Digestion and absorption of nutrients

Digestion of nutrients

• Digestion is a process of breaking down food into smaller, absorbable components through mechanical and chemical actions.

1. Mechanical Digestion

• Mouth: Food is physically broken down by chewing.
• Stomach: Food is mixed with gastric secretions and churned by stomach contractions.

2. Chemical Digestion

Carbohydrate Digestion

• Begins in the mouth with salivary amylase, which breaks down starches into maltose.
• In the small intestine, pancreatic amylase and brush border enzymes (maltase, sucrase, lactase) break down disaccharides into monosaccharides (glucose, fructose, galactose) for absorption.

Protein Digestion

• Starts in the stomach with pepsin, breaking proteins into smaller peptides.
• Pancreatic enzymes (trypsin, chymotrypsin) in the small intestine further break down peptides into amino acids for absorption.

Fat Digestion

• Begins in the stomach with gastric lipase.
• In the small intestine, pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids. Bile emulsifies fats to increase surface area for enzyme action.

Absorption of nutrients

1. Carbohydrate Absorption

   • Monosaccharides are absorbed in the small intestine via specific transport proteins: glucose and galactose via SGLT1, fructose via GLUT5, and all are transported into the bloodstream via GLUT2.

2. Protein Absorption

   • Amino acids are absorbed into enterocytes using specific transporters.
   • Small peptides are absorbed via PepT1, and both are then transported into the bloodstream.

3. Fat Absorption

   • Monoglycerides and free fatty acids are absorbed by simple diffusion.
   • Inside enterocytes, they are reassembled into triglycerides and packed into chylomicrons, which enter the lymphatic system and eventually the bloodstream.

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