T1 L7: Macronutrients

• What is the influx of TAG into adipose tissue largerly mediated by?

  tissue lipoprotein lipase LPL

  upregulated in presence of insulin and fatty acids during fasting
• What are the functions of dietary fat?

  TAG primary energy substrate stored in adipose tissue

  carries essential fat soluble vitamins A, D, E, K

  cell membrane component

  precursor of steroid hormones & vitamin D
• Via what does CHO digestion occur?

  salivary amylase (limited activity)
• How does fat digestion occur in stomach?

  gastric lipase secreted by gastric cells in fundic mucosa
• What is gastric lipase?

  an acid enzyme that breaks down fat and does not require bile acid or colipase for optimal functioning
• How does protein digestion occur in stomach?

  chief cells produce pepsinogen converted to pepsin in presence of HCl

  proteins -> peptides

  acid environment denatures proteins and reduces bacterial load
• How does liver aid in fat digestion?

  bile acids from liver - cholic and chenodeoxycolic acid

  amphipathic - act as detergent as have both hydrophobic and hydrophilic properties

  solubilise lipids to form mixed micelles
• How does pancreas aid in fat digestion?

  secretes pancreatic lipase & cofactor pro-colipase into small intestine

  • pancreatic lipase hydrolyses TG -> monoglycerol, fatty acids, glycerol
  • pro-colipase activated by trypsin to form co-lipase in intestinal lumen
• What is the function of colipase?

  stabilise pancreatic lipase, increasing its efficiency
• How does pancreas aid in carbohydrate digestion?

  secretes pancreatic alpha amylase; begin breakdown of starch and glycogen into maltose and glucose

  (starch: 30% amylose, 70% amylopectin)
• How does pancreas aid in protein digestion?

  secrete pancreatic proteases:

  • trypsin
  • chymotrypsin
• How does pancreatic protease secretion occur?

  synthesise trypsinogen & chymotrypsinogen -> package with secretory vesicles + trypsin inhibitor (to prevent activation in pancreas) -> duodenum -> enterokinase -> activation of proteases:

  • trypsinogen -> trypsin
  • chymotrypsinogen -> chymotrypsin
• What is the link between phenylalaine, phenylalanine hydroxylase and tyrosine?

  phenylalanine (essential) is the essential precursor for tyrosine (conditionally non-essential)

  if individual lacks phenylalanine hydroxylase enzyme, phenylalanine needs to be excluded from diet

  in that case tyrosine becomes essential in diet
• Glutamine is depleted in hyper-catabolic states (sepsis, burns, COVID-19).
• How does disaccharide digestion occur in the small intestine?

  brush border enzymes:

  • gluco-amylase, alpha-dextrinase, sucrase, maltase

  form monosaccharides glucose, fructose, galactose
• Which glucose transporter transports fructose into small intestine cells?

  GLUT5
• How does fat absorption occur in the small intestine?

  water soluble micelles release fatty acids and monoglycerol

  diffuse across apical membrane + lipid transport mechanisms
• What are the 2 important transporters for amino acid absorption in the small intestine?

  Human peptide transporter 1 (PEPT1)

  sodium-hydrogen exchanger 3 (NHE3)
• How is sodium absorbed in the large intestine?

  various methods:

  • Sodium-hydrogen antiporter on luminal membrane
  • epithelial sodium channels

  enhanced by absorption of short-chain fatty acids via specialised symporters
• How does chloride/bicarbonate transport work in the large intestine?

  electrochemical gradient produced by movement of sodium into plasma

  allows absorption of chloride

  chloride ions exchanged for bicarbonate ions

  causing net bicarbonate secretion
• How is water absorbed in the large intestine?

  absorption of electrolytes creates osmotic gradient

  water diffuses by osmosis

  aldosterone increases net absorption of water and electrolytes
• How does aldosterone increase the net absorption of water and electrolytes?

  stimulate baseolateral sodium-potassium ATPase
• How is potassium absorbed in the large intestine?

  absorption of water -> high concentration of potassium in lumen -> electrochemical gradient -> net movement of potassium into plasma

  secretion/absorption depends on net concentration

  secretion usually occurs when lumina K+ concentration <25mM
• Which vitamins and fats are absorbed in the large intestine and how?

  short chain fatty acids

  crucial B vitamins (eg B6 & B12)

  vitamin K (for blood clotting): produced by digestion of chyme by commensal microbial flora of colon
• What are NSP?

  non-strach polysaccharides (fibre) and most oligosaccharides are not digested & absorbed in small intestine

  insoluble make up cellulose roughage in diet

  soluble fibres (pectin/gum) are fermented by bacteria in colon
• What does microbial fermentation lead to the production of?

  gas

  short chain fatty acids

  enhance microbial growth