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This question already has an answer here:

Why do ADULTS need protein in their diet, assuming they are not growing?

What happens to the amino acids already present in the body? Why don't our bodies conserve them?

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marked as duplicate by Bryan Krause, iayork, mgkrebbs, Remi.b, Community Sep 12 '18 at 8:10

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  • $\begingroup$ Why the downvote? I researched the Q, and the answers I found are really silly ("You need AAs to create enzymes/proteins/etc") $\endgroup$ – MaxB Sep 11 '18 at 4:12
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    $\begingroup$ You should read more on the role of protein in the organism (antibodies, enzyme, hormones....etc). Amino acids are constantly implicate in diverse metabolic reaction and are degraded. $\endgroup$ – Nico Coallier Sep 11 '18 at 4:13
  • $\begingroup$ @NicoCoallier you really missed the point of the Q, but thanks for explaining your downvote at least. $\endgroup$ – MaxB Sep 11 '18 at 4:16
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    $\begingroup$ The point of the Q is therefore not well explained. 1. you suggest direct link between proteine and growing.... 2. What happens to amino acids already present in the body ? They are used ! See a simple explanation here bioinfo.org.cn/book/biochemistry/chapt17/bio4.htm 3. Why don't our bodies conserve because once the amino acids are degraded it generated molecule that is no longer useful. $\endgroup$ – Nico Coallier Sep 11 '18 at 4:18
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    $\begingroup$ @MaxB Make a list of keywords relevant to your question and search PubMed database for relevant papers. Keywords: amino acid, protein, intake, synthesis, degradation, turnover, metabolism, oxidation. You will find out, for example, that "amino acid oxidation in muscle leads to an appreciable amount of ATP generation" ncbi.nlm.nih.gov/pmc/articles/PMC3854183 $\endgroup$ – Martin Klvana Sep 11 '18 at 9:24
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There are several reasons first is we lose some protein to the environment, sloughed skin, worn hair and nails, even mucus contain protein.

second proteins are not infinitely recyclable, some digestive and alteration processes irreversibly alter amino acids thus the body needs a a further source of those amino acids. Often an amino acid is transformed into something else that cannot be recycled into proteins.

third many digestive enzymes or their components are lost and not recovered during digestion.

Lastly some amino acids just break down naturally and have to be replaced sometimes just due to the presence of oxygen, some organisms can build every amino acid they need humans are not among them.

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  • $\begingroup$ Another reason is that parts of the body often ARE growing. For instance, if you take up running or weightlifting, the muscles &c that you use will grow. (As will less visible parts like the heart & lungs.) If you don't exercise and continue to eat a high-calorie diet, other parts of your body will grow. and those fat cells need some proteins too. Then there's the extra skin... $\endgroup$ – jamesqf Sep 12 '18 at 4:22

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