Digestion and Absorption: From Food to Nutrients
Complete overview of the digestive process - mechanical and chemical digestion, absorption sites for each nutrient, digestive enzymes, and nutrient transport mechanisms. From the NIH and anatomy resources.
This content is for informational purposes only. Always consult a healthcare professional.
Overview
Digestion is the process of breaking down food into absorbable units. Mechanical digestion involves physical breakdown (mastication, churning, segmentation), while chemical digestion involves enzymatic hydrolysis of macromolecules into monomers. Absorption is the transport of these monomers across the intestinal epithelium into blood or lymph.
Process
Definition
Location
Ingestion
Intake of food
Oral cavity
Mastication
Chewing (mechanical)
Oral cavity
Deglutition
Swallowing
Pharynx, esophagus
Mechanical digestion
Physical breakdown (mixing, churning, segmentation)
Stomach, small intestine
Chemical digestion
Enzymatic hydrolysis of macromolecules
Stomach, small intestine (lumen + brush border)
Absorption
Transport of nutrients across epithelium
Small intestine (primarily), colon (water, electrolytes)
Elimination
Excretion of indigestible residue
Large intestine, rectum, anus
Overview of Digestive Enzymes
Major Digestive Enzymes
Enzyme
Source
Substrate
Product(s)
Optimal pH
Site of Action
Carbohydrate-digesting
Salivary α-amylase
Salivary glands (parotid)
Starch (amylose, amylopectin)
Maltose, maltotriose, limit dextrins
6.5-7.5
Mouth, stomach (transient)
Pancreatic α-amylase
Pancreas (acinar cells)
Starch
Maltose, maltotriose, limit dextrins
6.5-7.5
Small intestine lumen
Maltase
Intestinal brush border
Maltose
2 Glucose
5.5-7.0
Small intestine
Sucrase
Intestinal brush border
Sucrose
Glucose + Fructose
5.5-7.0
Small intestine
Lactase
Intestinal brush border
Lactose
Glucose + Galactose
5.5-7.0
Small intestine
Isomaltase (debranching)
Intestinal brush border
Limit dextrins
Glucose
5.5-7.0
Small intestine
Trehalase
Intestinal brush border
Trehalose
2 Glucose
5.5-7.0
Small intestine
Protein-digesting
Pepsin (pepsinogen → HCl)
Stomach (chief cells)
Proteins
Polypeptides
1.5-3.0
Stomach
Trypsin (trypsinogen → enteropeptidase)
Pancreas (acinar cells)
Polypeptides
Smaller peptides
7.5-8.5
Small intestine lumen
Chymotrypsin
Pancreas (acinar cells)
Polypeptides
Smaller peptides
7.5-8.5
Small intestine lumen
Elastase
Pancreas (acinar cells)
Elastin, polypeptides
Smaller peptides
7.5-8.5
Small intestine lumen
Carboxypeptidase A and B
Pancreas (acinar cells)
C-terminal of peptides
Free amino acids + smaller peptides
7.5-8.5
Small intestine lumen
Aminopeptidase
Intestinal brush border
N-terminal of peptides
Free amino acids + smaller peptides
7.0-8.0
Small intestine
Dipeptidases
Intestinal brush border
Dipeptides
2 Free amino acids
7.0-8.0
Small intestine
Tripeptidases
Intestinal brush border
Tripeptides
3 Free amino acids
7.0-8.0
Small intestine
Fat-digesting
Lingual lipase
Lingual glands (tongue)
Triglycerides (MCT)
Fatty acids + diglycerides
4.0-6.0
Mouth, stomach
Gastric lipase
Stomach (chief cells)
Triglycerides (MCT)
Fatty acids + diglycerides
4.0-6.0
Stomach
Pancreatic lipase
Pancreas (acinar cells)
Triglycerides
2-Monoglyceride + 2 Fatty acids
7.0-8.0
Small intestine lumen
Colipase
Pancreas (acinar cells)
— (activates lipase)
—
7.0-8.0
Small intestine lumen
Phospholipase A₂
Pancreas (acinar cells)
Phospholipids
Lysophospholipid + Fatty acid
7.0-8.0
Small intestine lumen
Cholesterol esterase
Pancreas (acinar cells)
Cholesteryl esters
Cholesterol + Fatty acid
7.0-8.0
Small intestine lumen
Nucleic acid-digesting
Deoxyribonuclease (DNase)
Pancreas (acinar cells)
DNA
Deoxyribonucleotides
7.0-8.0
Small intestine lumen
Ribonuclease (RNase)
Pancreas (acinar cells)
RNA
Ribonucleotides
7.0-8.0
Small intestine lumen
Nucleotidases
Intestinal brush border
Nucleotides
Nucleosides + Phosphate
7.0-8.0
Small intestine
Nucleosidases
Intestinal cells
Nucleosides
Purines/Pyrimidines + Pentose
7.0-8.0
Small intestine
Digestion and Absorption by Organ
Oral Cavity
Process
Detail
Mechanical
Mastication (chewing) — teeth reduce particle size, tongue mixes with saliva
Saliva
1-1.5 L/day; contains: water (99.5%), salivary amylase, lingual lipase, mucin, lysozyme, IgA, electrolytes, bicarbonate
Deglutition
Oral phase (voluntary) → pharyngeal phase (involuntary) → esophageal phase (peristalsis)
Absorbed
Nitrates (converted to NO in oral microbiome), some lipid-soluble compounds (sublingual)
Salivary Glands:
Gland
Location
Contribution
Secretion Type
Parotid
Lateral cheeks, over masseter
25%
Serous (watery, enzyme-rich)
Submandibular
Posterior to chin, below mandible
60%
Mixed (serous + mucous)
Sublingual
Floor of mouth
5%
Mucous (thick, lubricating)
Minor glands
Throughout oral mucosa
10%
Mucous
Esophagus
Process
Detail
Transport
Bolus moves via peristalsis (5-6 seconds)
Sphincters
Upper esophageal (UES, cricopharyngeus) — skeletal muscle control; Lower esophageal (LES) — smooth muscle, tonically contracted
Secretion
Mucus only (no digestive enzymes)
No absorption
Stratified squamous epithelium, tight junctions prevent absorption
Stomach
Process
Detail
Mechanical
Mixing, churning (3 peristaltic waves/min), trituration of solids
Gastric emptying
Liquids: 5-15 minutes; Semi-solids: 1-2 hours; Solids: 2-4 hours; High-fat meals: slowest
Volume
Empty: ~50 mL; After meal: 1-1.5 L; Maximum: 3-4 L
pH
1.0-3.0 (fasting), 3.0-5.0 (after meal), 1.0-2.0 (late digestion)
Gastric secretions
2-3 L/day
Gastric Cell Types and Secretions:
Cell Type
Location
Secretion
Function
Parietal (oxyntic)
Body, fundus
HCl, intrinsic factor
HCl: pH 1-3, activates pepsinogen, bactericidal; IF: B₁₂ absorption
Chief (peptic)
Body, fundus
Pepsinogen I and II
Pepsinogen → pepsin (autocatalytic, pH <4) — protein digestion
Mucous neck cell
Neck regions
Mucus (acidic mucin)
Protection against acid/pepsin
Surface mucous cell
Surface epithelium
Mucus (neutral mucin) + bicarbonate
Gastric mucosal barrier
G cell
Antrum
Gastrin
Stimulates parietal cells (HCl), growth of gastric mucosa
D cell
Antrum, body, fundus
Somatostatin
Inhibits G cells, parietal cells, chief cells
Enterochromaffin-like (ECL)
Body
Histamine
Potent stimulator of parietal cells (H₂ receptor)
Enterochromaffin (EC)
Throughout
Serotonin, melatonin
Motility regulation
Absorption in Stomach (Minimal):
Substance
Absorption
Mechanism
Alcohol (ethanol)
20-30%
Passive diffusion (significant, especially on empty stomach)
Aspirin (NSAIDs)
Some
Passive diffusion (this is why gastric irritation occurs)
Water
Minimal
Small amount
Iron (non-heme)
Minor (Fe²⁺ in acidic pH)
DMT1
Vitamin B₁₂
None (needs IF and ileal receptor)
—
Small Intestine
The small intestine is the primary site of digestion and absorption. Its absorptive surface is amplified ~600-fold by three structural features:
Plicae circulares (Kerckring folds): 3x increaseVilli: 10x increaseMicrovilli (brush border): 20x increaseTotal surface area: ~250-400 m² (size of a tennis court)
Section
Length
Transit Time
Primary Functions
Duodenum
25 cm
~30 min
Chyme neutralization, bile and pancreatic enzyme mixing, Fe²⁺/Ca²⁺ absorption
Jejunum
2.5 m
~1-2 hours
Most nutrient absorption (carbohydrates, proteins, fats, most vitamins)
Ileum
3.5 m
~2-4 hours
Bile salt absorption, B₁₂-IF complex absorption
Total small intestine
6-7 m
~3-6 hours (total)
Brunner’s Glands (Duodenum): Secrete alkaline mucus (pH 8-9) — neutralize gastric acid, protect duodenal mucosa.
Peyer’s Patches (Ileum): Lymphoid follicles — immune surveillance, antigen sampling (M cells).
Carbohydrate Absorption
Step
Location
Mechanism
Monosaccharide transport into enterocyte
Brush border
SGLT1 (Na⁺-glucose/galactose cotransport), GLUT5 (fructose facilitated diffusion)
Monosaccharide export to blood
Basolateral membrane
GLUT2 (all monosaccharides — facilitated diffusion)
Route
Portal vein → Liver
First-pass hepatic metabolism
Lactose Intolerance:
Type
Cause
Population Prevalence
Primary (adult-onset)
↓ Lactase expression with age (genetic)
Affects ~65% of world population; highest in East Asians (90-100%), lowest in Northern Europeans (5-10%)
Secondary
Damage to brush border (celiac, gastroenteritis, Crohn’s)
Transient, resolves with underlying condition treatment
Congenital
Lactase deficiency from birth (extremely rare)
Autosomal recessive
Developmental
Prematurity — immature enterocytes
Resolves with maturation
Protein Absorption
Step
Location
Mechanism
Peptide transport into enterocyte
Brush border
PepT1 (H⁺-coupled di/tripeptide cotransport) — quantitatively most important route
Free amino acid transport
Brush border
Multiple sodium-dependent (B⁰AT1, ASCT2, ATB⁰⁺) and sodium-independent transporters
Intracellular hydrolysis
Enterocyte cytoplasm
Peptidases hydrolyze di/tripeptides to free amino acids (some intact di/tripeptides enter portal blood)
Amino acid export to blood
Basolateral membrane
Multiple transporters (LAT1, y⁺LAT1, TAT1)
Amino Acid Transporter Families:
Transporter
Amino Acids Transported
Na⁺ Dependence
B⁰AT1 (SLC6A19)
Neutral amino acids
Yes
ASCT2 (SLC1A5)
Alanine, serine, cysteine, threonine
Yes
ATB⁰⁺ (SLC6A14)
Neutral + cationic amino acids
Yes (Cl⁻ also)
PepT1 (SLC15A1)
Di/tripeptides
H⁺-coupled
y⁺LAT1 (SLC7A7)
Cationic amino acids (efflux)
No (exchanges with neutral AA + Na⁺)
LAT1 (SLC7A5)
Large neutral amino acids
No (exchanges with intracellular AA)
Neonatal Protein Absorption: In neonates, intact proteins can be absorbed via pinocytosis (transcytosis) — important for passive immunity (maternal IgA in breast milk).
Fat Absorption
Step
Location
Mechanism
Emulsification
Duodenum lumen
Bile salts reduce fat droplet size → micelle formation
Lipolysis
Duodenum/jejunum lumen
Pancreatic lipase (+ colipase) → 2-monoglyceride + 2 FFA; phospholipase A₂ → lysophospholipid + FFA; cholesterol esterase → cholesterol + FFA
Micelle formation
Duodenum/jejunum lumen
Mixed micelles (bile salts + monoglycerides + FFAs + cholesterol + lysophospholipids + fat-soluble vitamins) — water-soluble carriers
Uptake into enterocyte
Brush border
Diffusion of monomers from micelle across enterocyte membrane
Re-esterification
Enterocyte (SER)
Triglycerides reassembled (2-monoglyceride + fatty acyl-CoA → TG)
Chylomicron assembly
Enterocyte (Golgi)
TGs + cholesterol esters + phospholipids + apoB-48 → nascent chylomicrons
Export
Basolateral → lacteals
Chylomicrons enter lymphatics (not portal vein) — due to size
Lymphatic transport
Thoracic duct → left subclavian vein
→ Systemic circulation (bypasses liver first pass)
Medium-Chain Triglycerides (MCT) Exception:
MCTs (C6-C12) do not require bile salts or micelles
Absorbed directly into portal vein (not lymphatics)
Do not require chylomicrons
Provide rapid energy (used in ketogenic diets, malabsorption syndromes)
Bile Salt Enterohepatic Circulation:
Component
Details
Primary bile acids
Cholic acid, chenodeoxycholic acid (synthesized from cholesterol in liver)
Conjugation
Glycine or taurine → bile salts (more water-soluble)
Storage
Gallbladder (30-50 mL, concentrates 5-20x)
Release
CCK stimulates gallbladder contraction + relaxation of Sphincter of Oddi
Recycling
~95% reabsorbed in ileum (active transport) → portal vein → liver
Daily loss
~5% (500 mg/day) — replaced by hepatic synthesis
Total pool
~3-4 g, recycles 6-10 times/day (20-30 g bile salts enter duodenum daily)
Large Intestine
Section
Length
Transit Time
Primary Functions
Cecum
6-8 cm
—
Ileocecal valve prevents reflux; appendix (immune)
Ascending colon
15-20 cm
—
Water and electrolyte absorption
Transverse colon
45-50 cm
—
Microbial fermentation, water absorption
Descending colon
25-30 cm
—
Storage, further water absorption
Sigmoid colon
35-40 cm
—
Storage
Rectum
12-15 cm
—
Fecal storage, defecation reflex
Anal canal
3-4 cm
—
Continence (internal/external sphincters)
Total colon
1.5 m
18-72 hours
What is absorbed in the colon:
Substance
Mechanism
Amount
Water
Passive osmotic gradient
1.0-1.5 L/day (from 2 L entering cecum → ~100 mL in stool)
Sodium (Na⁺)
Active (ENaC, Na⁺/H⁺ exchange)
~200 mEq/day
Potassium (K⁺)
Passive (secretion and absorption)
Balanced
Chloride (Cl⁻)
Coupled with Na⁺ and HCO₃⁻ exchange
~200 mEq/day
Short-chain fatty acids
Passive diffusion (SCFA transporters SMCT1, MCT1)
Butyrate: colonocyte fuel; Acetate, Propionate: systemic
Vitamin K (menaquinones)
Produced by bacteria, absorbed via passive diffusion
MK-4 to MK-13 (K₂)
Biotin
Produced by bacteria, absorbed via SMVT
Small amount
Folate
Produced by bacteria, absorbed via PCFT
Small amount (may contribute to folate status)
Colonic Microbiome:
Characteristic
Detail
Number of bacteria
10¹⁴ (100 trillion) — 10x more than human cells
Number of species
500-1,000 species (mostly Firmicutes and Bacteroidetes)
Metabolic capability
Ferment non-digestible carbohydrates → SCFAs; synthesize vitamins; metabolize bile acids; break down oxalates
Immune function
Development of gut-associated lymphoid tissue (GALT), immune tolerance
Protective role
Colonization resistance (prevents pathogen growth)
Nutrient Transport Pathways
Portal Vein (Hepatopetal Circulation)
Nutrients absorbed into blood capillaries (not lymph):
Nutrient
Transporter(s)
Monosaccharides (glucose, galactose, fructose)
GLUT2 (basolateral)
Free amino acids
LAT1, y⁺LAT1, TAT1, others
Di/tripeptides
Small amount intact
Short-chain fatty acids
MCT1, SMCT1
Medium-chain triglycerides
Direct (as free fatty acids)
Water-soluble vitamins
Various transporters
Minerals
Various channels and transporters
Glycerol
Diffusion
Lymphatic System (Lacteals)
Nutrients absorbed initially into lymph (bypassing liver first-pass):
Nutrient
Carrier
Chylomicrons (dietary fats, fat-soluble vitamins)
Chylomicron particles
Long-chain fatty acids
As chylomicron triglycerides
Fat-soluble vitamins (A, D, E, K)
Within chylomicrons
The liver receives portal blood and processes absorbed nutrients:
Glucose: Storage as glycogen (glycogenesis) or releaseAmino acids: Deamination, transamination, protein synthesisFats: Lipoprotein assembly (VLDL), fatty acid oxidationVitamins/Minerals: Storage (vitamins A, D, B₁₂, iron as ferritin)Toxins: Detoxification (Phase I and Phase II reactions)
Hormonal Regulation of Digestion
Hormone
Source
Stimulus
Target
Effect
Gastrin
G cells (stomach antrum)
Gastric distension, peptides, ACh, Ca²⁺
Stomach (parietal cells), ECL cells
↑ HCl secretion, ↑ gastric motility
Secretin
S cells (duodenum)
↓ pH (acid) in duodenum
Pancreas (ductal cells), stomach
↑ HCO₃⁻ secretion, ↓ gastric acid secretion
Cholecystokinin (CCK)
I cells (duodenum, jejunum)
Fatty acids, amino acids
Gallbladder, pancreas, stomach
↑ Gallbladder contraction, ↑ pancreatic enzyme secretion, ↓ gastric emptying
Gastric inhibitory peptide (GIP)
K cells (duodenum, jejunum)
Glucose, fats
Pancreatic β-cells, stomach
↑ Insulin secretion (incretin effect), ↓ gastric acid/motility
Glucagon-like peptide-1 (GLP-1)
L cells (ileum, colon)
Glucose, fats, fiber fermentation
Pancreatic β-cells, α-cells, brain
↑ Insulin secretion (incretin effect), ↓ glucagon, ↓ appetite, ↓ gastric emptying
Peptide YY (PYY)
L cells (ileum, colon)
Calories (fats, carbs, proteins)
Brain (hypothalamus), gut
↓ Appetite (anorexigenic), ↓ gastric emptying, ↓ pancreatic secretion
Motilin
M cells (duodenum, jejunum)
Fasting state (interdigestive)
Stomach, duodenum
Initiation of migrating motor complex (MMC) — “housekeeping contractions”
Ghrelin
X/A-like cells (stomach fundus)
Fasting, ↓ calories
Hypothalamus (arcuate nucleus)
↑ Appetite (orexigenic), ↑ gastric acid secretion, ↑ growth hormone
Amylin
Pancreatic β-cells
Meal (with insulin)
Brain (area postrema)
↓ Gastric emptying, ↓ glucagon, ↑ satiety
Somatostatin
D cells (stomach, intestine), δ-cells (pancreas)
Acid, nutrients
Paracrine (local)
↓ Gastrin, ↓ HCl, ↓ insulin/glucagon, ↓ pancreatic enzymes, ↓ biliary secretion
Vasoactive intestinal peptide (VIP)
Enteric neurons
Neural stimulation
Intestinal cells, smooth muscle
↑ Intestinal secretion, ↑ vasodilation, ↓ smooth muscle tone
Digestive Timeline
Event
Time
Cephalic phase (sight, smell, thought)
Seconds before eating
Gastric phase (stomach filling, digestion)
2-4 hours
Intestinal phase (nutrient arrival in duodenum)
1-3 hours
Stomach emptying (complete)
2-6 hours (varies by meal composition)
Small intestine transit (complete)
3-6 hours
Colonic transit (complete)
18-72 hours
Total GI transit
24-72 hours
Key Takeaways
Digestion begins in the mouth (mechanical + salivary amylase) and continues through stomach (pepsin, acid) and small intestine (pancreatic enzymes, bile)
The small intestine is the primary site of digestion and absorption (surface area: 250-400 m²)
Carbohydrates are absorbed as monosaccharides via SGLT1 (glucose/galactose) and GLUT5 (fructose)
Proteins are absorbed primarily as di/tripeptides via PepT1 (not just free amino acids)
Fats require bile salts for emulsification and micelle formation; absorbed as chylomicrons into lymphatics
Medium-chain triglycerides are absorbed directly into portal blood (no bile, no chylomicrons)
The colon absorbs water, SCFAs (from fiber fermentation), and some vitamins (K, biotin)
Bile salts undergo enterohepatic circulation (95% reabsorbed in ileum)
Pancreatic enzymes require activation (trypsinogen → enteropeptidase → trypsin → activates others)
Hormonal regulation (gastrin, CCK, secretin, GIP, GLP-1, PYY) coordinates digestion across organs
MCTs provide an alternative absorption route for fat malabsorption syndromes
Total GI transit time varies from 24-72 hours depending on fiber, hydration, and individual factors