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The process of protein digestion is a step-by-step process of breaking down food's large, complex molecules into small, absorbed amino acids. It begins as mechanical degradation in the mouth, progresses to acid denaturation and primary enzymatic cleavage by pepsin in the stomach, and is finished almost entirely by a combination of pancreatic and brush border enzymes in the small intestine.
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Figure 1: Protein
Think of proteins as small LEGO blocks that our body uses to construct nearly everything: muscles, hair, skin, enzymes (which make chemical reactions occur), and even messengers called hormones (1). When we consume foods such as chicken, fish, eggs, beans, or lentils, we're consuming proteins.
But, these proteins in food are typically large and complicated, such as a completed LEGO castle. Our body cannot utilize this large castle directly. It must disassemble it into individual LEGO bricks – these are referred to as amino acids (1,2). Our body then constructs its own unique structures using these amino acids. This entire process of breaking down is digestion, and getting those bricks into our bloodstream is absorption. Figure 2: Protein
is highly acidic (pH of 1.5-3.5) since it secretes hydrochloric acid (HCl) (3). This potent acid does two principal things:
folded structures of proteins, such as uncurling a ball of knotted string. This lays bare the long chains of amino acids (2,3).
enzyme known as pepsinogen. HCl transforms pepsinogen into its active state, pepsin (1,3).
pair of scissors. It begins cutting those long, uncoiled protein chains (now referred to as polypeptides) into smaller segments – shorter polypeptides and some free amino acids (1,2).
Pepsin functions optimally in the stomach's acidic environment.
proteins are unfolded by the stomach acid. Next, pepsin begins to slice these opened proteins into bite-sized pieces.
A majority of the protein digestion takes place in the small intestine, and that includes the first segment known as the duodenum (2). When the half-digested food (a soupy concoction now known as chyme) passes from the stomach to the small intestine, two crucial things occur:
organ, the pancreas, discharges a mixture of potent protein-eating enzymes (proteases) into the small intestine.
Polish The cells of the small intestine (enterocytes) possess a "brush border" – a surface coated with minute projections. The brush border hosts additional enzymes, such as aminopeptidases and dipeptidases (1,2). The last "polishing" step is carried out by these enzymes that convert most of the remaining small peptides into individual amino acids. Example: The chicken protein chunks, already made smaller by pepsin, are now further diced by pancreatic enzymes into tiny bits. The brush border enzymes finish the job, resulting mostly in individual amino acids.
the Bloodstream
Now that proteins are digested to primarily single amino acids, along with some di- and tripeptides, they can now be absorbed into the body. This primarily occurs in the jejunum and ileum (sections of the small intestine).
amino acids are absorbed into intestinal cells via active transport. That is, it takes energy and specialized carrier proteins, some of which are sodium-dependent (such as a co- transporter system – consider that sodium is assisting in opening the door for the amino acid) (2,5). There are various carriers for various amino acid types.
quite surprising that di- and tripeptides can be absorbed directly into intestinal cells as well.
Constructing new proteins (e.g., muscle repair after exercise, creating new enzymes) Producing other significant molecules (e.g., hormones, neurotransmitters) Supplying energy if your body lacks sufficient carbohydrates or fat (1).
The process of protein digestion is a step-by-step process of breaking down food's large, complex molecules into small, absorbed amino acids. It begins as mechanical degradation in the mouth, progresses to acid denaturation and primary enzymatic cleavage by pepsin in the stomach, and is finished almost entirely by a combination of pancreatic and brush border enzymes in the small intestine. These amino acids (and certain short peptides) are then transported into the
body through active transport into the blood (1,2,4). References Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism. 7th ed. Boston, MA: Cengage Learning; 2018. Guyton AC, Hall JE. Textbook of Medical Physiology. 13th ed. Philadelphia, PA: Elsevier; 2016. Chapter 66, Digestion and Absorption in the Gastrointestinal Tract. Schubert ML. Gastric acid secretion. Curr Opin Gastroenterol. 2017;33(6):426-432. Whitcomb DC, Lowe ME. Human pancreatic digestive enzymes. Dig Dis Sci. 2007;52(1):1-17. Adibi SA. The path-breaking studies on peptide absorption. J Parenter Enteral Nutr. 2003;27(5):366-370.
