Digestion:
The process of breaking down food into smaller, absorbable components through mechanical and chemical actions.
a. Mechanical digestion:
Mouth: Food is physically broken down by chewing.
Stomach: Food is mixed with gastric secretions and churned by stomach contractions.
b. Chemical digestion:
This process involves breaking down macronutrients (proteins, carbohydrates, and fats) into their smaller building blocks using enzymes and other substances.
1. 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.
2. 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.
3. 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:
The process of taking up digested nutrients into the bloodstream or lymphatic system for distribution to cells.
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.
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.
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.