Summary: Food Digestion: Nutrient Breakdown And Absorption

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  • 1 NBA1 (molecular aspects of) protein digestion and absorption

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  • Which proteins can be found in our GI tract?

    Dietary proteins 

    • different sources (meat, dairy, legumes, nuts etc)

    Endogenous proteins 
    • salivary and gastric secretions 
    • pancreatic and bile secretions 
    • small intestinal secretions 
    • sloughed epithelial cells 
    • mucus 
    • microbial protein 

  • Which enzymes are there for the protein digestion

    Endopeptidases - internal bonds 
    Exopeptidases - amino acid from the carboxy- or amino- terminus 

    • Pepsin - endo
    • Trypsin - endo 
    • Chymotrypsin - endo 
    • Elastase - endo
    • Carboxypeptidase A - exo
    • carboxypeptidase B - exo
    • Aminopeptidases - exo
  • But .... Enzyme digestion is not exactly straight forward, why?

    • Specificity for a cleavage site 
    • but not all cleavage sites on a protein are always utilised by an enzyme 
    • peptide composition from hydrolysis is difficult to predict, it depends on selectivity of enzyme towards cleavage site 

    spontaneous non-enzymatic breakdown 
    • whey protein hydrolysate: 13/77 identified peptides from a-specific cleavage, even in mild conditions 
  • What is the rol of PEPT1 in the protein absorption?

    Peptide transport protein PEPT1

    • Transport/absorption of di- & tri-peptides
    • Very broad substrate specificity
      • >400 dipeptides and >8000 tripeptides
    • Low affinity
    • Secondary active transport (no direct ATP)

    NHE3 is important for the proton gradient --> Na+ wil go into the cell and H+ will go out of the cell
  • What is the role of the AA transport system for protein absorption?

    • Many different transporters
    • Transporters have:
      • stereospecificity --> L-amino acids
      • substrate specificity --> multiple amino acids
      • overlap in specificity --> multiple transporters per AA
    • Mechanism:
      • facilitated diffusion
      • secondary active transport

  • Which inherited diseases influence the absorption?

    Hartnup disorder:
    • Defective intestinal transport: B0AT1 (neutral AA)
    • Effect: poor nutrition results in pellagra-like skin changes, cerebellar ataxia, and psychiatric abnormalities
    • Treatment: sometimes only a high protein diet!

    • Defective intestinal transport: rBAT/b0,+AT (cationic AA)
    • Effect: kidney stones made of Sistine (transporter also absent in kidneys for reabsorption)
    • Treatment: multiple options to prevent kidney stones
  • Which parts of the intestine are involved in the absorption (along the intestine)

    Duodenum  (4)
    Jejunum  (1)
    Ileum (2) 
    Proximal colon (5)
    Distal colon  (3)
  • What happens in the after protein absorption - in enterocytes?

    Di & tripeptides --> AA
    • by intracellular peptidases

    Free amino acids (AA)
    • metabolism (in enterocyte)
      • glutamine, glutamate and aspartate oxidised
      • protein synthesis
    • transport over basolateral membrane --> blood

  • What happens after protein absorption in the circulation?

    • Release of dietary protein-derived amino acids in circulation
    • From intestinal capillaries to peripheral circulation
      • via portaal vein to liver
      • first pass metabolism - splanchnic amino acid extraction ± gut + liver

    • Fast vs. Slow protein concept
      • whey vs casein
        • whey has a high increase and after that a decrease
        • casein stays 'stable' 
        • whey is better than casein
      • influence anabolic effect
  • What are the key points in the fast vs slow protein concept

    • Speed of protein digestion depends not only on proteins sources, but also on food matrix, texture, technological processes and associated calories
    • Fast digestive proteins are efficient for improving muscle mass and muscle function in older individuals through the postprandial changes in amino acid availability
    • The anabolic effect of fast digestive proteins, especially whey protein, is amplified by physical exercise

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