Absorption and Assimilation of Proteins: Diffusion, Transport, Digestion

Introduction

The digestion of protein in the body entails first breaking this complex molecule into peptides, each having a number of amino acids, then into individual amino acids and using those building blocks to make the proteins that our body needs for daily life.

Absorption of proteins refers to the body’s ability to break down all the protein sources that we consume into individual building blocks (amino acids) and then use those amino acids to create the proteins in the form that our body can utilise for everyday activities, such as maintaining and improving muscle strength. During protein digestion and absorption, the body must break down protein to be able to absorb it and use it effectively for body processes.

Certain conditions, such as ageing and strenuous exercise, can increase a person’s protein requirement. The body may absorb proteins more slowly as we get older. Furthermore, athletes require a greater amount of protein in order to maintain and rebuild their muscles. Our bodies absorb protein better if we take positive steps to improve our diet, but it is not always enough. Maintaining good digestive health is essential in ensuring that your body can use protein efficiently.

What is a protein?

The human body relies heavily on proteins. Among the proteins that make up our body are muscles, eyes, hair, organs, enzymes and hormones. Our body also depends on proteins for repair and maintenance.

People can make their body consume protein more effectively by ensuring that the protein they consume is made up of the best quality.

Amino acids, which make up proteins, are smaller substances. Our body can make only 11 amino acids out of the 20 amino acids. Nine of the other amino acids are called essentials and must be consumed as part of a balanced diet.

The nine essential amino acids are all present in foods like meat, eggs, fish and dairy products. Therefore, they are referred to as complete or whole proteins.

In addition to nuts, beans and seeds, there are other sources of protein that include some essential amino acids. In fact, some of these sources of protein can be combined, such as rice and beans, to create a complete protein for the consumption of all essential amino acids.

Protein Digestion and Absorption 

Acids and enzymes in the stomach interact to digest protein. Proteins are broken down by the enzymes activated by the stomach acid. 

• In the first place, pepsin, an enzyme in the stomach, breaks down and splits the  proteins in the food into shorter polypeptides
• Enzymes are further released by the pancreas into the small intestine, where the polypeptide chains are broken and the process of breaking down peptides and amino acids begins
• Following this, the small intestine releases more peptidases to digest the food by breaking down the proteins into amino acids
• Finally, the amino acids are carried across the small intestinal wall and into the bloodstream, where they are taken up and utilised by the tissues in the body to maintain normal, healthy functioning

Digestion and Absorption of Carbohydrates, Proteins, and Fats

Digestion of carbohydrates

Monosaccharides are the only carbohydrates that are absorbed. Consequently, carbohydrates must undergo digestion into glucose, galactose and fructose before they can be absorbed.

The enzymes involved:

• Pancreatic and Salivary amylases hydrolyze 1,4-glycosidic bonds to form maltose, maltotriose and α-limit dextrins
• It is then hydrolyzed to glucose by maltase, ɑ-dextrinase, and sucrase at the intestinal brush border
• The disaccharides; lactose, trehalose and sucrose are degraded by the enzymes lactase, trehalase and sucrose to monosaccharides
• Lactose is degraded into glucose and galactose by lactase
•  Trehalose is degraded by trehalase into glucose
•   Glucose and fructose are formed when sucrose is degraded by sucrase

Absorption of carbohydrates

1. Glucose and Galactose

• Na+-dependent co-transporters (SGLT 1) in the luminal membrane transport the compounds into the cells from the intestinal lumen.
• Na+ is transported “downhill” and sugar is transported “uphill.”
• By facilitating diffusion (GLUT 2), they are transported from the cells to the blood.
• Consequently, Na+-K+ pumping in the basolateral membrane maintains the Na+ gradient across the luminal membrane to keep the intracellular [Na+] low.

2. Fructose

• Consequently, the fructose cannot be absorbed against a concentration gradient since it is transported entirely by the facilitated diffusion.

Digestion of proteins

• Consuming protein in the diet provides amino acids which are used to make cellular substances
• A protein’s absorption requires it to be broken down into amino acids. Dipeptides, tripeptides and amino acids are digested products of protein
• A peptide-degrading enzyme called endopeptidase hydrolyzes the interior peptide bonds of proteins, while an exopeptidase enzyme hydrolyzes one amino acid at a time from the C-terminus of proteins
• The small intestine and the stomach are involved in digestion

The enzymes involved are:

Pepsin

• The chief cells of the stomach secrete pepsin in its zymogen form as pepsinogen
• Gastric H+ triggers the conversion of pepsinogen into pepsin. A pH of 1- 3 is ideal for pepsin
• Proteins are hydrolyzed by pepsin into peptides and proteoses
• Pepsin denatures when the pH exceeds 5. HCO3 thereby increases pH in the duodenum and inactivates pepsin in the intestines as it is secreted by the pancreas

Pancreatic proteases

• With the action of the pancreatic and intestinal juice, the digestion is completed in the small intestine
• Trypsin, elastase, chymotrypsin, carboxypeptidase A and carboxypeptidase B are among the proteases
• Inactive forms of these enzymes are secreted into the small intestine which activates them as follows:
  • By using enterokinase, trypsinogen is converted to trypsin
  • The enzyme trypsin converts the inactive forms of proelastase, chymotrypsinogen and procarboxypeptidase A and B into their active forms

Absorption of Proteins

The absorption of proteins occurs in the small intestine with various enzymes acting upon them.

1. Free amino acids

•  In the luminal membrane, co-transport of Na+ dependent amino acids takes place. Glucose and galactose co-transporters are analogous to this transporter
• The facilitated diffusion transports amino acids from the cells to the blood
• Four different carriers exist for neutral, acidic, basic and imino amino acids, respectively

2. Dipeptides and tripeptides

• As compared to free amino acids, they are absorbed faster
• The luminal membrane is also capable of cotransporting dipeptides and tripeptides
•  The cytoplasmic peptidases hydrolyse the dipeptides and tripeptides into amino acids after they are transported into intestinal cells
• The facilitated diffusion transports the amino acids from the cells to the blood

Digestion of Fats

• Due to their insoluble nature in water, fats are difficult to digest and absorb. The stomach and intestinal contents cannot mix with the fat
• Among the many lipids in our bodies are triglycerides, phospholipids, cholesterol, steroids and fat-soluble vitamins
• It is essential for lipid digestion to emulsify large lipid droplets in order to form tiny lipid droplets
• It decreases the droplet size of the lipid-exposed to digestion enzymes by emulsifying it

The enzymes involved:

1. In the mouth

• As part of digestion, lingual lipases can transform some ingested triglycerides into monoglycerides and fatty acids
• The pancreatic lipases digest the majority of ingested lipids in the intestine

2. Stomach

• The mixing increases the surface area for pancreatic enzymes to digest lipids in the stomach by breaking them into droplets

3. Small intestine

• The small intestine is enriched with bile acids, which emulsify the lipids and increase the surface area in the digestive system. The bile acids dissolve hydrophobic products in micelles after the digestion of lipids
• During the hydrolysis of lipids, pancreatic lipases produce fatty acids, cholesterol, lysolecithin and monoglycerides. Pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase are the enzymes involved

Absorption of Fats

• The absorptive surface of intestinal cells is in contact with the products of lipid digestion through micelles
• Over the luminal membrane, fatty acids, monoglycerides and cholesterol diffuse into the cells. Hydrophilic Glycerol does not form micelles
• The cholesterol ester, phospholipids and triglycerides are re-esterified from lipids in intestinal cells, along with apoproteins, forming chylomicrons
• Exocytosis is the process by which chylomicrons are transported from intestinal cells
• Due to the chylomicrons’ size, they cannot enter capillaries. This causes them to bypass lymph nodes and enter the bloodstream through the thoracic duct

 Assimilation: It is a process by which absorbed nutrients are utilized to resynthesise complex molecules like carbohydrates, proteins and fats inside the cells.

Conclusion

The body depends on protein for nearly every function. Before the amino acids are released into the bloodstream, they are digested in the mouth, stomach and small intestine.

By eating complete proteins and by chewing thoroughly before swallowing, you can maximise your intake of nutrients from protein sources. In our older years, our bodies have a harder time absorbing protein. Athletes also require a higher amount of protein to maintain their muscle health. It is true that eating a healthy diet can improve our bodies’ ability to absorb protein, but a diet alone may not be enough. You would need a balanced diet for the absorption of proteins in the body.