Digestion and Absorption of Carbohydrates, Proteins and Fats

Carbohydrates, fats, and proteins are the major nutrients the body needs for growth, repair, movement and maintaining tissue and organ function.
These macromolecules are broken down and absorbed into the body at different rates and into specific forms as they travel through the organs in the digestive system.

Digestion of carbohydrates

Among carbohydrates, only the monosaccharide forms are absorbed. Hence, all carbohydrates must be digested to glucose, galactose, and fructose for absorption to proceed.

Enzymes Involved

a-Amylases (salivary and pancreatic) hydrolyze 1,4-glycosidic bonds in starch, yielding maltose, maltotriose, and α-limit dextrins.
Maltase, a-dextrinase, and sucrase in the intestinal brush border then hydrolyze the oligosaccharides to glucose.
Lactase, trehalase, and sucrase degrade their respective disaccharides lactose, trehalose and sucrose to monosaccharides.
- Lactase degrades lactose to glucose and galactose.
- Trehalase degrades trehalose to glucose.
- Sucrase degrades sucrose to glucose and fructose.

Absorption of carbohydrates

1. Glucose and Galactose

They are transported from the intestinal lumen into the cells by a Na+-dependent co-transport (SGLT 1) in the luminal membrane.
The sugar is transported “uphill” and Na+ is transported “downhill.”
They are then transported from cell to blood by facilitated diffusion (GLUT 2).
The Na+–K+ pump in the basolateral membrane keeps the intracellular [Na+] low, thus maintaining the Na+ gradient across the luminal membrane.

2. Fructose

Fructose is transported exclusively by facilitated diffusion; therefore, it cannot be absorbed against a concentration gradient.

Digestion of proteins

Dietary proteins are a source of amino acids which are utilized for formation of various cellular substances.
Mostly, proteins must be broken down into amino acids for absorption. Digestive products of protein can be absorbed as amino acids, dipeptides, and tripeptides
Both endopeptidases enzymes which degrade proteins by hydrolyzing interior peptide bonds and exopeptidases enzyme that hydrolyzes one amino acid at a time from the C-terminus of proteins and peptides are involved in the digestion of proteins.
Digestion takes place in the stomach and the small intestine.

Enzymes Involved

Pepsin
- Pepsin is secreted in its zymogen form as pepsinogen by the chief cells of the stomach.
- Pepsinogen is activated to pepsin by gastric H+. The optimum pH for pepsin is between 1 and 3.
- Pepsin hydrolyzes proteins into peptones and proteoses.
- When the pH is >5, pepsin is denatured. Thus, in the intestine, as HCO3− is secreted in pancreatic fluids, duodenal pH increases and pepsin is inactivated.

Pancreatic proteases
- The digestion is completed in the small intestine by the action of pancreatic and intestinal juice.
- The proteases include trypsin, chymotrypsin, elastase, carboxypeptidase A, and carboxypeptidase B.
- They are secreted in inactive forms that are activated in the small intestine as follows:
- Trypsinogen is activated to trypsin by a brush border enzyme, enterokinase.
- Trypsin then converts chymotrypsinogen, proelastase, and procarboxypeptidase A and B to their active forms.

Absorption of Proteins

1. Free amino acids

Na+-dependent amino acid cotransport occurs in the luminal membrane. It is analogous
to the cotransporter for glucose and galactose.
The amino acids are then transported from cell to blood by facilitated diffusion.
There are four separate carriers for neutral, acidic, basic, and imino amino acids, respectively.

2. Dipeptides and tripeptides

They are absorbed faster than free amino acids.
H+-dependent cotransport of dipeptides and tripeptides also occurs in the luminal membrane.
After the dipeptides and tripeptides are transported into the intestinal cells, cytoplasmic peptidases hydrolyze them to amino acids.
The amino acids are then transported from cell to blood by facilitated diffusion.

Digestion of Fats

Fats not being soluble in water by their nature are both difficult to digest and absorb. They do not mix with the stomach or intestinal contents.
Lipids include triglycerides, phospholipids, cholesterol, steroids, and fat-soluble vitamins.
The first step in lipid digestion is emulsification, which is the transformation of large lipid droplets into much smaller droplets.
The emulsification process increases the surface area of the lipid-exposed to the digestive enzymes by decreasing the droplet size.

Enzymes Involved

1. In the mouth

Lingual lipases digest some of the ingested triglycerides to monoglycerides and fatty acids.
However, most of the ingested lipids are digested in the intestine by pancreatic lipases.

2. Stomach

In the stomach, mixing breaks lipids into droplets to increase the surface area for digestion by pancreatic enzymes.

3. Small intestine

Bile acids emulsify lipids in the small intestine, increasing the surface area for digestion. The hydrophobic products of lipid digestion are solubilized in micelles by bile acids.
Pancreatic lipases hydrolyze lipids to fatty acids, monoglycerides, cholesterol, and lysolecithin. The enzymes are pancreatic lipase, cholesterol ester hydrolase, and phospholipase A2.

Absorption of Fats

Micelles bring the products of lipid digestion into contact with the absorptive surface of the intestinal cells.
Then, fatty acids, monoglycerides, and cholesterol diffuse across the luminal membrane into the cells. Glycerol is hydrophilic and is not contained in the micelles.
In the intestinal cells, the products of lipid digestion are re-esterified to triglycerides, cholesterol ester, and phospholipids and, with apoproteins, form chylomicrons.
Chylomicrons are transported out of the intestinal cells by exocytosis.
Because chylomicrons are too large to enter the capillaries, they are transferred to lymph vessels and are added to the bloodstream via the thoracic duct.