Digestion and Absorption of Carbohydrates, Proteins, and Fats

Carbohydrates, proteins, and fats are the primary nutrients a human body requires to grow, move, maintain, and repair organ and tissue function. These are the three macromolecules broken down for the body to absorb at various rates. They move through the different organs in the body in different forms. This article will discuss how the digestion and absorption of carbohydrates, proteins, and fats occur in human beings.

How Does the Digestion of Carbohydrates Take Place?

Carbohydrate digestion and absorption take place in monosaccharide form absorption. Thus, the carbohydrates need to digest fructose, glucose, and galactose to allow the absorption process to continue smoothly. a-Amylases, maltase, lactase, sucrase, and trehalose are the enzymes required for carbohydrate digestion and absorption in human beings. a-Amylases are found mainly in the saliva and pancreas of human beings, and it hydrolyses the 1,4-glycosidic bonds present in starch. Next, maltase, sucrase, and a-dextrinase hydrolyse oligosaccharides into glucose. Moreover, sucrase is known to degrade sucrose into fructose and glucose. Lastly, lactase breaks lactose into glucose and galactose, while trehalase works on trehalase and converts it into glucose.

Absorption of Carbohydrates in Human Beings

• Fructose

Since the absorption of fructose takes place through facilitated diffusion, the conduction cannot occur across the concentration gradient.

• Galactose and Glucose

The intestinal lumen transfers the glucose and galactose into the cells through Na+-dependent co-transport in the lumen membrane. The Na+ is transported downhill while the sugar transportation takes place uphill. The blood receives sugar through facilitated diffusion. The Na+ gradient of the lumen membrane remains maintained to the Na+-K+ pump.

Digestion of Proteins

The primary source of amino acids is dietary proteins. The utilisation of these amino acids results in various cellular substances. Proteins are broken down into amino acids before absorption can take place. The dietary products are broken down into amino acids, tripeptides, and dipeptides. The exopeptidases enzyme is known to hydrolyse one amino acid from the C-terminus in proteins. Endopeptidases enzymes are known to degrade proteins by hydrolysing interior peptide bonds. Peptides are responsible for digesting proteins. 

Enzymes Involved in the Digestion of Proteins

Pepsin and pancreatic proteases are the main enzymes involved in digesting proteins. Pepsin gets activated to peptic by the H+ gastric. It hydrolyses proteins into proteases and peptones. 

On the other hand, pancreatic proteases are responsible for digestion in the small intestine – several pancreatic proteases. They include chymotrypsin, trypsin, carboxypeptidase A, elastase, and even carboxypeptidase B. For example, Trypsinogen activation generates trypsin. It further activates prolapse, procarboxypeptidase A and B, and chymotrypsinogen.  

Absorption of Proteins

Tripeptides and Dipeptides

Unlike free amino acids, they are absorbed by the body faster. The faster absorption occurs in the lumen membrane because of the H+-dependant cotransport of tripeptides and dipeptides. The cytoplasmic peptidase hydrolyses them in the intestinal cells, after which they are transported to blood through facilitated diffusion.

Free Amino Acids

The Na+-dependent cotransport responsible for amino acids takes place in the lumen membrane. Through facilitated diffusion, the amino acids are transported from cell to blood. There are different carriers for acidic, basic, neutral, and imino amino acids in our body. 

Digestion of Fats

Since fats are not soluble in water, they are difficult to digest and absorb. They neither mix with the stomach nor the intestinal contents. Fat lipids include steroids, cholesterol, phospholipids, triglycerides, and fat-soluble vitamins. The process of fat digestion starts with emulsification. Here, the big droplet of fat lipids is broken down into smaller droplets. The process through the reduction of drop size helps the digestion process by increasing the surface area of the lipids and exposing them better to the enzymes.

Enzymes Involved in the Digestion of Fats and Lipids

Lingual lipases, pancreatic enzymes, bile acids, and pancreatic lipases are the major enzymes involved in lipid digestion. Although most lipids get digested through pancreatic enzymes, the triglycerides are converted into fatty acids and monoglycerides with the help of lingual lipases. Additionally, the stomach contains pancreatic enzymes, which increase lipids’ surface area, further breaking down the lipid drops. The emulsification of lipids in the small intestine takes place through bile acids. The solubilisation process of lipids’ hydrophobic products is done in micelles through bile acids. The hydrolysation process of monoglycerides, lysolecithin, cholesterol, and even fatty acids occurs in the pancreatic lipases. The enzymes include phospholipase A2, cholesterol ester hydrolase, and even pancreatic lipase.

Absorption of Fats

The products of digested lipids come in contact with the intestinal cell surface by micelles: the cholesterol, fatty acids, and monoglycerides in the lumen membrane in the cells. However, micelles don’t bring in glycerol since they are hydrophilic. Along with that, exocytosis brings chylomicrons out of the intestinal cells. The re-digestion of the lipids cells into cholesterol ester, triglycerides, and phospholipids occurs in the intestinal cells. Chylomicrons are transferred to lymph vessels since they are too large to get absorbed by the capillaries. They get into the bloodstream through the thoracic duct. 

Conclusion

The human digestion process is complex since the food we consume contains complex nutrients that require severe and routine breakdown before the bloodstream absorbs them. For example, since fats cannot dissolve in water, the digestion and absorption process is complex and unique. On the other hand, proteins break down into amino acids, while carbohydrates break into oligosaccharides before further digestion.