Digestion & Absorption
of Macronutrients
Explore how your body breaks down carbohydrates, proteins, and fats into absorbable molecules through mechanical and chemical digestion.
Anatomy & Physiology
What Are Macronutrients?
Macronutrients are the large organic molecules your body needs in substantial quantities for energy, growth, and cellular repair. There are three major classes:
Carbohydrates
Primary energy source. Found in grains, fruits, and vegetables. Yield \(\approx 4\) kcal/g.
Proteins
Build and repair tissues. Found in meat, legumes, and dairy. Yield \(\approx 4\) kcal/g.
Lipids (Fats)
Energy storage, insulation, hormone synthesis. Found in oils, nuts, and butter. Yield \(\approx 9\) kcal/g.
Mechanical vs. Chemical Digestion
Digestion occurs in two complementary ways. Both must work together to reduce food into molecules small enough for absorption.
Mechanical Digestion
Physical breakdown: chewing (mastication), churning in the stomach, and segmentation in the intestines. Increases surface area for enzymes.
Chemical Digestion
Enzymatic breakdown of covalent bonds via hydrolysis: adding \(\text{H}_2\text{O}\) across bonds to cleave polymers into monomers.
The GI Tract at a Glance
Carbohydrate Digestion Pathway
Protein Digestion Pathway
Lipid Digestion Pathway
Absorption: Blood vs. Lymph
After digestion, monomers must cross the intestinal epithelium. The route they take depends on their chemical nature:
| Blood Capillaries | Lacteals (Lymph) | |
|---|---|---|
| Nutrients | Monosaccharides, amino acids, short-chain FAs | Long-chain fatty acids, monoglycerides |
| Transport form | Free in plasma | Chylomicrons |
| First destination | Hepatic portal vein → Liver | Thoracic duct → Subclavian vein |
| Mechanism | Active transport (SGLT1) + facilitated diffusion (GLUT2) | Simple diffusion + exocytosis |
Key Enzymes and Their Targets
| Enzyme | Source | Substrate | Product |
|---|---|---|---|
| Salivary Amylase | Salivary glands | Starch | Maltose |
| Pepsin | Stomach (chief cells) | Proteins | Polypeptides |
| Pancreatic Amylase | Pancreas | Starch | Maltose |
| Trypsin | Pancreas | Polypeptides | Peptides |
| Pancreatic Lipase | Pancreas | Triglycerides | Monoglycerides + FAs |
| Lactase | Brush border | Lactose | Glucose + Galactose |
Why pH Matters for Digestion
Each digestive enzyme has an optimal pH range. Outside this range, the enzyme denatures and loses catalytic function. The GI tract creates distinct pH zones:
Stomach: pH ~ 2
HCl creates an acidic environment ideal for pepsin. Also kills pathogens and denatures food proteins.
Duodenum: pH ~ 8
Bicarbonate (\(\text{HCO}_3^-\)) from the pancreas neutralizes acidic chyme. Optimal for trypsin, lipase, and amylase.
How to Trace a Nutrient's Journey
1
Identify the macronutrient class: Is the molecule a carbohydrate, protein, or lipid?
2
Determine the starting polymer: Starch, polypeptide, or triglyceride.
3
Match organ to enzyme: Mouth (amylase), stomach (pepsin), duodenum (trypsin, lipase, pancreatic amylase).
4
Identify the final monomer: Monosaccharide, amino acid, or fatty acid + monoglyceride.
5
Determine absorption route: Blood capillary (sugars, amino acids) or lacteal (long-chain fats).
Example: Digesting a Slice of Bread
Bread is primarily starch (a polysaccharide). Let's trace its journey:
1
Mouth: Salivary amylase begins hydrolysis of starch into maltose. Chewing increases surface area.
2
Stomach: Low pH inactivates salivary amylase. No significant carbohydrate digestion here.
3
Duodenum: Pancreatic amylase resumes starch → maltose conversion.
4
Brush border: Maltase cleaves maltose into two glucose molecules.
5
Absorption: Glucose enters enterocytes via SGLT1, exits into blood via GLUT2 → hepatic portal vein → liver.
Example: Digesting a Steak
Steak contains primarily protein and fat. Two pathways run simultaneously:
Protein Pathway
Stomach: HCl denatures proteins; pepsin cleaves into polypeptides.
Duodenum: Trypsin + chymotrypsin → small peptides.
Brush border: Peptidases → free amino acids.
Absorbed via active transport → blood → liver.
Duodenum: Trypsin + chymotrypsin → small peptides.
Brush border: Peptidases → free amino acids.
Absorbed via active transport → blood → liver.
Fat Pathway
Duodenum: Bile salts emulsify fat globules.
Pancreatic lipase → monoglycerides + fatty acids.
Micelles carry products to enterocytes.
Reassembled into triglycerides → chylomicrons → lacteals.
Pancreatic lipase → monoglycerides + fatty acids.
Micelles carry products to enterocytes.
Reassembled into triglycerides → chylomicrons → lacteals.
Example: A Glass of Milk
Milk contains all three macronutrients: lactose (carb), casein (protein), and milkfat (lipid).
Lactose
Disaccharide digested by lactase at the brush border into glucose + galactose. Absorbed via SGLT1.
Casein
Protein digested by pepsin (stomach) then trypsin (duodenum) into amino acids. Absorbed into blood.
Milkfat
Emulsified by bile, cleaved by lipase. Absorbed as chylomicrons via lacteals.
Common Mistakes
✕ "Protein digestion starts in the mouth"
No protein enzyme exists in saliva. Protein digestion begins in the stomach with pepsin. The mouth only starts carbohydrate digestion (amylase).
✕ "Bile digests fat"
Bile emulsifies fat (breaks large globules into small droplets) but does NOT chemically digest it. Pancreatic lipase performs the actual hydrolysis.
✕ "All nutrients go to the liver first"
Only water-soluble nutrients (sugars, amino acids) travel via the hepatic portal vein. Long-chain fats enter the lymphatic system and bypass the liver initially.
Summary Comparison
| Carbohydrates | Proteins | Lipids | |
|---|---|---|---|
| Digestion begins | Mouth | Stomach | Duodenum |
| Key enzyme(s) | Amylase, Maltase | Pepsin, Trypsin | Lipase |
| Accessory help | None | HCl | Bile salts |
| Final monomer | Monosaccharides | Amino acids | FAs + Monoglycerides |
| Absorption route | Blood | Blood | Lymph (lacteals) |
| Energy yield | \(\approx 4\) kcal/g | \(\approx 4\) kcal/g | \(\approx 9\) kcal/g |
Question 1 of 5
Which enzyme initiates the chemical digestion of starch in the oral cavity?
Salivary amylase (also called ptyalin) is secreted by the parotid glands and begins hydrolyzing starch into maltose in the mouth. Pepsin acts on proteins in the stomach, lipase acts on fats, and trypsin acts on proteins in the duodenum.
Question 2 of 5
What is the primary role of bile salts in lipid digestion?
Bile salts are amphipathic molecules that emulsify fats, breaking large globules into tiny droplets. This dramatically increases the surface area available for pancreatic lipase to perform the actual chemical hydrolysis.
Question 3 of 5
After digestion, long-chain fatty acids are absorbed into which structure within the intestinal villi?
Long-chain fatty acids and monoglycerides are reassembled into triglycerides inside enterocytes, packaged into chylomicrons, and released into lacteals (lymphatic capillaries). They eventually enter the bloodstream at the subclavian vein.
Question 4 of 5
Pepsinogen is converted to active pepsin by which substance in the stomach?
HCl secreted by parietal cells lowers the stomach pH to ~2, which cleaves the inactive zymogen pepsinogen into active pepsin. Enterokinase activates trypsinogen (not pepsinogen) in the duodenum. Bicarbonate neutralizes acid, and bile emulsifies fats.
Question 5 of 5
A patient has their gallbladder removed. Which macronutrient's digestion would be most impaired?
The gallbladder stores and concentrates bile produced by the liver. Without it, bile drips continuously into the duodenum but cannot be released in large, concentrated amounts needed for efficient fat emulsification. Carbohydrate and protein digestion are unaffected because they don't require bile.