Will an animal that is not eating any aminoacids stop excreting urea (or whichever other form of excreting nitrogen)? If the body is not in a position to lose nitrogenated compounds, in which processes is nitrogen inevitably turned into waste products like amonia/urea?

  • $\begingroup$ A hypothetical question from a new user who hasn't taken the Tour looks suspiciously like a homework question for which there is a specific policy on SE Biology. For all questions, but especially homework questions you are required to show evidence of trying to answer the question yourself. In addition your standpoint seems unclear. Your "why not?" seems to be responding to some unspoken answer. Please clarify. $\endgroup$
    – David
    Jul 19 '18 at 17:40
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    $\begingroup$ The reason I am asking is because I am looking at the optimal protein intake from ratios of macronutrients for a healthy low-empty calorie diet. I want to understand what the smallest replacement amount of protein is that won't trigger autophagy and why even under autophagy one will still lose protein. I am sorry if I posed the question in a way that begs for a specific answer, but I could not really find a concrete answer to why it is that there is always some urea around even under nitrogen deficit. Seems to be because aminoacid reactions produce byproducts like ammonia that can't be reused. $\endgroup$
    – jpf137
    Jul 19 '18 at 22:32

Yes, you will still excrete urea, but the amount you excrete will be diminished. As you might expect, when the body is low on protein, there's a physiologic drive to hold onto as much nitrogen products as possible, in order to conserve the limited supply.

Normally, your kidneys excrete approximately 40% of the urea that is filtered through the glomerulus. (It's slightly more complicated, because there's both secretion and reabsorption that happens along the nephron, but that's approximately the end result.) However, in protein malnutrition, the kidneys reabsorb more of the urea, and that number can drop to excreting only 10% of the filtered urea. (Source) It appears that this is an active process to reabsorb the urea, rather than a passive effect from other effects that malnutrition has on the body.

However, the body isn't capable of stopping excretion of urea completely... there will still be some that is excreted. Urea transporters in the kidney aren't 100% efficient, but probably the bigger issue is that the body needs to use that urea to regulate other functions. Urea is important to the body's ability to concentrate urine, which in turn is related to the body's regulation of blood volume and sodium concentration. And in general, the body cares more about maintaining proper blood volume and tissue perfusion than anything else.

  • $\begingroup$ also not eating protein will not stop proteins in the body from wearing out and having to be recycled. $\endgroup$
    – John
    Jul 19 '18 at 16:43
  • $\begingroup$ Despite the title I would have thought the biochemical tag indicated an interest in whether urea was actually synthesized in starvation. Irrespective I find the information you provide about urea resorption interesting — I was not aware of it. Do you know whether the resorbed urea can be converted to useful biochemical intermediates either by reversal of the urea cycle or by other means (urease?). $\endgroup$
    – David
    Jul 19 '18 at 17:50
  • $\begingroup$ What I don't understand is why when a protein is recycled, it won't just return to being made of its original aminoacids. Can't the body keep those aminoacids around for further usage? What is the specific reaction family that releases ammonia/urea (not the ones triggered when the body runs out of energy storage and has to use aminoacids as an energy source). Essentially, where does deamination occur in the aminoacid cycle and how can it be reversed? $\endgroup$
    – jpf137
    Jul 19 '18 at 22:46

If an animal that is not eating amino acids is still eating enough nucleic acids to degrade some, that will produce ammonia and urea.

Plus some amino acids will still get degraded.

  • $\begingroup$ The scenario I was thinking of is one where you have plenty of sugar/fat around, so no energy deficit, but you are eating too little protein, so that the losses of nitrogen can be somehow reversed. If they can't, why is such a reaction necessary and irreversible, even with availability of energy to reduce entropy increases from protein degradation into aminoacids? $\endgroup$
    – jpf137
    Jul 19 '18 at 22:55

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