04/11/2025
Digestion made "simple": where each nutrient gets in
Your gut isn’t just one tube. Different nutrients are absorbed at different checkpoints. Here’s the simple breakdown:
1️⃣ Mouth & Stomach
Carbs begin breaking down with salivary amylase.
Proteins start with stomach acid + pepsin.
Vitamin B12 binds to intrinsic factor here.
🟢 Example: Without stomach acid, B12 can’t be absorbed properly.
2️⃣ Liver & Gallbladder
Bile salts emulsify fats, making them easier for enzymes to digest.
🟢 Example: Bile acts like dish soap, breaking fats into smaller droplets.
3️⃣ Pancreas
Sends enzymes into the small intestine: amylases for carbs, proteases for protein, lipases for fat.
🟢 Example: These enzymes finish the job of turning food into absorbable units.
4️⃣ Duodenum (first section of small intestine)
Carbs → disaccharides.
Proteins → amino acids.
Fats → monoglycerides + fatty acids.
🟢 Example: This is where most chemical digestion happens.
5️⃣ Jejunum (middle small intestine)
Main absorption zone: sugars, amino acids, iron, calcium, folate, and fat-soluble vitamins (A, D, E, K).
🟢 Example: Low stomach acid or no bile means nutrients here get poorly absorbed.
6️⃣ Ileum (end of small intestine)
Vitamin B12 + intrinsic factor absorbed.
Bile salts recycled back to the liver.
🟢 Example: People with Crohn’s disease affecting the ileum often end up B12 deficient.
7️⃣ Colon (large intestine)
Absorbs water, sodium, potassium, and leftover bile salts.
🟢 Example: This is why diarrhea causes dehydration. Water absorption fails here.
Each nutrient has its own “checkpoint” along the gut. Carbs, proteins, and fats mostly absorb in the jejunum, B12 in the ileum, and water + electrolytes in the colon. Smooth digestion depends on stomach acid, bile, enzymes, and every section of the gut working in sync.
