Digestion and absorption of food

Digestion and absorption of food

1. When I prayed for success , I forgot to ask for sound sleep and good digestion. - Mason Cooley  

2. Happiness: a good bank account, a good cook, and a good digestion . # Jean-Jacques Rousseau  

3. Digestion and Absorption of Food - Dr. Ganesh

4. Digestion and Absorption of Food Contents: Biological Importance Medical Importance Digestion and Absorption – General Aspects Digestion and Absorption of Carbohydrates Digestion and Absorption of Proteins Digestion and Absorption of Lipids

5. Biological Importance Digestion a chemical process – large molecules in the food are broken down to smaller molecules so that they can be absorbed

6. Biological Importance large molecules small molecules small molecules Food Digestion Absorption vitamins, minerals, monosaccharides and free amino acids BLOOD

7. Biological Importance Absorption is transport of molecules , either digested products or other small molecules which do not require digestion, from the intestinal lumen into blood across the intestinal mucosal cells .

8. Medical Importance Lactose intolerance deficiency of lactase intake of milk causes diarrhea Hartnup’s disease genetic defect in the absorption of neutral amino acids, especially tryptophan.

9. Medical Importance Steatorrhea excess fat is excreted in feces and is seen in diseases of pancreas , biliary obstruction , etc . Chronic diarrhea can cause malabsorption as seen in celiac disease, Sprue , Crohn’s disease, etc.

10. Digestion – General Aspects major foodstuffs that require digestion carbohydrates , proteins fats and oils ( triacylglycerol ) Amino acids macromolecules monosaccharides glycerol and fatty acids smaller molecules digestion

11. Digestion takes place in the aqueous medium of Site of Digestion Digestive Juices Mouth Stomach Small intestinal lumen Saliva Gastric juice Intestinal juice various digestive juices – Digestion – General Aspects Pancreatic juice, Bile

12. Digestion – General Aspects Digestion involves action of enzymes that are present in different digestive juices. All digestive enzymes are hydrolases that hydrolyze the anhydride linkages –

13. Digestion – General Aspects carbohydrates , proteins fats and oils ( triacylglycerol ) Glycosidic linkage Peptide linkage Ester linkage anhydride linkages

14. Digestion – General Aspects Bile , synthesized in liver and entering into duodenum, helps in fat digestion and neutralizes acidic stomach contents when it enters the duodenum.

15. Digestion – General Aspects Cooking hydration of polysaccharides and denaturation of proteins helps digestion of these molecules . Mastication helps in breaking down of food particles increases solubility and surface area for enzyme action. Peristalsis also important in breaking down of food particles and mixing them with enzymes.

16. Absorption– General Aspects small intestine main absorptive organ. About 90% of the ingested foodstuffs absorbed through the small intestine Considerably more water is absorbed in the large intestine, so that the contents, gradually become more solid in the colon.

17. Absorption – General Aspects Absorption of substances into mucosal cells involves the passage across the plasma membrane simple diffusion c arrier-mediated transports passive (no energy expenditure) No carrier protein facilitated transport (passive) (requiring energy expenditure) active transport

18. Absorption – General Aspects two pathways for the transport of nutrients hepatic portal blood lymphatic vessels water-soluble nutrients lipid-soluble nutrients blood thoracic duct liver absorbed by the intestine blood

19. Digestion and Absorption of Carbohydrates Contents :   Introduction Digestion of Starch Digestion of Disaccharides Absorption of Carbohydrates Clinical Significance

20. Digestion and Absorption of Carbohydrates Introduction major carbohydrates in the diet Monosaccharides Disaccharides Polysaccharides fructose (present in fruits) starch Starch - more than 50% of carbohydrates sucrose lactose maltose (present in malt, beer) Small amounts dietary fibers

21. Digestion and Absorption of Carbohydrates - Introduction Dietary fibers – cellulose, hemicellulose , pectin, lignins , etc. are indigestible. For ‘ Dietary fibers’ , see Chapters – ‘Chemistry of Carbohydrates’ and ‘Nutrition’ )

22. Digestion and Absorption of Carbohydrates - Introduction Human food also contains small amounts of pentoses , glucose, trehalose (disaccharide present in mushroom), and glycogen (present in liver in animal foods).

23. Digestion and Absorption of Carbohydrates - Introduction All enzymes of carbohydrate digestion cleave glycosidic bonds by hydrolysis. Disaccharides and polysaccharides are digested to their respective constituent monosaccharides units and absorbed in the small intestines along with free monosaccharides present in the food

24. Digestion and Absorption of Carbohydrates - Introduction The absorbed monosaccharides hepatic portal circulation liver cells systemic circulation.  

25. Fructose Sucrose Starch Lactose FOOD DIGESTION ABSORPTION (Small Intestine) Glucose Galactose Fructose Digestion and Absorption of Carbohydrates - Introduction

26. Digestion of Starch 1, 2 Starch on complete digestion yields glucose Digestion of starch takes place in mouth, small intestinal lumen and small intestinal brush border (luminal surface of intestinal mucosal cells).

27. Digestion of Starch 1, 2 The enzymes and the steps of digestion of glycogen are same as those of amylopectin component of starch as both have similar structure . Cooking hydrates the starch granules making it more susceptible to digestion.

28. Digestion of Starch 1, 2 Enzymes required for complete digestion of starch – amylase (salivary and pancreatic amylase), maltase and isomaltase . Amylase and maltase cleave  -1,4 glycosidic linkages and isomaltase cleaves  -1,6 glycosidic linkages of starch. (Amylase hydrolyzes internal  -1,4 glycosidic linkages.)

29. Digestion of Starch Starch is mainly digested by pancreatic amylase. Contribution by salivary amylase is very little since the food remains in the mouth for a very short period of time and the enzyme is inactivated by gastric HCl as it enters stomach. Maltase and isomaltase are present on the luminal surface of small intestinal epithelial cells (brush border cells).

30. Starch ( amylose and amylopectin ) Salivary amylase (in mouth) 3 Or Pancreatic amylase (in small intestinal lumen Limit dextrins 4 Small unbranched oligosaccharides (e.g., Maltose, Maltotriose , etc) Isomaltase (brush border cells) Glucose Isomaltose Maltase Maltase (brush border cells) Digestion of Starch Isomaltase (brush border cells)

31. Cl - is an activator of Salivary amylase (ptyalin) Limit dextrins derived from amylopectin component by the action of amylase and contain eight glucosyl units with one or two branches with α-1,6-glycosidic bonds. Digestion of Starch

32. Digestion of Disaccharides Major disaccharides present in the human diet are – sucrose and lactose Small amounts of free maltose (present in malt, beer) and trehalose (disaccharide present in mushroom)

33. Digestion of Disaccharides However, quantitatively the major disaccharide digested in the gut is maltose Most of the maltose in the gut is derived from digestion of starch . Isomaltose is another disaccharide derived from starch and is digested by isomaltase . (See ‘Digestion of Starch’ ).

34. Digestion of Disaccharides Disaccharidases are attached to the surface of the small intestinal brush border cells. (For ‘digestion of maltose’, see ‘Digestion of Starch’ .)

35. Digestion of Disaccharides Site of digestion of disaccharides – small intestinal brush border maltose sucrose lactose trehalose disaccharidases maltase sucrase (also called invertase ) lactase trehalase disaccharides attached to the surface of the small intestinal brush border cells

36. 2 glucose Digestion of Disaccharides maltose sucrose lactose trehalose maltase sucrase lactase trehalase glucose + galactose 2 glucose glucose + fructose Isomaltose isomaltase 2 glucose

37. Absorption of Monosaccharides The major monosaccharides resulting from carbohydrate digestion are – D-glucose, D- galactose and D-fructose. Absorption is carrier mediated . Pentoses are absorbed by simple diffusion. Monosaccharides are first transported from the lumen to the small intestinal epithelial cells and then into capillaries of portal venous system.

38. Absorption of Glucose from the small intestinal lumen into the intestinal epithelial cells by carrier mediated mechanism involving transporter proteins situated on the luminal surface of intestinal epithelial cells.  Glucose is absorbed mainly by 1) Na+-dependent transporter by secondary active transport and to a less extent by 2) Na + -independent transporter by passive transport.

39. Absorption of Glucose from the small intestinal lumen by carrier mediated mechanism involving transporter proteins 1 ) Na+-dependent transporter by secondary active transport and to a less extent by 2 ) Na + -independent transporter by passive transport into the intestinal epithelial cells

40. 1. Na + -dependent transporter (SGLT) This carrier protein carries glucose or galactose along with sodium ion from the lumen. The driving force for the Na + -dependent transport is derived from the maintenance of low intracellular levels of Na + by the action of the Na + - K + ATPase (secondary active transport).

41. 2) Na + -independent transporter Small amounts of glucose, transported by facilitated transport utilizing glucose transporter-5 (GLUT-5).

42. Transport of glucose from cells to portal venous capillaries: Glucose is transported from the intestinal epithelial cells into portal venous capillaries by glucose transporter-2 (GLUT-2). It is a uniport facilitated transport system, which is sodium independent .

43. Intestinal Epithelial Cell Absorption of Glucose Glucose Glucose Glucose GLUT-5 Intestinal Lumen Na + Na + Na + K + K + Na + -dependent transporter (SGLT) GLUT-2 Portal Capillary Blood ATP Na + –K+ ATPase ADP + Pi secondary active transport facilitated transport

44. Absorption of Glucose Galactose GLUT-5 Na + Na + Na + K + K + Na + -dependent transporter (SGLT) GLUT-2 ATP Na + –K+ ATPase ADP + Pi Galactose Galactose Fructose Fructose Fructose

45. Absorption of Other Monosaccharides Fructose facilitated trasporter GLUT-5, sharing with glucose. Galactose Na + -dependant trasporter (SGLT) secodary active transport sharing with glucose  Any pentose present in food is absorbed by simple diffusion.

46. Absorption of Other Monosaccharides Both fructose and galactose transported from the intestinal epithelial cells into portal venous capillaries by glucose transporter-2 (GLUT-2), sharing with glucose.

47. Clinical Significance Lactose Intolerance This is a common condition gastrointestinal symptoms like diarrhea , abdominal cramps and flatulence after ingestion of milk or milk-based foods

48. Clinical Significance Lactose Intolerance Cause deficiency of lactase 1 . Deficiency may be due to genetic (primary/inherited) or acquired ( secondaryto other causes) The reason for acquired lactose intolerance may be damage to intestinal epithelial cells due to colitis, gastroenteritis, alcohol consumption or sudden change into a milk-based diet.

49. Clinical Significance Lactose Intolerance Cause lactase enzyme defective at birth early onset lactose intolerance ( inherited lactase deficiency). A significant number of adults exhibit late onset lactase deficiency ( primary low lactase activity ) especially Asian-, Native- and African-Americans.

50.  Osmotic movement of water from the intestines to the lumen Biochemical basis of Clinical Manifestation Lactase deficiency Accumulation of lactose , (Produced by action of bacteria on lactose in the gut) organic acids and gases ( CO 2 and H 2 ) diarrhea Abdominal cramps flatulence

51. Lactose Intolerance Treatment avoiding milk Curd is an effective treatment, because Lactobacilli in curd contains the enzyme Lactase

52. Digestion and Absorption of Proteins Contents:   Digestion of protein – general aspects Reactions of protein digestion Absorption of amino acids Clinical significance

53. Digestion of Proteins – General Aspects Contents:   Introduction Sites of protein digestion, GIT juices and proteases Endopetidases and exopetidases Specificity of proteases Proteases - zymogen form and activation Role of HCl in protein digestion

54. Digestion of Proteins – General Aspects   Introduction Digestion of dietary proteins – hydrolysis of peptide bonds catalyzed by a group of enzymes called proteases or peptidases in the gastrointestinal tract   Complete digestion of proteins yields amino acids .

55. Digestion of Proteins – General Aspects   Introduction Dietary proteins denatured on cooking and therefore, cooked proteins more easily digested 1 . During the process of denaturation unfolding of protein molecule takes place and thus peptide bonds become more accessible for enzyme action .

56. Digestion of Proteins – General Aspects Sites of Protein Digestion, GIT Juices and Proteases Protein digestion takes place in stomach and intestinal lumen . Enzymes of protein digestion are secreted in gastric juice , pancreatic juice and intestinal juice .

57. Digestion of Proteins – General Aspects GIT Juices and Proteases GIT Juices Gastric juice Pancreatic Juice Intestinal Juice Proteases Present Pepsin ( chief cells of stomach ), Rennin 2 Trypsin Chymotrypsin Elastase Carboxypeptidases Aminopeptidases Dipeptidases Tripeptidases

58. Rennin Rennin , a protease , active in infants and involved in curdling of milk . Rennin denatures casein of milk to paracasein irreversibly , which then is acted upon by pepsin .

59. Endopeptidases and Exopeptidases hydrolyze terminal bond releasing one amino acid at a time. carboxy terminal amino terminal Pepsin Trypsin Chymotrypsin Elastase Carboxypeptidases Aminopeptidases hydrolyze peptide bonds in the interior of the protein chain to cleave the protein molecule into more than one smaller polypeptides and peptides.

60. Digestion of Proteins – General Aspects Specificity of Proteases Endopeptidases hydrolyze specific peptide bonds in protein molecules. Specificity differs from one protease to another 3 .

61. Enzyme Hydrolysis of petide bonds formed by carboxyl groups of Pepsin Phe , Tyr, Trp , Met Trypsin Arg, Lys (basic amino acids) Chymotrypsin Phe , Tyr, Trp , Val, Leu ( Aromatic, uncharged amino acids ) Elastase Ala, Gly , Ser (small amino acid residues) Specificity of Proteases

62. Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Proteolytic enzymes are secreted as inactive zymogens/ proenzymes , which are converted to their active form in the intestinal lumen 4 .

63. Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Activation of enzymes involves cleavage of small peptides so that active sites are exposed . prevents auto-digestion of the secretory acini .

64. Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Active Enzyme Inactive Zymogen Pepsin Trypsinogen Pepsinogen Trypsin Chymotrypsin Chymotrypsinogen Elastase Proelastase Carboxypeptidase Procarboxypeptidase

65. Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Pepsinogen Pepsin (Parietal cells of stomach) HCl Autoactivation Trypsin Trypsinogen Enterokinase Autoactivation (a protease present on the intestinal mucosal membranes)

66. Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Chymotrypsinogen Chymotrypsin Proelastase Elastase Procarboxypeptidase Carboxypeptidase Trypsin

67. Activation of pepsinogen Denaturation of dietary proteins and Providing optimum pH (pH 2 to 3) for the action of pepsin . ( HCl also kills microorganisms present in food).   Digestion of Proteins – General Aspects Role of HCl in Protein Digestion

68. Reactions of Protein Digestion (In Stomach and Small Intestinal Lumen ) Dietary Proteins In Stomach Pepsin HCl Gastric Juice Proteoses + Peptones (polypeptides and smaller polypeptides) In Small Intestinal Lumen Trypsin Chymotrypsin Elastase Pancreatic Juice Small Polypeptides + Peptides

69. Dietary Proteins In Stomach Pepsin HCl Gastric Juice Proteoses + Peptones (polypeptides and smaller polypeptides) In Small Intestinal Lumen Trypsin Chymotrypsin Elastase Pancreatic Juice Small Polypeptides + Peptides

70. HCl Proteoses + Peptones (polypeptides and smaller polypeptides) In Small Intestinal Lumen Trypsin Chymotrypsin Elastase Pancreatic Juice Small Polypeptides + Peptides Carboxypeptidases Pancreatic Juice Aminopeptidases Dipeptidases Intestinal Juice Amino acids In Small Intestinal Lumen

71. Absorption of Amino Acids absorbed from the intestine into portal blood .   transported by a number of carriers many by secondary active transport – Na + -dependent carriers similar to glucose transporter system.

72. Absorption of Amino Acids Different Na + -dependent carriers are: Neutral amino acid carrier Phenylalanine and methionine carrier Carrier specific for imino acids ( proline and hydroxy proline ) There are also Na + -independent carriers specializing in the transport of Neutral and lipophilic amino acids (e.g. Phe , Leu ) Cationic amino acids (e.g. Lys)

73. Clinical Significance Allergy to certain food proteins (milk, fish) believed to result from absorption of partially digested proteins.

74. Clinical Significance Defect in non-tropical sprue is located within the mucosal cells of the intestine and permits the polypeptides (resulting from the peptic and tryptic digestion of gluten , the principal protein of wheat ) to be absorbed into the circulation and thus elicit the production of antibodies.

75. Clinical Significance Defect in the intestinal amino acid transport systems seen in -- Hartnup’s disease, [defect in intestinal neutral amino acid ( Trp ) carrier], I -- iminoglycinuria , cystinuria etc.

76. Clinical Significance Acute pancreatitis (acute inflammation of pancreas) caused by autodigestion of pancreas by its proteolytic enzymes a life threatening disorder. Autodigestion is due to unusual conversion of proenzymes into active enzymes by trypsin .

77. Digestion of Fat ( Triacylglycerols ) Contents Digestion of fat Digestion of other lipids Absorption of lipids Clinical significance

78. Digestion of Fat ( Triacylglycerols ) main site Small intestinal lumen Hydrolysis of ester bonds Fatty acids Glycerol Monoacyl glycerols (MAGs) Lipases - Pancreatic lipase main enzyme for digestion of most of the fat Colipase a protein secreted by pancreas cofactor other lipases lingual lipase, gastric lipase and intestinal lipase their contribution is negligible Bile salts help fat digestion by emulsifying fat Fat

79. Role of Bile Salts in Fat Digestion Bile salts lower the surface tension emulsify fat in the intestine. Intestinal peristalsis also helps in this. present in the bile Emulsification increases the surface area of fat droplets enabling more enzyme (lipase) molecules to act and thus speeding up digestion

80. Reactions of Digestion of Fat ( Triacylglycerol ) 1 Colipase Pancreatic Juice Fatty Acid Triacylglycerol (Fat) Diacylglycerol (DAG) Lipase Colipase Lipase Fatty Acid Monoacylglycerol (MAG) Glycerol Colipase Lipase Fatty Acid

81. Reactions of Digestion of Fat ( Triacylglycerol ) 1 Digestion of fat requires another enzyme also an isomerase which isomerzes 2-MAG into 1-MAG , as lipase cannot hydrolyze 2-MAG .

82. The major end products of digestion of fat are – monoacylglycerols (MAGs), glycerol and fatty acids Reactions of Digestion of Fat ( Triacylglycerol ) 1

83. Digestion of Other Lipids Pancreatic secretion also contains cholesterol esterase phospholipase A 2 cholesterol ester cholesterol Fatty acid lysophospholipid phospholipid Fatty acid

84. Water-insoluble Digested Products of Lipids Water-soluble Absorption of Digested Products of Lipids Monoacylglycerols Long-chain fatty acids Cholesterol Micelles Fat-soluble vitamins Mixed micelles Monoacylglycerols Long-chain fatty acids Cholesterol Fat-soluble vitamins Phospholipids Apolipoproteins Triacylglycerol Chylomicron Chylomicron LYMPHATIC VESSELS Glycerol (SMCFA) Short- and Medium-chain fatty acids(SMCFA) SMALL INTESTINAL LUMEN PORTAL BLOOD Glycerol Intestinal Epithelial Cell

85. Absorption of Digested Products of Lipids Normally over 98% of the dietary lipid is absorbed.

86. Clinical Significance Steatorrhea   Chyluria and Chylothorax When daily excretion of fa t in feces more than 6g per day may be due to defective digestion or defective absorption of fat. - Defective digestion may be due to absence of or deficiency of pancreatic lipase as in chronic diseases of pancreas or surgical removal of pancreas . bile salt  

87. Clinical Significance Steatorrhea Defective absorption of fat occurs when bile salts do not enter the intestine as in biliary obstruction (e.g. due to biliary stone ) - Defective absorption may also be due to malabsorptive diseases e.g. celiac disease , Sprue , Crohn’s disease , etc or surgical removal of large lengths of the intestine .

88. MCQS on Digestion and Absorption 1. All of the below are true about digestion, EXCEPT, All digestive enzymes are hydrolases . Digestion ensures the absorption of nutrients. Sites of digestion in the GIT are mouth, stomach, and the lumen of small and the large intestines. Vitamins, minerals, monosaccharides and free amino acids do not need digestion.

89. 2. The anhydride linkages that are broken during digestion are: glycosidic , peptide and ester linkages of carbohydrates, proteins and fats, respectively. ester, peptide and glycosidic linkages of carbohydrates, proteins and fats, respectively. glycosidic , peptide and ester linkages of carbohydrates, fats and proteins, respectively. peptide, glycosidic and ester linkages of carbohydrates, proteins and fats, respectively.

90. 3. Factors, other than enzymes that help in digestion are the following, EXCEPT, Cooking Exercise Mastication Peristalsis

91. 4. All the following are true about absorption, EXCEPT, Considerably more water is absorbed in the large intestine than in the small intestines. Absorption of all substances require carrier-mediated transports. Water-soluble nutrients are absorbed through hepatic portal system. Lipid-soluble nutrients are absorbed through the lymphatic vessels .

92. 5. The full set of enzymes required for complete digestion of starch are – amylase, maltase and isomaltase . amylase, maltase and sucrase . amylase, maltase, isomaltase and sucrase . amylase, sucrase and isomaltase .  

93. 6. Contribution by salivary amylase in digestion of starch is very little because: activity of the enzyme is very low. the food remains in the mouth for a very short period of time. the food remains in the mouth for a very short period of time and the enzyme is inactivated by gastric HCl . the enzyme is inactivated by gastric HCl .

94. 7. Which of the following is FALSE about digestion of starch? The enzymes and the steps of digestion of glycogen are same as those of amylopectin component of starch. Cooking hydrates the starch granules making it more susceptible to digestion. Cl - is an activator of salivary amylase. Limit dextrins are derived from amylose component of starch by the action of amylase.

95. 8. Quantitatively the major disaccharide digested in the gut is sucrose maltose lactose isomaltose

96. 9. The major monosaccharides resulting from carbohydrate digestion are glucose, galactose and ribose. glucose, mannose and fructose. glucose, ribose and fructose. glucose, galactose and fructose.

97. 10. Glucose is absorbed mainly by simple diffusion Na + -dependent transporter Na + -independent transporter passive transport  

98. 11. Enzymes of protein digestion are secreted in gastric juice, pancreatic juice and intestinal juice. saliva, gastric juice and pancreatic juice. saliva, gastric juice and intestinal juice. saliva, gastric juice and intestinal juice.

99. 12. Proteases secreted in the pancreatic juice are pepsin, trypsin , chymotrypsin and carboxypeptidases . trypsin , chymotrypsin , elastase and carboxypeptidases . trypsin , chymotrypsin , elastase and aminopeptidases . trypsin , chymotrypsin , aminopeptidases and carboxypeptidases .

100. 13. HCl has following roles in protein digestion, EXCEPT. kills microorganisms present in food activation of pepsinogen denaturation of food proteins providing optimum pH for the action of pepsin

101. 14. Intestinal absorption of amino acids involves all of the following, EXCEPT, simple diffusion Na + -dependent transporter Na + -independent transporter passive transport

102. 15. Which of the following is false about action of bile salts? They help both in digestion of fat and absorption of digested products of lipids. They lower the surface tension and emulsify fat in the intestine. They stimulate pancreatic secretion. Emulsification increases the surface area of the fat droplets.

103. 16. Digestion of fat produces: fatty acids, glycerol and diacyl glycerols . fatty acids and glycerol. glycerol and monoacyl glycerols . fatty acids, glycerol and monoacyl glycerols .

104. 17. Lipases other than pancreatic lipase present in the human GIT are: salivary lipase, gastric lipase and intestinal lipase lingual lipase, gastric lipase and intestinal lipase lingual lipase, gastric lipase and hepatic lipase gastric lipase, biliary lipase and intestinal lipase

105. 18. Steatorrhea is a condition when daily excretion of fat in feces is more than: 6g 8g 10g 12g