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Part of what we eat are proteins,
and our body is in part build of proteins.

Are the proteins of the body build based on proteins in food at all?

Are proteins in food directly reused in the body, or are proteins first disassembled?

How far are they disassembled, randomly in various pieces, or systematically to keep what can optimally be used to build new proteins, while nothing is wasted for energy?

(The question Can proteins/peptides pass through the intestine? and it's answers are related, and provide some context and relevant parts, but is not a duplicate.)

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    $\begingroup$ Does this answer your question? Can proteins/peptides pass through the intestine? $\endgroup$ – tyersome Jan 18 at 0:35
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    $\begingroup$ @tyersome Not sure that's the best dupe, since it doesn't cover the "normal" way of business. I know variations of this question have come up before, though. $\endgroup$ – Bryan Krause Jan 18 at 0:46
  • $\begingroup$ @tyersome It gives some insight about part of the answer, so it is good that it is linked. $\endgroup$ – Volker Siegel Jan 18 at 11:35
  • $\begingroup$ It's not a duplicate of " Can proteins/peptides pass through the intestine?" $\endgroup$ – Volker Siegel Jan 24 at 19:51
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Short answer: Indeed the proteins in our body are based on amino acids from external food sources. BUT, proteins up-taken from food are ALWAYS disassembled first into amino acids, through specialized enzymes, proteases, (for instance Pepsin in the stomach's gastric juices and Tripsin in the pancreatic juices), during digestion, in the alimentary canal, (gut). This enables the body's liver to build the proteins most needed by the organism itself, through the processes of transamination, that allows conversion betwixt amino acids, and deamination, that removes N2 from the amino acid, (let's say the "amino" part is removed, and then expelled as urea), to excrete amino acids in excess. In addition this breaking down of external proteins is necessary, since they can act as labels for pathogens, and external organisms in general, and thus would soon be destroyed by the immune system if reused straight away.

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    $\begingroup$ Proteins are made on the ribosome using amino-acids as 'building blocks' that may come from ingested proteins, from the breakdown of existing proteins, or (in some cases) from 'simple' precursors by intermediary metabolism (such as alanine from pyruvate by transamination). I think you need to distinguish between 'essential' and 'non-essential' amino acids, and Schoenheimer and the dynamic state of body constituents comes to mind. $\endgroup$ – user1136 Jan 18 at 12:08

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