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How do stomach acid and pepsin begin protein digestion?
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human nutrition
MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.
Protein digestion begins in the stomach through the coordinated action of hydrochloric acid and the enzyme pepsin.
The process functions as follows:
- Activation: The stomach lining stores an inactive precursor called pepsinogen. When stimulated by hormonal or neural signals, pepsinogen is released into the stomach, where it mixes with hydrochloric acid. This acidic environment rapidly converts the inactive zymogen into the active enzyme, pepsin.1
- Optimal Conditions: Pepsin is most effective in the highly acidic environment of the stomach, typically functioning at a pH between 1.5 and 3.5.12
- Proteolysis: Once active, pepsin performs proteolysis, breaking down complex dietary proteins into smaller units known as peptides. This initial phase accounts for approximately 10 to 15 percent of total protein digestion.12
While pepsin initiates the breakdown, the majority of protein digestion is completed later in the small intestine by more powerful pancreatic enzymes.2