Digestion and Absorption of Food

Digestion is a vital physiological process that breaks down food into smaller molecules for absorption and utilization by the body. This article explores the human digestive system and the mechanisms involved in the digestion and absorption of food, key to maintaining life and health.

What is Digestion?

Digestion is the mechanical and chemical breakdown of food into absorbable units:

A. Mechanical digestion: Physical breakdown (e.g., chewing, churning).

B. Chemical digestion: Enzymatic reactions that convert complex molecules into simple ones.

1. Ingestion: 

It is the process of taking food into the mouth. It's the first step of the digestive process which initiates digestion. Mouth Contains teeth for chewing and tongue for mixing food.  In the mouth salivary glands secrete saliva which contains salivary amylase begins chemical digestion of starts. In the mouth food is lubricated by the mucus for easier swallowing. Then the food is shaped into a bolus and swallowed, passing through the pharynx and esophagus via peristalsis (wave-like muscle contractions).

2. Digestion: It involves two processes

A. Mechanical Digestion: It is the physical breakdown of food. It is completed in mouth where chewing increases surface area and in stomach where muscular churning mixes food with gastric juices.

B. Chemical Digestion: It is the enzymatic breakdown of macromolecules into absorbable units. Each macronutrient is broken down by specific enzymes:

➤ Carbohydrates: It is breakdown as follows-

1. Salivary amylase: Begins starch digestion in the mouth.

2. Pancreatic amylase: Continues digestion in the small intestine.

3. Brush border enzymes (maltase, sucrase, lactase): Convert disaccharides into monosaccharides.

➤ Proteins:

1. Pepsin (from pepsinogen, activated by HCl in stomach): Begins digestion.

2. Trypsin & chymotrypsin (from pancreas): Act in the small intestine.

3. Peptidases: Convert peptides into amino acids.

➤ Fats:

1. Bile salts (from liver via gallbladder): Emulsify fats.

2. Pancreatic lipase: Breaks fats into fatty acids and monoglycerides.

After chemical digestion food is converted into absorbable molecules such as Glucose, Amino acids and Fatty acids.

Key Organs and Enzymes:

Organ	Enzyme/Secretion	Function
Mouth	Salivary amylase	Begins starch digestion
Stomach	Pepsin (from pepsinogen), HCl	Begins protein digestion
Pancreas	Trypsin, Amylase, Lipase	Digests proteins, carbs, fats
Small Intestine	Brush border enzymes	Final digestion of all macronutrients

3. Absorption: 

    

This is the process of uptake of digested nutrients into the bloodstream or lymph. It occurs mainly in the small intestine (duodenum, jejunum, illeum). The villi and microvilli increase surface area forming the brush border. The capillaries absorb amino acids and sugars into the bloodstream and lacteals (lymph vessels) absorb fatty acids into the lymphatic system.

How Nutrients Travel:

Nutrient Type	Absorption Site	Transport Pathway
Glucose, Amino Acids	Small intestine (villi)	Bloodstream → Liver via portal vein
Fatty Acids, Glycerol	Small intestine (lacteals)	Lymphatic system → Bloodstream
Vitamins, Minerals	Various intestinal sites	Blood (fat-soluble ones via lymph)

4. Defecation: 

The elimination of undigested foods residues as feces through the large intestine and rectum. In the large intestine water and electrolytes are absorbs whereas Gut microbiota synthesizes certain vitamins (e.g., vitamin K). The residue becomes semi-solid feces and stored in the rectum until expelled through the anus via voluntary and involuntary reflexes.

Summary Table:

Stage	Major Organs Involved	Functions
Ingestion	Mouth, Salivary glands	Intake of food, initial starch digestion
Digestion	Mouth, Stomach, Pancreas, Small Intestine	Breakdown of food mechanically and chemically
Absorption	Small Intestine, Blood, Lymph	Uptake of nutrients into the body
Defecation	Large Intestine, Rectum, Anus	Elimination of waste and indigestible matter

  

References

1. Tortora GJ, Derrickson BH. Principles of Anatomy and Physiology, 15th ed. Wiley; 2017.
2. Guyton AC, Hall JE. Textbook of Medical Physiology, 14th ed. Elsevier; 2021.
3. National Institute of Diabetes and Digestive and Kidney Diseases. niddk.nih.gov