The first step in protein breakdown is to change the shape of protein, but not its primary structure. Factors such as temperature, pH, and enzymes play a role in protein digestion and absorption. For most protein-dense foods this begins with cooking your food. Applied heat while cooking denatures proteins. Further denaturing occurs within the body by mastication (chewing) and the acidic environment of the stomach.
Chewing food causes the stomach wall to release gastrin in anticipation of the digestion process along with hydrochloric acid and the hormone pepsinogen. Together they release active enzymes that begin the enzymatic breakdown of protein in the stomach. Very long polypeptide chains begin breaking into smaller ones. Those smaller ones move to the small intestine for the next step in digestion.
In the small intestine, various protease enzymes further the breakdown by breaking peptide chains into single amino acids to be absorbed by the body.
Within the liver, amino acids may be used for protein synthesis, broken down into urine waste, converted to carbohydrate of fat, metabolized for energy, or released into the peripheral blood stream for use throughout the body. In one meal, 85% of plant protein and 95% of animal protein is actually absorbed. Any remaining proteins and peptides make it past the small intestine and into the large intestine is excreted as waste.
Protein Function
While fats and carbohydrates may be thought of as energy macronutrients, protein is more of a structural and functional macronutrient like forming bodily structures. Proteins and amino acids are primarily used to create bodily tissue. But when theirs an abundant amount of protein in the diet and/or when there isn't enough carbs to support normal energy expenditure protein can be metabolized for energy...
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