19 Nutrition of Carbohydrates, Proteins, and Lipids (2)

Question 1

Protein Digestion and Absorption

• although an adult may ingest about 100 g of protein per day, the quantity digested and absorbed/
• Desquamation of mucosal cells contributes/
• the enzymes in the digestive juices contribute/
• The nitrogen remaining in daily fecal material is equivalent to/
• protein digested and absorbed each day

Answer 1

• In terms of the quantity of protein digested by the gastrointestinal tract each day, although an adult may ingest about 100 g of protein per day, the quantity digested and absorbed is much greater.
• Desquamation of mucosal cells contributes 35 g
• the enzymes in the digestive juices contribute a further 35 g per day.
• The nitrogen remaining in daily fecal material is equivalent to about 10 g,
• therefore, 160 g of protein are digested and absorbed each day.

Question 2

Protein Digestion and Absorption:
Stomach (p.39)

• Protein digestion first begins in/
• In order to initiate protein digestion, first it must be/
• Exposure to an acidic environment will/
• The stomach therefore provides/
• The gastric chief cells secrete/
• the parietal cells secrete/
• The bicarbonate buffered mucus secretions of the stomach surface cells become a necessity to/

Answer 2

• Protein digestion first begins in the stomach.
  
  • The stomach employs a complementary system of digestion.
• In order to initiate protein digestion, first it must be denatured.
• Exposure to an acidic environment will uncoil 4o and 3o structure.
• The stomach therefore provides
  
  • the acidic compartment
  • a peptidase, pepsin, that is enzymatically active at an acidic pH.
• The gastric chief cells secrete the inactive proenzyme pepsinogen,
• the parietal cells secrete HCl.
• The bicarbonate buffered mucus secretions of the stomach surface cells become a necessity to prevent self-digestion.

Question 3

Protein Digestion and Absorption:
Pancreas (p.40)

• Pancreatic proenzymes are emptied into/
  
  • Pancreatic proenzymes are emptied into/
  • Trypsin then activates/
• Pancreatic secretions contain
  
  • Endopeptidases
    
    • Trypsin
    • Chymotrypsin
    • Elastase
  • Exopeptidases
    
    • Carboxypeptidases A and B
    • Carboxypeptidase A
    • Carboxypeptidase B

Answer 3

• Pancreatic proenzymes are emptied into the duodenum.
  
  • They must encounter enterokinase, an intestinal brush border enzyme that cleaves trypsinogen.
  • Trypsin then activates the remainder of the pancreatic enzymes, as well as the brush border enzymes.
• Pancreatic secretions contain
  
  • Endopeptidases
    
    • Trypsin: cleaves peptide bonds on the carboxyl side of basic amino acids (lysine and arginine)
    • Chymotrypsin: cleaves peptide bonds on the carboxyl side of aromatic amino acids (tryosine, phenylalanine and tryptophan)
    • Elastase: cleaves peptide bonds on the carboxyl side of aliphatic amino acids (alanine, leucin, glycine, valine, isoleucine)
  • Exopeptidases
    
    • Carboxypeptidases A and B: zinc-containing metallo-enzymes that remove single amino acids from the carboxyl-terminal ends of proteins and peptides.
    • Carboxypeptidase A: polypeptides with free carboxyl groups are cleaved to lower peptides and aromatic amino acids
    • Carboxypeptidase B: polypeptides with free carboxyl groups are cleaved to lower peptides and dibasic amino acids

Question 4

Protein Digestion and Absorption:
Small Intestine (p.41-44)

• Peptidases within the enterocyte brush-border glycocalyx/
• About 20 peptidases have been identified, acting as/
• Also present in the lateral edges of the microvilli are/
• Imported di- and tri-peptides are digested in/
  
  • Ninety percent of absorbed amino acids are/
  • The remaining 10% are used for/
• Protein digestion and absorption in the small intestine/

Answer 4

• Peptidases within the enterocyte brush-border glycocalyx hydrolyze the oligo peptides generated by intraluminal digestion into free amino acid and di- and tri-peptides.
• About 20 peptidases have been identified, acting as endopeptidases, exopeptidases, aminopeptidases and carboxypeptidase.
• Also present in the lateral edges of the microvilli are the transport proteins for amino acid assimilation, often coupled with Na+ import.
• Imported di- and tri-peptides are digested in lysosomes and exported as single amino acids.
  
  • Ninety percent of absorbed amino acids are released into the portal blood.
  • The remaining 10% are used for local protein synthesis and as the major respiratory fuels for the small intestine (especially glutamine, glutamate and aspartate).
• Protein digestion and absorption in the small intestine
  
  • 1. brush-border membrane peptidases
  • 2. brush-border membrane amino acid transporters
  • 3. brush-border membrane di- and tripeptide transporters
  • 4. intracellular peptidases
  • 5.basolateral-membrane amino acid carriers
  • 6. basolateral membrane di- and tripeptide carriers

Question 5

Protein Digestion and Absorption:
Portal Vein and Liver (p.45-46)

• Assimilated amino acids are carried away by/
• Amino acids are absorbed first by/
• The liver synthesizes/
• The liver also deaminates amino acids to form/

Answer 5

• Assimilated amino acids are carried away by veins that coalesce to become the hepatic portal vein.
• Amino acids are absorbed first by liver transporters.
• The liver synthesizes all the major plasma proteins (albumin, prealbumin, transferrin, apoproteins, fibrinogen, prothrombin and alpha & beta globulins).
• The liver also deaminates amino acids to form ammonia (NH3), which it then combines with glutamine and converts to the less toxic form, urea.

Question 6

Utilization of Protein as an Energy Source (p.47)

• If all the components of the body are considered strictly for their caloric value, in the average 70 kg man, the break down would be:
• the first to be mobilized
  
  • However, after simply an overnight fast, these/
• In mild starvation, liver carbohydrate reserves/
  
  • This limits energy supplies to/
• In the early post absorptive period, 8-16 hours after eating, glucagon levels/
• Although fatty acids are the highest caloric yield per weight, they cannot/
  
  • This means that those organs dependant on glucose metabolism: the brain, red blood cells, peripheral nerves and the renal medulla, must rely on/
• As there are no true reserve stores of protein, only half of the potential caloric value/
  
  • Depletion of total body protein below 50%/
• The first organs to manifest inadequate protein/
• the most common cause of death in an epidemic of starvation is typically/
• A nutritional evaluation for protein status will include/

Answer 6

• If all the components of the body are considered strictly for their caloric value, in the average 70 kg man, the break down would be:
  
  • Triglycerides (adipose) 135,000 Calories
  • Protein (viscera & muscles) 54,000 Calories
  • Carbohydrate (liver & muscle) 1,200 Calories
• the first to be mobilized are the glycogen stores of the liver.
  
  • However, after simply an overnight fast, these are nearing depletion.
• In mild starvation, liver carbohydrate reserves are totally oxidized within three days.
  
  • This limits energy supplies to fats and proteins.
• In the early post absorptive period, 8-16 hours after eating, glucagon levels begin to rise and increase lipolysis for energy, thus sparing proteins.
• Although fatty acids are the highest caloric yield per weight, they cannot be converted to glucose.
  
  • This means that those organs dependant on glucose metabolism: the brain, red blood cells, peripheral nerves and the renal medulla_, must rely on_ glucose produced via amino acid conversion through gluconeogenesis.
• As there are no true reserve stores of protein, only half of the potential caloric value can be extracted.
  
  • Depletion of total body protein below 50% is incompatible with life.
• The first organs to manifest inadequate protein are the liver and the immune system.
• the most common cause of death in an epidemic of starvation is typically simple bacterial pneumonia.
• A nutritional evaluation for protein status will include assessments of liver proteins, immune function and lean body mass.

Question 7

Assessment of Protein Nutrition:
Liver (p.48)

• Albumin
  
  • accounts for/
  • contributes/
  • half-life
• Transferrin
  
  • half-life
  • is increased during/
• Prealbumin
  
  • half-life
  • can be used to/
  • has been more accurately renamed/

Answer 7

• Albumin
  
  • accounts for 50% of the proteins synthesized in the liver, totaling 20 g of albumin per day.
  • contributes essential functions as a carrier protein and as the major contribution to plasma oncotic pressure.
  • has a relatively long half-life of 20 days in circulation, and does not reflect recent alterations in nutritional status, but rather long-term shifts.
• Transferrin
  
  • has a half-life of 8.8 days,
  • is increased during iron deficiency, which may accompany a decrease in protein intake.
• Prealbumin
  
  • has a half-life of only 24-48 hours,
  • can be used to reflect changes in nutritional status over the short-term as patients receive therapeutic nutritional support.
  • has been more accurately renamed transthyretin to reflect its unique functions in the plasma transport of thyroxin and retinol-binding protein.

Question 8

Assessment of Protein Nutrition:
Immune Function and Lean Body Mass (p.49-50)

• Immune function:
  
  • Inadequate protein/
  • Absolute lymphocyte counts/
  • hypersensitivity reactions using skin test allergens/
  • Routine skin allergens tested are chosen based on/
  • caution must be added for those individuals with/
• Lean Body Mass:
  
  • The status of lean body mass can be assessed by measuring/
  • Creatinine production is directly proportional to/

Answer 8

• Immune function:
  
  • Inadequate protein impairs the immune system.
  • Absolute lymphocyte counts decrease significantly,
  • hypersensitivity reactions using skin test allergens are delayed.
  • Routine skin allergens tested are chosen based on the wide spread exposure to common antigens that could therefore be expected to elicit a skin reaction (tuberculin PPD, mumps, Candida albicans).
  • caution must be added for those individuals with disease states that alter the ability of the immune system to respond, such as Hodgkin’s disease or HIV.
• Lean Body Mass:
  
  • The status of lean body mass can be assessed by measuring urinary creatinine excretion over 24 hours.
  • Creatinine production is directly proportional to skeletal muscle mass, provided there is no rapid turnover due to severe sepsis or trauma and verifying appropriate kidney function.

Question 9

Lipids (p.51-53)

• Lipids
  
  • ?
  • In a typical Western diet, an average adult consumes/
  • These consist primarily of/
• Lipids serve many functions :
• The challenge in the assimilation of lipids
  
  • Absorption
  • subsequent distribution of absorbed lipids throughout the body/

Answer 9

• Lipids
  
  • hydrophobic molecules that are more soluble in organic solvents than in water.
  • In a typical Western diet, an average adult consumes 120-150 g of lipid a day.
  • These consist primarily of triglycerides (with fatty acids of at least 12 carbons) , and lesser amounts of phospholipids, plant sterols and cholesterol.
• Lipids serve many functions :
  
  • Denser in caloric value than carbohydrates or protein (9 Kcal/g vs 4 Kcal/g) and serve as an important storage form of energy
  • Vitamins A,D, E, K are lipids
  • Biological membranes are built of lipids
  • Signaling molecules are derived from arachadonic acid ( a long-chain fatty acid)
  • Volatile lipids make food palatable and tasty
• The challenge in the assimilation of lipids, is how water–soluble lipolytic enzymes access these hydrophobic molecules for digestion.
  
  • Absorption becomes less of a challenge because the small products of lipolysis readily diffuse across plasma membranes.
  • However, subsequent distribution of absorbed lipids throughout the body occurs via a different mechanism than carbohydrates and proteins.

Question 10

Lipid Digestion:
Gastric Digestion (p.54-55)

• Digestion of dietary lipids begins/
• Shearing action, especially of the antrum, produces/
  
  • This increased surface area is then/
• Gastric lipase
  
  • functions at/
  • is resistant to/
  • functions independent of/
  • cleaves/
• At the acidic stomach pH, however, the free fatty acids/
• Thus, gastric digestion of lipids/
• In an adult, gastric digestion of lipids/
  
  • In neonates, however, pancreatic maturity/
  • Steatorrhea

Answer 10

• Digestion of dietary lipids begins in the stomach with the powerful tituration of the gastric musculature.
• Shearing action, especially of the antrum, produces a fine emulsion of small lipid droplets.
  
  • This increased surface area is then attacked by gastric lipase, a secretion of chief cells in the body/fundus of the stomach.
• Gastric lipase
  
  • functions at a low pH of 4.0-5.5
  • is resistant to proteolytic cleaveage by pepsin (also released by the chief cells).
  • functions independent of any cofactors
  • cleaves triglycerides at the C1 of the glycerol backbone to produce a free fatty acid and a 2,3-diglyceride.
• At the acidic stomach pH, however, the free fatty acids become protonated and migrate into the center of oil droplets.
• Thus, gastric digestion of lipids is incomplete, and shields the very products that could diffuse through a plasma membrane.
• In an adult, gastric digestion of lipids is not essential; pancreatic enzymes alone can accomplish lipid digestion.
  
  • In neonates, however, pancreatic maturity is often delayed and there is a much greater dependence on gastric lipase for lipid breakdown.
  • Steatorrhea (fat in the stools) is therefore common in newborns as well as in patients with pancreatic diseases.

Question 11

Lipid Digestion:
Intestinal Digestion (p.56)

• Acidic chyme/
• Fatty acids now become/
• Fatty acids in the lumen of the duodenum are a potent stimulus for the release of cholecystokinin (CCK):
  
  • Secreted in response to/
  • Secondary stimulus for/
  • Stimulates/
  • Decreases/
  • Relaxes/
• Diet can affect CCK release:
  
  • Long-chain triglycerides/
  • Medium-chain triglycerides/
  • Oleic acid/

Answer 11

• Acidic chyme is slowly released into the duodenum where it is neutralized by bicarbonate secretions from the pancreas, biliary ducts and Brunner’s glands.
• Fatty acids now become ionized and shift to the outer surface of the oil droplet, with a few molecules dissociating and diffusing through the intestinal mucosa.
• Fatty acids in the lumen of the duodenum are a potent stimulus for the release of cholecystokinin (CCK):
  
  • Secreted in response to fats (and polypeptides) in the duodenum
  • Secondary stimulus for exocrine pancreas secretion of digestive enzymes (via CCK receptors on afferents nerves to acetylcholine released from the vagus)
  • Stimulates gallbladder contraction, releasing bile into the small intestine
  • Decreases stomach contractions (slows delivery)
  • Relaxes sphincter of Oddi
• Diet can affect CCK release:
  
  • Long-chain triglycerides increases CCK
  • Medium-chain triglycerides increases CCK
  • Oleic acid (major FA in olive oil) most potent stimulus for CCK release

Question 12

Lipid Digestion:
Intestinal Digestion:
Pancreatic acinar cells produce two proteins necessary for fat digestion (p.57-58)

• Pancreatic lipase
  
  • active at/
  • cleaves/
  • Of the pancreatic enzymes, lipase is most susceptible to/
  • in a patient with pancreatic diseases/
  • colonic bacteria can digest/
  • Any reason for fat malabsorption in the small intestine results in/
• Both gastric and pancreatic lipases are inhibited by/
  
  • sphincter of Oddi
• pancreatic colipase

Answer 12

• Pancreatic lipase
  
  • active at a neutral pH.
  • cleaves triglycerides at both the 1 and 3 site of the glycerol base forming esterified fatty acids and a 2-monoglyceride.
  • Of the pancreatic enzymes, lipase is most susceptible to degradation at an acidic pH.
  • in a patient with pancreatic diseases, where delivery of bicarbonate is restricted and lipase becomes inactivated, the earliest clinical symptom is steatorrhea.
  • colonic bacteria can digest any remaining carbohydrates, but the anaerobic nature of the colon prevents bacterial oxidation and digestion of fatty acids.
  • Any reason for fat malabsorption in the small intestine results in steatorrhea.
• Both gastric and pancreatic lipases are inhibited by bile salts.
  
  • This is usually not an issue in the stomach, but pancreatic enzymes and bile share a common delivery system through the sphincter of Oddi.
• When bile salts coat the lipid droplet, lipase is displaced.
  
  • The solution for this dilemma is pancreatic colipase.
  • Colipase binds to both bile acids and to lipase, locking the digestive enzyme on to the lipid surface.

Question 13

Lipid Digestion:
Intestinal Digestion:
Two other lipolytic enzymes are synthesized by the pancreas (p.59-61)

• Like colipase (but not lipase), they are/
• Phospholipase A2/
• Cholesterol esterase

Answer 13

• Like colipase (but not lipase), they are secreted as pro-peptides that become activated within the duodenum.
• Phospholipase A2 cleaves the fatty acid from the 2-position of glycerol.
• Cholesterol esterase
  
  • a nonspecific enzyme capable of degrading esters of cholesterol, esters of vitamins A, D and E, as well as the total cleavage of all three fatty acids from triglycerides.
  • requires the presence of bile salts to assemble into its active tetramer.

Question 14

Lipid Absorption:
Bile Salts (p.58)

• Bile again plays an important role in/
• These lipids are truly in solution within/
• Bile mixed micelles
  
  • on their outer face
  • easily mix into/
• Critical micellar concentrations of bile are the rate limiting factor in/

Answer 14

• Bile again plays an important role in solubilizing the end products of lipid digestion.
• These lipids are truly in solution within the mixed micelles, not merely emulsified into smaller droplets, as was the case in the stomach.
• Bile mixed micelles
  
  • are hydrophilic on their outer face
  • easily mix into the aqueous environment of the brush border, presenting high concentrations of lipid products for uptake.
• Critical micellar concentrations of bile are the rate limiting factor in assimilation of lipids and the fat-soluble vitamins, A, D, E and K.

Question 15

Lipid Absorption:
Enterocyte Absorption & Re-esterification (p.61-64)

• The products of lipid digestion diffuse/
• Fatty acids are re-esterified into/
• Short-chain fatty acids (< 10 C) diffuse/
• Cholesterol uptake is also facilitated by/
• The NPC1L1 is the target of/
• The bile salts remain/

Answer 15

• The products of lipid digestion diffuse across the apical membrane of the enterocytes.
• Fatty acids are re-esterified into triglycerides and onto cholesterol.
  
  • FA + CoA + ATP –> FA-CoA + AMP + PPi (fatty acyl CoA synthetase)
  • FA-CoA + 2-monoglyceride –> TG (fatty acyl CoA transferase)
  • FA-CoA + cholesterol –> cholesterol ester (ACAT)
• Short-chain fatty acids (< 10 C) diffuse directly into the portal blood and are carried by albumin.
• Cholesterol uptake is also facilitated by a specific carrier, the Niemann Pick C1 Like1 (NPC1L1) transporter.
• The NPC1L1 is the target of ezetimibe, a recently approved drug that blocks uptake of cholesterol from the small intestine.
• The bile salts remain behind in the intestinal lumen, until the terminal ileum, where a specific enterocyte receptor binds and transcytoses the bile

Question 16

Lipid Absorption:
Chylomicrons & Transport of Dietary Fats (p.66+68)

• Chylomicrons are assembled in/
  
  • The inner core is composed of/
  • The hydrophilic outer shell is composed of/
  • The only apoprotein carried by chylomicrons upon secretion into the lymph
• Chylomicron Composition

Answer 16

• Chylomicrons are assembled in the enterocyte.
  
  • The inner core is composed of triglycerides, with smaller amounts of cholesterol esters and fat-soluble vitamins.
  • The hydrophilic outer shell is composed of phospholipids, free cholesterol and apoproteins.
  • The only apoprotein carried by chylomicrons upon secretion into the lymph is Apo B48.
• Chylomicron Composition
  
  • Triglyceride 88%
  • Phospholipid 8%
  • Cholesterol ester 3%
  • Cholesterol 1%
  • Apoprotein 1-2%

Question 17

Lipid Absorption:
Chylomicrons & Transport of Dietary Fats (p.69-70)

• Chylomicrons are carried/
• In the systemic circulation, the chylomicrons gain/
• the types of apoproteins on all the lipoprotein particles:
  
  • Apo A
  • Apo B
  • Apo C
  • Apo D
  • Apo E

Answer 17

• Chylomicrons are carried in the lymph to the thoracic duct and into the right atrium.
• In the systemic circulation, the chylomicrons gain Apo E and Apo CII from HDLs.
• the types of apoproteins on all the lipoprotein particles:
  
  • Apo A
    
    • HDL Reverse cholesterol transport
    • (LCAT cofactor)
  • Apo B
    
    • LDL receptor binding and clearance(B100)
    • Chylomicron assembly (B48)
  • Apo C
    
    • Cofactor for LpL
  • Apo D
    
    • HDL cholesterol ester transfer (CETP cofactor)
  • Apo E
    
    • Remnant receptor binding and clearance

Question 18

Lipid Absorption:
Chylomicrons & Transport of Dietary Fats (p.70)

• Lipoprotein lipase (LpL)
  
  • ?
  • requires/
• Triglycerides of chylomicrons (and VLDLs & IDLs)
  
  • ?
  • the products, FAs and glycerol, enter adipose tissue to be
• A chylomicron remnant
  
  • produced by/
  • Apo CII/
  • This remnant is taken into the liver/
  • ApoE- remnant receptors are found/

Answer 18

• Lipoprotein lipase (LpL)
  
  • an extracellular enzyme attached to the endothelial walls of capillaries, especially in adipose tissue, heart and skeletal muscle.
  • requires Apo C II as a co-factor.
• Triglycerides of chylomicrons (and VLDLs & IDLs)
  
  • hydrolyzed
  • the products, FAs and glycerol, enter adipose tissue to be reconstructed and stored as triglycerides, or enter muscle to be oxidized for energy.
• A chylomicron remnant
  
  • produced by further hydrolysis.
  • Apo CII is returned to HDL.
  • This remnant is taken into the liver via the ApoE- remnant receptor interaction.
  • ApoE- remnant receptors are found only in the liver.