How Digestion Works Explained (Step-by-Step)

Digestion is more than just what happens in your stomach. It’s a coordinated process that starts before you eat and ends once nutrients are absorbed and waste is removed.

Digestion starts outside of the body

1. The Preparation Phase (Seeing & Smelling Food)

Digestion begins before the first bite.

When you see or smell food, your brain activates what’s called the cephalic phase:

• Stomach acid starts being released

• Digestive enzymes are prepared

• Bile production is signalled

👉 Your body is preparing to digest efficiently.

Why this matters:

• Eating in a relaxed state improves digestion

• Rushing or eating stressed can reduce digestive efficiency

  
  

2. The Mouth (Chewing & Saliva)

Once you start eating, digestion continues in the mouth.

What happens:

• Food is broken down mechanically (chewing)

• Saliva mixes with food

• Enzymes begin the digestive process

Chewing is crucial because:

• Smaller food particles are easier to digest

• It reduces workload on the stomach

• It signals the rest of the digestive system to activate

👉 Poor chewing = harder digestion later

3. The Stomach (Acid & Protein Breakdown)

After swallowing, food enters the stomach.

Key functions:

• Strong acid (HCl) breaks down protein

• Activates digestive enzymes

• Kills harmful bacteria

The stomach turns food into a semi-liquid form called chyme.

Important:

• Protein digestion depends heavily on stomach acid

• Low acid can lead to bloating or discomfort

  
  

4. The Small Intestine (Main Digestion & Absorption)

This is where most digestion and nutrient absorption happens.

What happens here:

• Protein breaks down into→ amino acids

• Fat breaks down into→ fatty acids (with help from bile)

Organs involved:

• Liver → produces bile

• Gallbladder → releases bile

• Pancreas → releases digestive enzymes

What gets absorbed:

• Nutrients

• Vitamins

• Minerals

• Energy

👉 This is the most important stage of digestion

Intestines

5. The Large Intestine (Colon)

After most nutrients are absorbed, remaining material enters the colon.

Main roles:

• Absorb water

• Process leftover material

• House gut bacteria

On diets high in plant foods:

• Fibre is fermented by bacteria

On animal-based diets:

• Very little residue remains

👉 Less waste = smaller, less frequent stools

6. The Cecum (Fibre Fermentation Area)

The cecum is a small pouch at the start of the large intestine.

Its role:

• Ferment fibre

• Support bacteria that break down plant material

Important comparison:

Herbivores (cows, horses):

• Large cecum

• Depend on fibre fermentation for energy

Humans:

• Small cecum

• Limited ability to extract energy from fibre

👉 Fibre digestion is secondary, not essential as we can not break it down, just goes through you, causing damage in the intestines.

The Full Flow

Seeing food→ brain activates digestion→ chewing + saliva→ stomach acid breakdown→ small intestine absorption→ colon processes leftovers→ waste eliminated

Most digestion and nutrient absorption happens early—before food ever reaches the colon.

This means:

• The body is highly efficient at extracting nutrients

• Especially from nutrient-dense foods like animal products

  
  

Simple takeaway

Digestion is a front-loaded process—your body does most of the work early, making it highly efficient at extracting nutrients from food.