Physiology

Digestion and Absorption of Carbohydrates, Proteins, and Fats

Learn about the digestion of various nutrients

Amos Asamoah
October 2025
4 min read
Digestive System

After the first mouthful of food leaves your plate, it embarks on a journey of enzymatic disassembly. Each nutrient β€” carbohydrate, protein, and fat β€” meets a custom-designed set of enzymes and transporters that turn it into absorbable molecules. Let’s follow each one step by step.

🍞 1️⃣ Carbohydrate Digestion and Absorption

Carbohydrates make up 60–70% of our diet, mainly as starch, sucrose, lactose, and cellulose. But only monosaccharides (glucose, galactose, fructose) can be absorbed β€” so the body must break everything down to that level.

βš™οΈ Digestion of Carbohydrates

A. In the Mouth

  • Enzyme: Salivary amylase (ptyalin)
  • Action: Begins starch digestion β†’ breaks starch β†’ maltose + maltotriose + dextrins
  • Condition: pH ~6.8 (active only briefly; inactivated by gastric acid)

B. In the Stomach

  • No carbohydrate digestion (acid destroys amylase).

C. In the Small Intestine

  • Pancreatic amylase continues starch digestion β†’ maltose, maltotriose, Ξ±-limit dextrins
  • Brush-border enzymes (in enterocytes):
Enzyme Substrate Product
Maltase Maltose 2 Glucose
Sucrase Sucrose Glucose + Fructose
Lactase Lactose Glucose + Galactose
Isomaltase Ξ±-dextrins Glucose
Clinical pearl: Deficiency of lactase β†’ lactose intolerance (bloating, cramps, diarrhea after milk).

πŸ’§ Absorption of Carbohydrates

Occurs mainly in the jejunum.

Monosaccharide Transport Mechanism Transporter
Glucose & Galactose Active (Na⁺-dependent cotransport) SGLT-1
Fructose Facilitated diffusion GLUT-5
All monosaccharides (into blood) Facilitated diffusion GLUT-2
Mnemonic: β€œSGLT-1 sucks in sugars with sodium.”
These absorbed monosaccharides enter portal circulation β†’ liver β†’ glycogen storage or energy use.

πŸ₯© 2️⃣ Protein Digestion and Absorption

Proteins are large, complex molecules made of amino acids, and must be broken into smaller units before absorption. Let’s follow the breakdown from the plate to the plasma.

βš™οΈ Digestion of Proteins

A. In the Stomach

  • Chief cells secrete pepsinogen, activated by HCl β†’ pepsin.
  • Pepsin β†’ breaks large proteins into smaller peptides.
  • Works best at pH 1.5–3.5 (acidic environment).
Note: Pepsin is inactive in the duodenum (neutralized by pancreatic bicarbonate).

B. In the Small Intestine

Pancreatic enzymes (secreted into the duodenum):

Enzyme Activated Form Function
Trypsinogen β†’ Trypsin Activates other enzymes
Chymotrypsinogen β†’ Chymotrypsin Breaks peptide bonds
Procarboxypeptidase β†’ Carboxypeptidase Cleaves amino acids from peptide ends
Proelastase β†’ Elastase Acts on elastic fibers
Activation cascade: Enteropeptidase (from intestinal mucosa) converts trypsinogen β†’ trypsin, and trypsin activates all others.

C. Brush Border and Intracellular Enzymes

  • Peptidases at microvilli β†’ convert oligopeptides β†’ dipeptides, tripeptides, amino acids.
  • Cytosolic peptidases in enterocytes finish digestion.

πŸ’§ Absorption of Proteins

Occurs mainly in jejunum and ileum.

Type Transport Mechanism Notes
Amino acids Na⁺-dependent cotransport Similar to glucose transport
Dipeptides & tripeptides H⁺-dependent cotransport (PepT1) Hydrolyzed inside cells
Small peptides Endocytosis (minor pathway) In infants, allows antibody absorption from milk
Clinical note: Defects in amino acid transport β†’ Hartnup disease (neutral amino acids lost in urine) or cystinuria (kidney stones).

🧠 Summary of Protein Digestion

Stomach: Pepsin β†’ large peptides

Pancreas: Trypsin, chymotrypsin β†’ smaller peptides

Brush border: Peptidases β†’ amino acids β†’ absorbed

🧈 3️⃣ Fat Digestion and Absorption

Fats are energy-rich but water-insoluble β€” meaning they need special handling for digestion and transport. This is where bile and pancreatic enzymes shine.

βš™οΈ Digestion of Fats

A. In the Mouth and Stomach

  • Lingual lipase (from tongue) and gastric lipase (from stomach) begin fat digestion.
  • These enzymes are especially important in infants (milk fat digestion).

B. In the Small Intestine

Here’s where the real work happens πŸ‘‡

  1. Emulsification by bile salts Bile (from liver) contains bile salts + lecithin β†’ breaks large fat droplets into small micelles β†’ increases surface area.
  2. Enzymatic digestion by pancreatic lipase Converts triglycerides β†’ monoglycerides + free fatty acids. Requires colipase (anchoring coenzyme from pancreas).
Other lipid enzymes: Phospholipase Aβ‚‚: acts on phospholipids; Cholesterol esterase: acts on cholesterol esters.

πŸ’§ Absorption of Fats

Step 1: Micelle formation

  • Bile salts surround monoglycerides and fatty acids β†’ soluble micelles.
  • Micelles carry lipids to the brush border for absorption.

Step 2: Diffusion into enterocytes

  • Lipids leave micelles β†’ diffuse through cell membrane.

Step 3: Re-esterification

  • Inside enterocytes β†’ lipids reassembled into triglycerides.

Step 4: Chylomicron formation

  • Triglycerides + cholesterol + proteins β†’ chylomicrons.
  • Released into lacteals (lymphatics) β†’ thoracic duct β†’ bloodstream.
Short-chain fatty acids bypass lymphatics β†’ enter portal blood directly.

🧠 Clinical Correlations

Condition Effect
Bile salt deficiency ↓ fat emulsification β†’ steatorrhea
Pancreatic insufficiency ↓ lipase β†’ fat malabsorption
Celiac disease Villous atrophy β†’ ↓ absorption
Blocked lymphatics ↓ chylomicron transport β†’ fat loss in stool

🩸 Summary Table β€” Digestion & Absorption

Nutrient Main Enzymes End Products Absorption Site Mechanism
Carbohydrates Amylases, maltase, sucrase, lactase Glucose, galactose, fructose Duodenum, jejunum Active (SGLT-1), facilitated (GLUT-5/2)
Proteins Pepsin, trypsin, peptidases Amino acids, di/tripeptides Jejunum, ileum Na⁺ or H⁺ cotransport
Fats Lipase, colipase, bile salts Monoglycerides, fatty acids Jejunum Passive diffusion via micelles
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