BCHM

• Glycemic Index


  The increase in blood glucose after a test dose of a carbohydrate compared with that after an equivalent amount of glucose (as glucose or from a reference starchy food)
• Glucose and galactose have an index of 1 (or 100%), as do lactose, maltose, isomaltose, and trehalose, which give rise to these monosaccharides on hydrolysis
• Fructose, sucrose and the sugar alcohols are absorbed less rapidly and have a lower glycemic index
• Starch varies between near 1 and near 0 as a result of variable rates of hydrolysis, and that of nonstarch polysaccharides is 0
• Foods that have a low glycemic index are considered to be more beneficial since they cause less fluctuation in insulin secretion
• Resistant starch and nonstarch polysaccharides provide substrates for bacterial fermentation in the large intestine, and the resultant butyrate and other short chain FAs provide a significant source of fuel for intestinal enterocytes
• Carbohydrate digestion in the mouth

  1. Salivary alpha amylase starts digestion

  2. Amylase is an endoglucosidase that hydrolyzes alpha 1,4 glycosidic bonds in polysaccharide chain

  3. Little activity for 1,4 bond at the non reducing end of a chain

  4. Has NO activity on 1,6 bond branch
• Alpha dextrins, shortened polysaccharides, are initially formed
• Amylases


  Catalyze the hydrolysis of starch, including salivary and pancreatic amylases which catalyze random hydrolysis of α(1 → 4) glycoside bonds to yield dextrins, then a mixture of glucose, maltose and maltotriose and small branched dextrins
• In the stomach, alpha amylase is inactivated by acidity due to HCL content
• Carbohydrate digestion in the small intestine

  1. Pancreatic secretion is released, including bicarbonate which neutralizes acidic gastric content and pancreatic alpha amylase

  2. Pancreatic amylase hydrolyzes dextrins to oligosaccharide (limit dextrin), trisaccharide (maltotriose) and disaccharides (maltose, isomaltose, lactose and sucrose)
• Carbohydrate digestion in the intestinal brush border

  1. Alpha glycosidases cleave oligosaccharides from non reducing end, cleaving alpha 1,4 linkage of maltose to two glucose

  2. Alpha dextrinase cleaves alpha 1,6 linkages of branched oligosaccharides

  3. Disaccharidases (sucrase, lactase) convert lactose and sucrose to monosaccharides
• Disaccharidases


  Located on the brush border of the intestinal mucosal cells, where the resultant monosaccharides and those arising from the diet are absorbed. Includes maltase, lactase, trehalase, and sucrase-isomaltase
• Congenital deficiency of sucrase isomaltase occurs among the Inuit, leading to sucrose intolerance, with persistent diarrhea and failure to thrive when the diet contains sucrose
• Dietary fibers


  Portion of diet that is resistant to digestion by human digestives enzymes (for beta 1,4 linkage). Includes soluble fibers like pectin, mucilages and gums, and insoluble fibers like cellulose, hemicellulose and lignin
• Benefits of dietary fibers

  • Increase bowel motility
  • Delays gastric emptying by giving sensation of fullness
  • Lowers cholesterol by increasing fecal bile acid excretion
• Lactose intolerance


  Condition of abdominal pain, nausea and flatulence (sometimes diarrhea) after ingestion of food with lactose like dairy products, often caused by low level of lactase and intestinal injury
• Congenital deficiency of lactase occurs rarely in infants, leading to lactose intolerance and failure to thrive when fed on breast milk or normal infant formula
• In most mammals, and most human beings, lactase activity begins to fall after weaning and is almost completely lost by late adolescence, leading to lactose intolerance
• Lactose remains in the intestinal lumen, where it is a substrate for bacterial fermentation to lactate, resulting in abdominal discomfort and diarrhea after consumption of relatively large amounts
• Carbohydrate absorption


  1. Glucose and galactose are transported into small intestine absorptive epithelial cells by protein mediated Na+ dependent glucose active transport and facilitative transporters

  2. Fructose absorbed via facilitative diffusion
• Na+ dependent glucose transporters

  Located at luminal side of the intestinal absorptive cells, transport of glucose is promoted by secondary active transport via cotransport of glucose and sodium
• Glucose follows Na+ across the luminal epithelial membrane, and the Na+ gradient that drives this symport is established by Na+-K+ exchange at the basolateral membrane facing the extracellular fluid compartment via the action of the Na+-K+-ATPase
• Glucose at high concentration within the cell moves "downhill" into the extracellular fluid by facilitated diffusion via a uniport mechanism GLUT2
• The sodium-glucose symport actually carries 2 Na+ for each glucose, and low intracellular Na+ concentration is maintained by Na+, K+ ATPase
• Facilitative glucose transport


  Located on serosal and luminal sides of cell, move glucose from high concentration to low concentration without energy expenditure, via GLUT 1 -V transporters
• Glucose and galactose have an index of 1 (or 100%), same with lactose, maltose, isomaltose, and trehalose which give rise to these monosaccharides on hydrolysis
• Fructose and the sugar alcohols are absorbed less rapidly and have a lower glycemic index, same with sucrose
• The glycemic index of starch varies between near 1 (or 100%) and near 0 as a result of variable rates of hydrolysis, and the glycemic index of nonstarch polysaccharides is 0
• Resistant starch and nonstarch polysaccharides provide substrates for bacterial fermentation in the large intestine, and the resultant butyrate and other short-chain fatty acids provide a significant source of fuel for intestinal enterocytes
• Triacylglycerols (TAGs)


  Major fat in human diet, they contain glycerol backbone to which three fatty acids (FAs) are esterified
• Dietary lipid digestion


  1. Limited digestion in mouth (lingual lipase) and stomach (gastric lipase) due to low solubility and preference for short and medium chain FAs

  2. In small intestine, bile salts emulsify fats to increase surface area for pancreatic lipase and colipase to bind and digest, hydrolyzing TAGs at positions 1 and 3 to yield free FAs and 2 monoacylglycerol (2 MAG)

  3. Other pancreatic enzymes include phospholipase A2 and cholesterol esterase
• Absorption of lipid digestion products

  1. FAs and 2 MAGs with bile salts are packaged into micelles which travel to microvilli on surface of intestinal epithelial cells, where they are absorbed by diffusion, while bile salts are left behind and reabsorbed in ileum

  2. In intestinal epithelial cells, FAs and 2 MAGs are condensed in SE
• Secretin


  Signals liver, pancreas, and certain intestinal cells to secrete bicarbonate, which raises pH
• Lipid digestion


  1. Fats are emulsified by bile salts which increase surface area for pancreatic lipase and colipase to bind and digest

  2. Bile salts inhibit access of lipase to TAG

  3. Colipase relieves inhibition and allow TAG to enter active site of lipase

  4. Pancreatic lipase hydrolyzes TAG at positions 1 and 3

  5. Product: Free FAs and 2 monoacylglycerol (2 MAG)
• Other enzymes from pancreas

  • Phospholipase A2
  • Cholesterol esterase
• Phospholipase A2


  Hydrolyzes phospholipids, yields FAs and lysophospholipid
• Cholesterol esterase


  Hydrolyzes cholesteryl esters, forms free FAs and cholesterol
• Lipid absorption


  1. FAs and 2 MAGs with by bile salts are packaged into micelles

  2. Micelles travel to microvilli on surface of intestinal epithelial cells

  3. Where they are absorbed by diffusion

  4. Bile salts are left behind and reabsorbed in ileum

  5. In intestinal epithelial cells, FAs and 2 MAGs are condensed in SER to form TAG

  6. Chylomicrons take up and transport TAGs, apoprotein, phospholipids, and cholesterol

  7. Chylomicrons are secreted into chyle of lymphatic system and enter blood through thoracic duct
• Short and medium chain FAs do NOT REQUIRE BILE SALTS, they are directly absorbed in intestinal cells, enter portal blood and transported to liver BOUND to SERUM ALBUMIN