enzymes include dextrinase, maltase, lactase, sucrase, and glucoamylase. These enzymes convert complex sugars into absorbable monosaccharides.
• Dextrinase hydrolyzes alpha-1,6 bonds, while maltase and glucoamylase release glucose units.

• Lactase splits lactose into glucose and galactose, and sucrase splits sucrose into glucose and fructose.
• The final products of carbohydrate digestion are glucose, galactose, and fructose.
• Enzyme activity is highest in the duodenum and upper jejunum and decreases toward the ileum.

Absorption of Carbohydrates

• Carbohydrate absorption occurs mainly in the duodenum and jejunum and decreases toward the ileum.
• Only monosaccharides such as glucose, galactose, and fructose are absorbed efficiently.
• Glucose and galactose enter enterocytes through the sodium–glucose cotransporter 1 on the apical membrane. This transport depends on luminal sodium concentration and operates as secondary active transport.

• Sodium moves down its gradient into the cell and carries glucose or galactose along with it.
• The sodium gradient is maintained by the sodium–potassium pump located on the basolateral membrane. This pump actively removes sodium from enterocytes in exchange for potassium, sustaining low intracellular sodium levels.
• After entry, glucose exits the cell through glucose transporter 2 into the interstitial fluid. From there, it diffuses into the bloodstream.
• Galactose follows the same transport pathway as glucose.
• Fructose is absorbed differently by facilitated diffusion using glucose transporter 5 on the apical membrane. It then exits via glucose transporter 2 on the basolateral side.
• Efficient absorption depends on intact mucosa, adequate sodium availability, and proper function of transport proteins.

Applied Aspects

Carbohydrate Malabsorption Syndromes

• Carbohydrate malabsorption results from deficiency of brush border enzymes or transport proteins in enterocytes.
• Undigested carbohydrates remain in the lumen and draw water, leading to osmotic diarrhea.

Lactose Intolerance

• Lactose intolerance is caused by reduced lactase activity in the intestinal mucosa.
• Undigested lactose is fermented by colonic bacteria, producing gas and organic acids. This leads to abdominal distension, flatulence, and increased bowel movements.

Sucrase-Isomaltase Deficiency

• Sucrase–isomaltase deficiency is an inherited disorder affecting disaccharide digestion.
• Ingestion of sucrose results in diarrhea, bloating, and excessive gas formation.

Glucose-Galactose Malabsorption Syndrome

• Glucose–galactose malabsorption is due to defective sodium–glucose cotransporter 1.
• Glucose and galactose are not absorbed, causing severe diarrhea and dehydration.
• Fructose absorption remains intact and is usually well tolerated.
• Management includes dietary modification and avoidance of poorly absorbed carbohydrates.

Oral Sugar Tolerance Test

• The oral sugar tolerance test is used to detect carbohydrate malabsorption.
• A specific sugar is administered orally, and its levels are measured in blood and stool.
• In normal absorption, blood glucose rises after ingestion.
• In malabsorption, blood levels fail to increase, while stool sugar content rises.
• Symptoms such as diarrhea and flatulence may occur after ingestion.

Digestion and Absorption of Proteins

• Dietary protein intake varies with nutritional status and food habits.
• About 10 to 30 grams of endogenous protein enters the intestine from secretions and shed epithelial cells.
• Normally, proteins are efficiently digested and absorbed in the small intestine. ·  Excess protein intake may exceed absorptive capacity, leading to small amounts appearing in stool.

Digestion of Proteins

• Protein digestion begins in the stomach, where hydrochloric acid activates pepsin from its inactive precursor.
• Pepsin breaks proteins into smaller peptides.
• In the small intestine, pancreatic enzymes such as trypsin, chymotrypsin, elastase, and carboxypeptidases continue protein breakdown.
• Activation of these enzymes occurs when trypsinogen is converted to trypsin by enterokinase from intestinal mucosa.
• Trypsin further activates other pancreatic proteases, ensuring efficient digestion.
• Brush border enzymes, including aminopeptidases and dipeptidases, convert peptides into amino acids and small peptides. The final products of digestion are amino acids and short peptides.

Absorption of Proteins

• Protein absorption occurs mainly in the duodenum and jejunum.
• Multiple transport systems facilitate uptake of amino acids and peptides across enterocytes.
• Many of these transporters are sodium-dependent, similar to glucose transport mechanisms.
• Some peptide transporters operate independently of sodium.
• Absorption efficiency decreases toward the ileum.

Defects of Protein Digestion

Hartnup Disease

Hartnup disease is a hereditary disorder with impaired transport of neutral amino acids in intestinal and renal epithelial cells. This results in loss of amino acids in both stool and urine.

Cystinuria

Hartnup disease is a hereditary disorder with impaired transport of neutral amino acids in intestinal and renal epithelial cells. This results in loss of amino acids in both stool and urine.

Prolinuria

Prolinuria is a rare disorder with defective absorption and reabsorption of proline. It causes increased excretion of proline and hydroxyproline in urine.

Digestion and Absorption of Lipids

Digestion

• Lipid digestion begins in the oral cavity with secretion of lingual lipase.
• Gastric lipase contributes modestly to fat digestion in the stomach. These early enzymes are not essential for normal fat absorption.
• The major digestion of lipids occurs in the duodenum.
• Bile salts emulsify fats, increasing surface area for enzyme action.
• Pancreatic lipase hydrolyzes triglycerides at positions 1 and 3. This produces free fatty acids and 2-monoglycerides.
• Lipase activity is enhanced by colipase, which stabilizes enzyme binding to fat droplets.
• Bile salt–activated lipase assists in digestion of cholesterol esters and other lipids.
• Cholesterol esters are hydrolyzed by cholesteryl ester hydrolase.
• Efficient digestion depends on coordinated action of bile, pancreatic enzymes, and proper intestinal function.

Absorption

• Lipid absorption occurs mainly in the small intestine through passive diffusion and carrier-mediated processes.
• Emulsification by bile salts, lecithin, and monoglycerides increases the surface area of fats. These components form micelles, which solubilize fatty acids, monoglycerides, and cholesterol.
• Micelles transport lipids to the brush border of enterocytes, where they release their contents.
• Lipids then diffuse into enterocytes and are re-esterified to form triglycerides. This maintains a concentration gradient that favors continuous lipid uptake.
• Inside enterocytes, lipids combine with proteins to form chylomicrons.
• Chylomicrons enter lymphatic vessels and later reach systemic circulation.
• Short-chain fatty acids, produced by colonic bacteria, are absorbed directly in the colon. They are taken up in exchange for hydrogen ions and contribute to acid–base balance. These fatty acids also enhance sodium absorption in the colon.
• Cholesterol absorption is facilitated by bile salts and lipid digestion products. It is incorporated into chylomicrons for transport.

• Absorption of fat-soluble vitamins (A, D, E, and K) depends on normal fat absorption.
• Deficiency of bile salts or impaired lipid absorption leads to deficiency of these vitamins.

Disorders of Fat Digestion

Steatorrhea

• Steatorrhea is characterized by bulky, pale, and fatty stools due to impaired fat digestion or absorption.
• Deficiency of pancreatic lipase in exocrine pancreatic disorders reduces triglyceride breakdown.
• Excess gastric acid can lower duodenal pH, inhibiting lipase activity and precipitating bile salts.
• Impaired bile salt reabsorption in the distal ileum reduces fat emulsification.
• Intestinal mucosal diseases also contribute to fat malabsorption.

Tropical Sprue

  Tropical sprue involves structural damage to enterocytes and reduced microvilli. This decreases absorptive surface area and leads to defective lipid absorption.

Absorption of Water and Electrolytes

Absorption of Water

• The gastrointestinal tract handles a large daily water load from intake and secretions.
• More than 99 percent of water is reabsorbed, with only about 100 milliliters lost in stool.
• Minimal water absorption occurs in the mouth, esophagus, and stomach.
• In the duodenum, hyperosmotic contents draw water from blood into the lumen.
• As nutrients are absorbed in the small intestine, luminal osmolality decreases progressively.
• Water absorption occurs mainly in the jejunum, ileum, and colon. It follows osmotic gradients, moving from lumen to blood where osmolality is relatively constant.
• Absorption of water is closely linked to electrolyte transport, especially sodium and chloride.
• Active sodium absorption creates an osmotic gradient that facilitates water movement.
• In the colon, water can be absorbed even against a gradient by standing gradient osmosis.
• These mechanisms maintain fluid balance and prevent excessive fluid loss.

Absorption of Sodium

• Sodium absorption occurs throughout the intestine, especially in the small intestine and colon. It is mainly a secondary active transport driven by the sodium–potassium pump on the basolateral membrane. This pump lowers intracellular sodium, allowing sodium entry from the lumen.
• Sodium enters enterocytes via facilitated diffusion and through sodium–glucose cotransport.

Physiological Importance

Sodium and glucose absorption enhance each other. This principle is used in oral rehydration therapy, which contains glucose and sodium to improve fluid and electrolyte uptake during dehydration.

Absorption of Other Nutrients

Absorption of Potassium

• Potassium absorption occurs mainly in the jejunum and ileum by passive diffusion.
• The average daily intake is about 4 grams.
• As water is absorbed in the intestine, luminal potassium concentration increases. This creates a gradient that favors potassium diffusion into enterocytes.
• There is no significant active transport mechanism for potassium in the intestine.
• In the colon, potassium can be both absorbed and secreted.
• Hypokalemia may lead to serious cardiac disturbances and requires prompt correction.

Absorption of Cl– and HCO3–

• Chloride and bicarbonate are key ions involved in intestinal fluid balance.
• Bicarbonate is secreted mainly into the duodenum with pancreatic juice, while chloride is present in gastric and intestinal secretions.
• Both ions are largely reabsorbed in the jejunum.
• In the ileum, bicarbonate is secreted and chloride is absorbed.
• In the colon, chloride absorption occurs through specialized chloride channels.

Physiological Significance

• In secretory diarrhea, such as cholera, chloride secretion into the lumen is markedly increased.
• Bacterial toxins elevate intracellular cyclic adenosine monophosphate, which activates chloride channels. This drives sodium and water secretion, producing profuse watery stools.
• In cystic fibrosis, defective chloride channels reduce chloride secretion. ·  As a result, intestinal fluid loss is less pronounced compared to normal individuals.

Absorption of Vitamins

Absorption of Water Soluble Vitamins

• Water-soluble vitamins are generally absorbed in the small intestine by diffusion and specific carriers.
• Most are taken up efficiently in the jejunum, while some require specialized mechanisms.

Absorption of Vitamin B12

• Vitamin B12 absorption is a complex, multi-step process.
• In the stomach, gastric acid and pepsin release vitamin B12 from dietary proteins. Free vitamin B12 initially binds to R protein, a salivary glycoprotein.
• In the small intestine, pancreatic enzymes degrade this complex, releasing vitamin B12. The vitamin then binds to intrinsic factor, forming a stable complex resistant to digestion. This complex travels to the terminal ileum, where specific receptors recognize it.
• Binding is facilitated by receptor components such as cubilin, enabling endocytosis into enterocytes.
• Inside the cell, vitamin B12 separates from intrinsic factor and binds to transcobalamin II. This complex enters the bloodstream and is transported mainly to the liver.
• Absorption of vitamin B12 is slow, and plasma levels rise several hours after ingestion.
• In the absence of intrinsic factor, absorption is markedly reduced, though a small passive uptake may occur.
• Impaired absorption can result from gastric, pancreatic, or ileal disorders.

• Efficient vitamin absorption is essential for hematological function, neurological integrity, and overall metabolic health.

Clinical Significance

• Intrinsic factor deficiency impairs vitamin B12 absorption in the distal ileum.
• Vitamin B12 is essential for red blood cell maturation, and its deficiency leads to pernicious anemia, a form of megaloblastic anemia.
• Autoimmune pernicious anemia results from gastric mucosal atrophy with reduced intrinsic factor secretion.
• Circulating antibodies against parietal cells are commonly present.
• Congenital intrinsic factor deficiency involves reduced intrinsic factor secretion despite normal acid production.
• Congenital vitamin B12 malabsorption occurs due to absence of ileal receptors for the intrinsic factor–vitamin B12 complex.
• Gastric secretion and intrinsic factor production remain normal in this condition.

Absorption of Fat Soluble Vitamins

• Fat-soluble vitamins include A, D, E, and K. These vitamins are mainly absorbed in the upper small intestine. Their absorption depends on normal fat digestion and bile salts.
• Disorders of pancreas or biliary system impair fat absorption. This leads to deficiency of fat-soluble vitamins and associated clinical manifestations.

Absorption of Minerals

Absorption of Calcium

• Mineral absorption occurs mainly in the small intestine and is tightly regulated according to body requirements.
• About 50 percent of calcium is absorbed, primarily in the duodenum and upper jejunum.
• Calcium absorption occurs mainly by active transport regulated by vitamin D (1,25-dihydroxycholecalciferol).
• Vitamin D increases synthesis of calcium-binding proteins in enterocytes.
• Low serum calcium enhances vitamin D activation, increasing absorption.
• High serum calcium suppresses this mechanism.
• Calcium absorption is reduced by oxalates and phosphates, which form insoluble complexes.

Absorption of Iron

• Iron absorption is limited and usually ranges from 3 to 6 percent of dietary intake. It is absorbed mainly in the upper small intestine in the ferrous form.
• Dietary iron is often in ferric form and is converted to ferrous form by gastric acid and intestinal enzymes.
• Ascorbic acid enhances iron absorption by maintaining it in a soluble form.
• Iron is absorbed as both heme and nonheme iron through specific transport mechanisms.
• Inside enterocytes, iron may be stored as ferritin or transported into blood.

• Iron released into circulation binds to transferrin for transport.
• A portion of iron remains stored and is lost when intestinal cells are shed.
• Iron balance depends on absorption rather than excretion. ·  Impaired gastric function or poor dietary intake can lead to iron deficiency.

Hemosiderosis and Hemochromatosis

• Iron absorption increases when body stores are low or during increased erythropoiesis.
• Excess iron promotes formation of ferritin in enterocytes, which may be lost with cell shedding.
• Ferritin is the main intracellular storage form of iron.
• With persistent overload, ferritin aggregates into hemosiderin within tissues.
• Accumulation of hemosiderin without major damage is termed hemosiderosis.
• Extensive deposition leads to hemochromatosis, causing tissue injury.
• Common features include skin pigmentation, liver cirrhosis, endocrine dysfunction, and diabetes.
• Pancreatic involvement may result in bronze diabetes.
• Gonadal atrophy may occur due to hormonal impairment. ·  Most body iron is present in hemoglobin, while smaller amounts are stored as ferritin and in myoglobin.