Digestion + Absorption

Digestion

Chemical and mechanical breakdown of food into smaller, absorbable units

(General Definition)

Small bowel, mouth, stomach, pepsin

Digestion

• Primarily occurs in proximal __ __, some occurs in the __ and __

• __ is NOT obligatory for protein digestion

Absorption

Transport or diffusion across the epithelium

(General Definition)

Small bowel, large intestine

Absorption

• Primarily occurs in __ __

• Additional absorption of water and ions in __ __

Carbon, carbohydrates, lipids, protein, vitamins

Organic nutrients contain __ (and oxygen + hydrogen)

• (4 types…)

Minerals

Chemical elements (i.e. iron, potassium, sodium)

Minerals, water

Inorganic nutrients (2)

Essential Nutrients

Nutrients not made endogenously by body in sufficient quantities

Starch

Primary dietary carbohydrate

Oligosaccharides, glucose, cellulose

Starch

• Broken down into multiple other __ require other enzymes for conversion to __

• __ is a dietary fiber

Monosaccharides, disaccharides, sucrose, fructose, lactoses

Simple sugars

• __ and __

• Ex: __ (table sugar), __ (fruits), and __ (milk)

Monosaccharides, amylase

• Carbohydrates are digested to __

• Complex carbohydrates - Broken into smaller chains by salivary and pancreatic __

Maltase

Breaks down glucose and glucose

Sucrase

Breaks down glucose and fructose

Lactase

Breaks down glucose and galactose

Absorbed, glucose, fructose, galactose

Monosaccharides are __ (secreted/absorbed)

• Examples: (3…)

SGLT-1

Active transport of glucose and galactose across apical membrane

GLUT-5

Fructose entry to cell via facilitated diffusion from this channel

GLUT-2

Glucose, galactose, and fructose exit basolateral membrane via facilitated diffusion from this channel

Glucose, facilitated, basolateral

GI epithelial cells can concentrate __ within the cell

• Enables __ diffusion → Across the __ (apical/basolateral) membrane

Not, cellulose, dietary fiber, gut microbiome

Undigested polysaccharides__ (are/not) absorbed

• Humans lack enzymes required to break down __ and other polysaccharides

• Remain in lumen as “__ __”

• Some metabolism by __ __

Stimulate enteric NS, promote GI motility, stabilize blood sugar, probiotic

What are some potential benefits of dietary fiber?

Peptides, amino acids

Proteins are broken down into __ → then broken down into smaller peptides and __ __

Na+, H+, transcytosis, protein absorption

Most absorption occurs via __ or __ linked cotransporters

• Some small peptides move via __

• Many drugs are absorbed via __ __ transporters

Na+ amino acid, H+ peptide, aminoA

Protein Absorption

• Amino acids cross apical membrane via __ __ co-transporters

• Dipeptides and tripeptides cross apical membrane via PEPT1 __ __ cotransporter

• Redundancy* - Same __ can be carried by both types of transporters

Cystinuria

Congenital mutation in cystine transporter in GI tract and kidneys

Protein, PEPT1, short peptides

Cystinuria

• No significant __ deficiency

• Cystine still absorbed as di- and tri-peptides via __ and as __ __ chains

Triglycerides

Dietary fats (lipids) are in form of..

Saturated fatty acids

Butter, cheese, fatty meats, coconut oil, palm oil (fatty A)

Unsaturated fatty acids

Most vegetable oils (fatty A)

Triglycerides

Glycerol with 3 fatty acid chains

Cholesterol

4 ring component of cell membranes, steroids, and bile acids

Phospholipids: Amphipathic lipids that emulsify fats and water

Cholesterol, phospholipids

__ and __ are synthesized by liver; dietary intake is NOT essential

Phosphate group and choline

Phospholipids are different from triglycerides in that:

One glycerol binding site is occupied by __ __ and __

Fat droplets, recycled

Fat digestion + absorption

• Bile salts emulsify __ __

  • Bile salts are later __

Triglycerides, pancreatic, salivary, gastric, monoglycerides, free

Fat digestion + absorption

• Lipases break down __

  • __ lipases in duodenum, and __ and __ lipases

  • __ and __ fatty aids diffuse across epithelial cell membrane

Chylomicrons, fatty, monoglycerides

Fat digestion + absorption

• __ are absorbed in lymph vessels

  • Once in cell, __ acids recombine with __

Chylomicrons

Within cell triglycerides and cholesterol are packaged into these lipoproteins

Lacteals

Chylomicrons absorbed directly into these lymph vessels

Steatorrhea

How will lipid malabsorption affect stool composition?

Chronic pancreatitis

Less pancreatic lipase in the duodenum (disorder)

Cystic fibrosis

Thick, slow moving pancreatic secretions (with bouts of pancreatitis), less lipase (disorder)

Excessive acid in duodenum

pH too low for optimal enzyme function (disorder)

Choledocholithiasis

Secretion of bile obstructed, no emulsifying fat (disorder)

Celiac / Inflammatory bowel disease

Less anatomical absorption of monoglycerides + free fatty acids

Fat-soluble Vitamins

Vitamins A, D, E, K

(fat or water-soluble)

Water-soluble Vitamins

Vitamins B and C, with specific transporters

Bacteria, intrinsic factor, RBC

Vitamin B12

• From meat/eggs/dairy products, and gut __

• Transporter only recognizes B12 when it’s complexed with __ __

• __ synthesis is dependent on B12

Alcoholism, vegan diet, autoimmune, pernicious anemia, gastritis, Gi resection, bowel inflammation

What situations may promote B12 deficiency?

Bulk absorption, additional absorption

Electrolyte absorption

• Small bowel is for ___ ___

• Colon is for __ __ (fine-tuning)

Ion, co-transporters, Na+/K+ ATPase, aldosterone

Electrolyte absorption

• Electrolytes are secreted and reabsorbed via __ channels and __

• __ (channel) transports this in colon resembles principal cells of kidney

  • __ stimulates Na+ reabsorption by “kicking K to the curb”

1-2, 8, 100

GI fluid balance

• Ingest __ L/daily and secrete _ L/daily into GI lumen

• Normally absorb all but __ mL

Cl-, NKCC, CFTR, Na+, water, cAMP, CFTR

Intestinal __ secretion influences fluid secretion

• Cl- enters via basolateral __ and exits via apical __ Cl- channels

• Cl- in lumen attracts __ and __ (ion and compound)

• Factors for secretion are increasing __ or Ca2+ increase __ opening

Cl-, VIP, Ach

Factors increasing cAMP or Ca2+ increase CFTR opening

• More __ and thus fluid secretion

• ___ - Released from enteric nerves

• __ - Released from vagus nerve

Bacterial toxins, Cl-, Na+, water, secretion

__ __ (i.e. cholera toxin, E coli) secrete toxin to stimulate cAMP production

• Increases __ secretion

• __ and __ follow

• Massive __ (absorption/secretion) overwhelms GI tract capacity to reabsorb fluid

Secretory Diarrhea

GI fluid exceeds reabsorption

Cholera, E coli

Secretory Diarrhea

• __ and __ toxins - stimulate Cl- secretion

Osmotic Diarrhea

Unabsorbed solutes in lumen retain water

Lactase, pancreatitis, cystic fibrosis, fat

Osmotic Diarrhea

• __ deficiency, chronic __, __ __

• Determined malabsorptive when there is __ in stool and malnourishment

Inflammatory/exudative diarrhea

Damage to mucosal lining alters permeability

Blood/pus/protein, inflammatory bowel disease

Inflammatory Diarrhea

• __/__/__ may ooze into lumen

• I.e. __ __ __ or from bacterial infections

Hyperchloremic, 10, HCO3- wasting, Cl-

Non-anion gap (__) metabolic acidosis

• Normally at __ mEq/L

• Cause is from GI __ __ from diarrhea, also excess __ (ion)

Rehydration, treating underlying cause

Ideal approaches for diarrhea - (2)

Loperamide (Imodium)

Drug acts on mu opioid Rs in enteric nervous system; more absorption time

Water follows solutes (thru lumen to rehydrate patient)

Why are solutions with water, salt, and sugar ideal for rehydrating patients with diarrhea?

Fiber

Bulk-forming laxative; Add soft “bulk” increasing distension

Fiber, obstruction

___ (med)

• Retains water, but must take with water

• If not, will have an __

Polyethylene glycol, magnesium hydroxide, lactulose

Osmotic laxatives - (3)

Osmotic laxatives

Medications actively draw water into lumen

Mineral oil

Lubricating laxative for stool surface

Docusate

Stool softener laxative; Surfactant that facilitates mixing of stool, water, and fats

Stimulant laxatives

Act on enteric nervous system to stimulate secretions and motility

Senna and bisacodyl

Irritant-stimulant laxatives - (2)

Cisapride

Prokinetic stimulant laxative