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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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.