If proteins are building blocks of an organism then it makes sense why a growing organism would need an intake of them, but why would a fully grown organism need proteins (aside from those lost by shedding skin, growing hair etc.) and not recycle other proteins back into amino acids.

I understand that protein can be broken down to generate energy when needed or when there's too much of it in the body, but given enough energy from other sources (like glucose), it seems that proteins wouldn't need to be used for these processes. Is there perhaps a net loss of amino acids during protein biosynthesis?

My question was inspired after reading this study of a man who was fasting for 382 days and was given only water and mineral supplements: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2495396/pdf/postmedj00315-0056.pdf

  • $\begingroup$ I've checked both of these and they still don't answer my question. Protein degradation explains that proteins can be broken down into amino acids, which I am aware of, hence the question: If the body can do this, then why the need for external amino acids? Just supply energy for analysis and new synthesis. $\endgroup$
    – Ardath
    Commented Jul 24, 2017 at 20:39
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    $\begingroup$ Please note that the man was using/losing protein as well as using fat as evidenced by the continual excretion of creatinine (which comes from muscle protein breakdown.) This is not ideal and were he not morbidly obese and on a medically supervised fast, he most likely would have died. $\endgroup$ Commented Jul 24, 2017 at 21:07
  • $\begingroup$ I see you have edited your question to clarify that the question is one of why not all the amino acids used for breakdown are not available for reuse. I'll remove my close vote and delete my comment. $\endgroup$
    – David
    Commented Jul 24, 2017 at 21:09
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    $\begingroup$ @David The question hasn't been edited (unless it was deleted and reposted and I missed that) - it was always about why recycling doesn't provide enough amino acids. $\endgroup$
    – Bryan Krause
    Commented Jul 24, 2017 at 21:28
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    $\begingroup$ @anongoodnurse: And for an opposite extreme, consider the stereotypical 98 pound weakling who takes up bodybuilding :-) Or even me, as I shift from my winter skiing muscles to summer hiking/biking/horse riding ones and back again. $\endgroup$
    – jamesqf
    Commented Jul 25, 2017 at 6:44

2 Answers 2


Protein is polymerized amino acids.

Humans do not require protein intake, but certain essential amino acids are required. These essential amino acids are usually obtained through protein intake, but intake of the free amino acids themselves would suffice.

Amino acids, in free or protein form, are lost from the body due to urination (see Diverse characteristics of the urinary excretion of amino acids in humans and the use of amino acid supplementation to reduce fatigue and sub-health in adults), perspiration, menstruation, ejaculation, tear drops (see Amino Acid profiles in human tear fluids ), skin exfoliation, hair and nail growth, nasal mucus (see Identification of human nasal mucous proteins using proteomics), and vaginal mucus (see Proteins of Cervical Mucus). Additional loses can occur due to additional bleeding, lactation, child birth, spitting (and other activities causing loss of saliva), and coughing up mucus.

Synthesis of new proteins from amino acids is constantly required by the body (see NUTRITION AND PROTEIN TURNOVER IN MAN). Though there are chemical pathways to reuse amino acids, it is necessary for the concentration of each amino acid to be non-zero, and therefore some fraction is necessarily lost through excretion.

Beyond just direct excretion of amino acids and proteins, there is excretion of nitrogen-containing metabolites such as urea, uric acid, creatinine, ammonium salts and bilirubin.

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    $\begingroup$ Just as a note, the first study you cited reports an average excretion of 5.1 mmol amino acids per L of urine. The average weight of an amino acid is somewhere around 110 g/mol (Histidine is at 155 and Glycine at 75, so the two most prevalent amino acids nicely balance each other out to the average of 115 as well). That makes 0.0051 * 110 = 0.56 g of amino acid per L of urine. Daily urine excretion is between 800mL and 2L, so that's perfectly in line with medline's 80mg excreted per day. (Healthy) urination is therefore unable to account for any significant fraction of amino acid loss. $\endgroup$
    – Armatus
    Commented Jul 27, 2017 at 20:36
  • $\begingroup$ Which basically leaves us with losing about 50g of protein through the other things you mentioned. If you consider how much we sweat (10-14 L PER DAY), lose skin and hair, and how much enzyme juice we secrete into our food to digest it... $\endgroup$
    – Armatus
    Commented Jul 27, 2017 at 20:40
  • $\begingroup$ @Armatus 50 grams lost per day is only if you eat more than you need. 24 grams per day is enough to supply all the needed nitrogen for the body. books.google.com/… Only 6.3 grams per day of essential amino acids are required according to table 12.7 of that reference. $\endgroup$
    – DavePhD
    Commented Jul 27, 2017 at 20:58
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    $\begingroup$ @Armatus I don't think an average human sweats 10-14 L per day (possibly excluding extreme heat and such), because that means you would need to drink probably well over 16 L of water to account for this loss plus urine and water loss due to breathing. From what I understand most of the excess proteins end up as an additional fuel for the body. $\endgroup$
    – Ardath
    Commented Jul 27, 2017 at 22:06
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    $\begingroup$ @Armatus Urea excretion through urine is something like ~25 grams per day. That would be the primary method of excretion of amino acid metabolites. $\endgroup$
    – Bryan Krause
    Commented Jul 28, 2017 at 16:10

A fully grown organism is subject to senescence and on a narrow perspective one of the signs of aging is the accumulation of large amount of lipofuschin.

In unstressed situations protein homeostasis is balanced by folding and stabilization of proteins by chaperones of the Hsp family [1] and the controlled degradation of proteins by the proteasomal system.[source1] [see also]


One of the highlights of postmitotic aging is the intracellular accumulation of highly oxidized and cross-linked proteins, known as lipofuscin. Lipofuscin is insoluble and not degradable by lysosomal enzymes or the proteasomal system, which is responsible for the recognition and degradation of misfolded and oxidatively damaged proteins. These aggregates have been found in various cell types, including heart, liver, kidney, neuronal tissue, and dermal tissue, and are associated with the life span of a single postmitotic cell and, consequently, of the whole organism. Lipofuscin formation appears to depend on the rate of oxidative damage to proteins, the functionality of mitochondrial repair systems, the proteasomal system, and the functionality and effectiveness of the lysosomes. This review highlights the current knowledge of the formation, distribution, and effects of lipofuscin in mammalian cells.[source]

Lipofuschin is known to contain oxidised proteins,lipids and glycosides as follows:

This accumulation of aggregates is the final fate of protein damage under stress conditions. Under such circumstances oxidized proteins may not undergo appropriate proteolytic digestion but instead, cross-link with one another or form extensive hydrophobic bonds. It is believed that the cross-linked proteins react further with other cellular components, forming a fluorescent material referred to as lipofuscin (Fig. 1F). Lipofuscin is accepted to consist of oxidized proteins (30–70%) as well as lipids (20–50%) [23] and from the fifth decade of life, bound sugar residues were also detected in human lipofuscin. [source]

The lipofuschins are present in residual bodies of lysosomes ,the centre of protein recycling.

Lipofuschins render the non digestibility of the oxidised proteins as follows:

According to this model, an intralysosomal accumulation of lipofuscin can be considered as the long-term result of a decreased degradation of oxidized proteins and an increase in intracellular free radical formation. Metals, including Fe, Cu, Zn, Al, Mn, and Ca, comprise up to 2% of lipofuscin [26] and especially catalytic iron seems to be an important factor in further oxidation reactions of the initial protein aggregate. In mammalian cells iron is the most abundant cellular transition metal and a fundamental player in the above mentioned mitochondrial–lysosomal axis theory of aging [source]

Another major characteristic of lipofuscin is its ability to inhibit the degradation of oxidized proteins by competitively binding to proteolytic enzymes including the 20S proteasome as mentioned above as well as lysosomal proteases. How exactly the proteasomal substrate recognition functions is still under investigation, but one of the recognition motifs might be exposed hydrophobic patches from oxidatively damaged and partially unfolded proteins [5,28]. It seems likely that the proteasome also binds to such exposed hydrophobic oligopeptides on the lipofuscin surface but is then unable to completely degrade or to release these peptides, resulting in proteasomal inhibition [source]

The lipofuschins being the undigested remains,it can be said that the lysosome fails to recycle the entire endosomal protein specifically with aging (when it becomes very prominent).

Thus, some of the loss of amino acid may be attributed to its lack of recycling.

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    $\begingroup$ Can you try to add some scale of reference to how much waste protein really ends up as lipofuschin? Given how long it takes such material to build up, and given that it seems like most sources focus on the lipid, rather than protein content, I am doubtful that it consists of any substantial fraction of the day-to-day amino acid needs. A different answer here says that about 6 grams are needed daily; if this was all accumulated as irrecoverable wastes in cells, we'd be growing over 2kg of body weight annually just in those granules - that makes no sense. $\endgroup$
    – Bryan Krause
    Commented Jul 28, 2017 at 18:07
  • $\begingroup$ @Bryan Krause senescence takes years! OP asked:" but why would a fully grown organism need proteins (aside from those lost by shedding skin, growing hair etc.) and not recycle other proteins back into amino acids."aside from these phenomenon ( already discussed in the 1st answer)the only one i could find was senescence...i think i stated there will be some loss of amino acid during recycling not all...and speaking of accumulation...gaining weight? i think there will be age related degeneration of the cells rather than weight gain...apoptosis too. $\endgroup$
    – user 33690
    Commented Jul 28, 2017 at 19:00
  • $\begingroup$ @Bryan Krause as for "scale of reference"try the aging link already in the answer $\endgroup$
    – user 33690
    Commented Jul 28, 2017 at 19:04
  • $\begingroup$ What I mean by scale is: of the daily requirement for protein (that is, the inherent waste or use in the system), what percentage belongs to the mechanisms you are describing? I think that percentage is very low; I don't see in your references where that information is given, just a note that it accumulates with age. $\endgroup$
    – Bryan Krause
    Commented Jul 28, 2017 at 19:30

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