The digestive and absorptive processes within the gastrointestinal tract are integral to the body’s ability to derive nutrients from consumed food. These processes are complex and involve the sequential breakdown and uptake of proteins, carbohydrates, fats, electrolytes, and vitamins.
Protein Digestion and Absorption
- Stomach: Proteins begin their digestion in the stomach, where they are cleaved by pepsin and hydrochloric acid, breaking them down into smaller polypeptides.
- Duodenum: Upon entering the duodenum, the nearly neutral pH temporarily halts pepsin activity. Here, pancreatic enzymes such as trypsinogen, chymotrypsinogen, carboxypeptidase, and aminopeptidase further break down polypeptides into oligopeptides and individual amino acids. The activation of these enzymes in the duodenum is a critical step in protein digestion.
- Jejunum: The jejunum is the primary site for the absorption of amino acids, where they are rapidly transported across the enterocyte membranes into the bloodstream.
Carbohydrate Digestion and Absorption
- Mouth: The process begins in the mouth where salivary alpha-amylase starts the breakdown of complex carbohydrates into simpler sugars.
- Stomach: Carbohydrate digestion is paused in the stomach due to the acidic environment.
- Duodenum: Digestion resumes in the duodenum with the addition of pancreatic alpha-amylase, further breaking down carbohydrates into disaccharides and oligosaccharides.
- Small Intestine: Enzymes on the brush border of the small intestine, such as maltase, lactase, and sucrase, split these sugars into monosaccharides (glucose, fructose, and galactose), which are then absorbed and transported to the liver via the bloodstream.
Fat Digestion and Absorption
- Stomach: Lipase enzymes initiate the breakdown of triglycerides into diglycerides and free fatty acids.
- Duodenum and Jejunum: The process intensifies in the duodenum with the emulsification by bile acids and further enzymatic action by pancreatic lipase. This results in the formation of monoglycerides and free fatty acids, which form micelles for absorption.
- Absorption: The absorption predominantly occurs in the duodenum and proximal jejunum, where micelles facilitate the transfer of lipids across the enterocyte membrane.
Electrolyte and Water Absorption
- Source: The electrolytes in the small intestine predominantly originate from digestive secretions, with a smaller contribution from dietary intake.
- Jejunum: The jejunum plays a central role in the active and passive reabsorption of electrolytes, with water following osmotically to maintain fluid balance.
Vitamin Absorption
- Fat-soluble Vitamins (A, D, E, K): These vitamins are absorbed in the presence of dietary fats and follow the same micellar pathway as lipids for absorption.
- Water-soluble Vitamins: These are absorbed primarily in the jejunum through various mechanisms, including facilitated diffusion and active transport.
- Vitamin B12: Unique among vitamins, B12 requires the intrinsic factor for its absorption, which occurs in the ileum.
