Protein is an important macromolecule and the main source of energy. It supports tissue growth and repair, boosts immunity, enhances focus, and helps produce hormones and enzymes. For energy generation, proteins must be converted into their building blocks that are absorbed and broken down. Absorption also depends upon the source of protein you take. In this article, we will discuss protein digestion, absorption, and sources of proteins, either from natural or supplements.
What is digestion?
Food is broken down through digestion to obtain energy from it. It produces the energy from all the macromolecules. Our diet comprises three macromolecules e.g., Proteins, Carbohydrates, and Fats. One molecule of Protein and carbohydrates produces 4 while fats have 9 ATPs. Thus for the functioning of these molecules, they must be digested and absorbed into the body and functionally available to the body.
What is the digestion of Proteins?
Protein is broken down into peptides and amino acids throughout the mechanical and chemical processes. Thus large molecules are converted into smaller ones increasing their absorbility. Let’s discuss the parts involved in digestion and absorption.
Where does protein digestion take place?
Mechanical digestion
First, protein digestion takes place in the mouth. This is mechanical digestion. In this process, large food molecules are broken down into smaller ones, increasing their surface area, which provides a larger surface area for the enzyme action on the bolus. When the teeth have completed mechanical digestion, the peristalsis movement pushes this bolus down to the stomach.
Chemical digestion
Chemical digestion takes place in the stomach and small intestine. Let’s discuss how this works.
Chemical digestion in the Stomach:
When the protein is touched in the mouth, it signals to the stomach to release gastric juices. Hydrochloric acid (HCL) is present in the stomach, which activates the pepsin enzyme. It converts the pepsinogen into an active form (pepsin). This enzyme breaks down large pieces of protein into amino acids. HCL in the stomach also provides an acidic medium. The pH of the stomach drops, killing the bacteria in the food.
Gastrin regulates the secretions of the stomach for proper functioning.
The walls of the stomach convert the bolus into chyme. This chyme is then pushed to the small intestine for final digestion and absorption of protein.
Chemical digestion in the small intestine
The chyme is passed into the small intestine. In the small intestine, complete digestion of the proteins takes place. Pancreatic enzymes help break down the partially digested protein into the amino acids.
The pancreas releases many enzymes, such as proteases and peptidases. They are transported in an inactive form to prevent them from self-digestion.
The small intestine releases the enterokinase. This enzyme converted the chymotrypsinogen into the active form of chymotrypsin and procarboxypeptidase into carboxypeptidase. Carboxylase breaks the carboxyl group (-COOH) in the protein. It also converts the trypsinogen into active form trypsin. This enzyme breaks down the amino acids, especially arginine and lysine. Elastase breaks down the elastic and fibrous protein.
Enzymes that complete the digestion of the protein in the small intestine are three types of brush border.
Absorption of the protein
Amino acids are absorbed in the small intestine through these processes. These are given below:
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Active Transport
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H+ dependant transport
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Facilitated diffusion
Let’s discuss them one by one.
Active Transport:
Amino acids are carried into the blood flow through the sodium-dependent co-transporter.
H+ dependent transport:
Mostly dipeptides and tripeptides are transported through the PEPT1 transporter into the intestinal cells.
Facilitated diffusion
When the amino acids reach enterocytes they are transported through facilitated diffusion.
Thus the process of absorption of the amino acids is completed. Now these amino acids are transported to the liver.
Transport to the Liver (Hepatic Portal System)
These amino acids are carried to the liver for further action. The hepatic portal system is utilized for this function. In this system, amino acids enter the portal vein which carries them to the liver. The liver utilizes them according to the body's requirements.
Process of Amino Acids in the Liver:
These amino acids are processed in the liver. Here are the functions carried out by the liver.
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Proteins synthesized
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Energy Production
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Amino acids storage and distribution
Proteins synthesis:
The liver uses these amino acids to produce the plasma proteins e.g. such as albumin, Clotting factors, and transport proteins.
Energy Production:
Sometimes the liver removes the amino group from these amino acids, later utilized for urea production that leaves the body through urine. The remaining carbon group is
used for ATP production. ATP is the energy unit produced by the mitochondria of the cells through the process of gluconeogenesis.
Amino acids storage and distribution:
The liver temporarily stores these amino acids and releases them into the bloodstream as needed. These amino acids are later utilized for repairing muscles, enzyme production, and other body functions.
Circulation of the Amino acids:
These amino acids are then circulated to these parts of the body.
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Muscles
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Brain/Nervous system
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Immune Cells
Muscles
These amino acids are utilized to repair muscles and synthesis of protein.
Brain and Nervous System:
Nerve impulses travel through the nervous system, and neurotransmitters are made of proteins.
Immune System:
To kill the antigens (foreign particles), our immune system produces antibodies. These antibodies are made up of proteins.
Factors Affecting Protein Digestion and Absorption
Protein digestion and absorption depend upon many factors. Some of them are given below:
Digestive Secretions:
Stomach and pancreatic secretions do the chemical digestion of protein. Change in any secretion may alter the digestion and without proper digestion absorption of the amino acid will not take place.
Protein Sources:
The source of the proteins matters a lot. Animal protein is not difficult to digest compared to plant protein because animal protein has all the essential amino acids.
Cooking methods:
The structure of the protein is important, Well cooked protein is easier to digest than raw protein. Raw proteins are also difficult to digest. Broken-down proteins like shakes are easy to absorb.
Gut Health:
A healthy gut can easily digest and absorb proteins. Gut microbiota is necessary to carry out this process.
Inhibitors or Enhancers:
Inhibitors decrease digestion and absorption. Enhancers activate the enzymes to boost protein digestion and absorption.
Physiological and Metabolic Factors:
Age affects the digestion and absorption of protein. As you age, hormone changes affect the metabolism.
Medical Conditions:
Certain diseases affect the digestive tract and secretions e.g. Celiac Disease, GERD, Lactose Intolerance, cirrhosis, kidney disease. Such medical conditions disturb the digestive tract.
Fast-Digesting Protein Foods
Fast-digesting proteins can break down into amino acids easily. These proteins are best for post-workout. Some of the easily digested proteins are given below:
Whey Protein:
Whey protein can be digested within 30-60 minutes. It is considered the ideal for post-workout.
Egg White:
Egg white has maximum bioavailability. It can be digested within 30-45 minutes, its low portion of fat increases its absorption.
Protein Shakes (Hydrolyzed Whey or Pea Protein)
These proteins are predigested and absorbed quickly, this quality makes them the ideal choice for post-workout for muscle recovery.
Plant-Based Protein Powders (Pea, Rice, Soy, Hemp)
These proteins can also absorb very fast approximately 30-90 minutes. They can be made from Pea, Rice, Soy, etc.
Conclusion
Protein digestion is breaking down into amino acids for proper absorption. Digestion takes place in the mouth, stomach, and small intestine. The pancreas plays a supportive role in producing secretions.
The digested protein is absorbed into the body through the small intestine. After the absorption, these amino acids are used to perform different functions.
The source of the proteins, cooking methods, etc. affect the digestion and absorption of the proteins.
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