Digestion and Absorption of Carbohydrates and Proteins
Introduction
Many digestive processes are dependent upon enzymes supplied by the pancreas (more than 30 g/day is secreted)
Villi and microvilli produce a very large surface area for absorption; digestive enzymes are on the surface
digestion on the surface allows immediate absorption of monomers, preventing an increase in osmolality
any disease that reduces surface area (e.g., inflammatory disease) reduces digestion and absorption
Carbohydrates
– must be monomers to pass apical membrane
Digestion
– in order to be absorbed through the hydrophobic bilayer, all carbohydrates must be reduced to monomers
three polysaccharides make up 90% of carbohydrates in the diet:
(1) amylose
(starch) – glucose polymer, most common carbohydrate in diet (60%)
digested in lumen by amylase from pancreas or salivary glands to smaller oligosaccharides
maltase
and alpha-limit dextrinase break these down into glucose monomers
(2) sucrose
(table sugar) – glucose-fructose disaccharide – digested by sucrase
(3) actose
– glucose-galactose disaccharide specific to mammals – digested by lactase
there are also polymers that cannot be digested
certain carbohydrates in beans – digested by bacteria in the colon
cellulose
in plants is undigestable
peptidases
and phosphatases exist as well, for digestion of glycoproteins
on brush border = sucrase, glucoamylase, lactase
Absorption
glucose
and galactose are taken up by active transport through a Na+ dependent cotransporter (SGLT1)
fructose
is taken up by facilitated diffusion through (GLUT5 channels)
monosaccharides exit basolaterally by diffusion through GLUT2 channels – enter the portal circulation
trehalose – glucose + glucose (1-1)
Þ only in young mushrooms
Efficiency of Absorption
Efficiency of absorption – ileum > jejunum > duodenum
amount absorbed – jejunum > duodenum > ileum
Almost all carbohydrates are absorbed by the middle of ileum
presence of carbohydrate in the colon provides substrates for bacteria
Þ flatulence
lactose deficiency
– lactase levels fall, allowing lactose to remain in the intestine
lactose in the lumen causes diffusion of water into the lumen
Þ osmotic diarrhea
Proteins – dimers and trimers can pass apical membrane; only monomers enter blood
5 phases
:
(1) Gastric Phase – pepsin/HCl
(2) Pancreatic Phase – endo/carboxypeptidase
(3) Surface Phase – di/amino/carboxypeptidase
(4) Absorption
(5) Cellular Phase
Digestion
– since proteins are not as hydrophilic as carbohydrates, they can be absorbed in dipeptide or tripeptide form
many proteins are denatured by cooking and stomach enzymes (pepsin), but these are not essential
protease
enzymes – huge array for very specific peptidase reactions (cleave peptide bond)
can be endo- (cleave internal bonds), exo- (cleave from the end), amino-, or carboxypeptidases
most are serine proteases (trypsin, chymotrypsin); some require zinc (carboxypeptidases)
often must be activated at brush border – enteropeptidase (enterokinase) activates trypsin which activates everything else; prevents self-digestion of the pancreas
all proteases are pH dependent; pepsin (in the stomach) requires acid for its activation; intestinal proteases require neutral pH
oligopeptides are digested in the lumen and on the brush border to amino acids (40%) and dipeptides/tripeptides
Absorption
– oligopeptides (dimers and trimers) can enter the cell, but only single amino acids leave basolaterally
most amino acids are transported through the apical membrane by secondary active transport (with Na+ or glucose)
oligopeptides are digested inside the cell to single amino acids – through the basolateral membrane by active transport
into portal circulation
nutroaminoacidurea – no transporter for single amino acids on apical border
also lacking in proximal tubule – amino acids in urine
no malnutrition associated with this disorder because oligopeptides could still be transported
symptoms due to malabsorption of essential amino acids
Efficiency of Absorption
Only 75-85% of all proteins are digested by the terminal ileum – rest are undigestable (e.g., collagen)