We obtain amino acids from the hydrolysis, digestion and absorption of dietary food to build our own proteins. This process begins with cooking the food.
Animal muscle is our main source of our natural protein. The muscle fibers are orderly biological tissues composed of myosin and actin bundles that are wrapped by collagen-rich connective tissue, a tripe helix fibrous protein. The numerous hydrogen bonds allow muscle and collagen to withstand strong external forces and also hinder our digestion. Eating raw meat directly requires extra energy to break these hydrogen bonds. The fire invention marked a milestone in human civilization. These hydrogen bonds are destroyed directly, and a disordered, loose structure appears during high temperature. Thereby, cooked meat is softer and easier to chew. Cooking also kills microorganisms that parasitize animals. After all, diarrhea and parasite infections greatly reduce nutrient absorption.
Chewing is the second step in protein digestion. Your teeth cut meat into very small pieces to increase surface area. The larger surface area, the faster chemical reaction rate, so it is beneficial for quick breakdown of proteins in subsequent digestive process. At the same time, chewing is one of the signals to start your digestive system. Your stomach is prompted to secrete more gastric acid and pepsin. Its content is emptied by gastric peristalsis rapidly. Although it seems unrelated to protein digestion, chewing is an indispensable step. Tooth decay, missing teeth, or orthodontics may result in poor nutrient absorption.
When food enters stomach, powerful and rhythmic muscle contractions not only mix food with gastric juice evenly, but also crush them into smaller pieces through mechanical force. Since the very low pH, pepsin is activated and proteins carry positive charges that will repel each other to unfold them. It is easier for protein to be broken down into polypeptides. Only a few milliliters semi-fluid is pushed into small intestine through pylorus at a time; otherwise, the food influx will lead to indigestion.
The small intestine also undergoes peristalsis to mix semi-fluid food with digestive juices evenly. Since it moves only a few centimeters per minute, it takes several hours for food to achieve the large intestine. Pepsin becomes inactive in the alkaline small intestine, but trypsin and chymotrypsin are activated here to hydrolyze polypeptides into oligopeptides. The surface of villi in small intestine, as well as cytoplasm, also contains peptidases. The small peptides are hydrolyzed into free amino acids by them.
Traditional theory holds that proteins must be decomposed into amino acids before absorption. Recent studies have found that some oligopeptides can also be absorbed directly. Experiments have shown that animals fed oligopeptides have a higher lean growth rate than those fed amino acids. This phenomenon is attributed to independent transport mechanism for peptide. Amino acid uptake is dependent on multiple transport proteins that compete with each other to result in inefficient. For example, free lysine and arginine compete for same transport proteins. This explains why excess arginine hinders lysine absorption. However, peptide transporters don’t have this problem, ensuring the rapid and efficient absorption of oligopeptides.
During co-transport, the proton gradient is consumed and oligopeptides are actively transported into cells through peptide transporters (such as PEPT1) that are located on cell membrane. Most of the peptides that enter cells via this way are dipeptides and tripeptides; it’s very difficult for oligopeptides with more than 5 amino acids. After binding with CCP, longer peptides penetrate phospholipid bilayer directly or through endocytosis. Then they are delivered to lysosomes and quickly degraded into amino acids. Sometimes free amino acids in cytoplasm will inhibit their hydrolysis, so a few oligopeptides are found in blood. Organelles called peroxisomes are where they are processed. Oligopeptides also have specific biological activity. One is that collagen peptides activate our body to synthesize more collagen. This discovery not only provides new perspectives for research in nutrition and physiology, but it’s also a great significance for animal husbandry and human nutrient supplement. Thus, peptide supplements are becoming more and more popular on market.
These amino acids are raw materials for cells to build human proteins. Excess amino acids are deaminated in liver and converted into glucose that becomes energy for cells or synthesizing glycogen and fat.