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I am aware of the different ways animals excrete excess nitrogen (mainly ammonium, urea, uric acid). My question is: why do we excrete excess nitrogen instead of recycling it? How can we explain that there has been no selection for more nitrogen recycling in animals, as this would probably constitue a selective advantage?

I have read here that "In the quest for sufficient food energy to meet caloric requirements, animals ingest more nitrogen, largely as amino acids, than they require. Accordingly, the excess nitrogen ingested must be excreted in some form.". This is exemplified by this paper: a low protein diet comes with lower urea levels.

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    $\begingroup$ Herbivores and carnivores have vastly different nitrogen intakes, so it depends how much excess or paucity there is in the diet, i.e. ruminants recycle nitrogen more than carnivores. google.com/… I would say they excrete it if they have a major excess, especially given the pH considerations of nitrogen, urea and ammonia. $\endgroup$ Apr 12, 2023 at 5:55
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    $\begingroup$ An analogy with human recycling may be helpful: For millenia we've been tossing our garbage because it's easier than saving and recycling it. Recycling is hard, and we've only recently started recycling because we've been using some resources faster than we can replace them, and the environmental impact of tossing garbage has gotten exorbitant. $\endgroup$
    – Barmar
    Apr 12, 2023 at 15:24
  • $\begingroup$ Helpful answer: biology.stackexchange.com/questions/111475/… $\endgroup$ Apr 12, 2023 at 15:53
  • $\begingroup$ Recently asked: biology.stackexchange.com/q/111880/16927 $\endgroup$ Apr 12, 2023 at 15:55

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I think your quote more or less explains it: what advantage is there is recycling nitrogen when you're ingesting more than required?

Additionally, we recycle nitrogen all the time, especially in amino acid form. Proteins are constantly broken down into their amino acid constituents and recycled into new proteins.

When you talk about waste nitrogen, that involves the excess (no need to recycle an excess - excess means there's already too much).

Further recycling may make some sense in conditions where nitrogen intake is not in excess, such as hibernation.

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  • $\begingroup$ Since liver converts excess amino acids into glucose and subsequently glycogen. Won't it be better to store proteins as a reserve food material in form of muscles and other structures etc. ? That should be better than storing it up as fats which beyond a limit cause hundreds of health problems $\endgroup$
    – Aurelius
    Sep 6, 2023 at 18:07
  • $\begingroup$ @Aurelius Fat is more energy-dense than protein, and it costs energy to move that stored stuff around. Negative health impacts from fats are coming from over-storage that's only made possible by our extensive technological advancements over incredibly rapid time evolutionarily. And, in any case, storing nitrogenous materials as energy would hardly make a dent in the excess nitrogen in an adult human with a protein-rich diet. $\endgroup$
    – Bryan Krause
    Sep 6, 2023 at 18:15
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I would add to the points made by @BryanKrause by stressing that the poster is misusing the word ‘recycling’. In metabolism you can only recycle something that was previously present, such as protein (as in turnover) or nucleic acid bases (for which there are salvage pathways).

The alternatives for dealing with dietary excess of a metabolite are either to reject (excrete) it or to store it for use in future conditions of deprivation. Presumably because the demands for energy are greater, more immediate and more crucial for life than that for protein precursors, animals have evolved to prioritize the storage of the carbohydrate portion of amino acids (as fat predominantly), rather than store both carbon and nitrogen as protein.

Indeed, when muscle protein ‘wastes’ in extreme starvation, this is to provide energy — and particularly glucose and ketone bodies for nervous tissue. The nitrogen portion cannot be utilized too, so that it also excreted under these conditions.

As with many questions of evolutionary rationale, the simplistic “this would probably constitute a selective advantage” is unsupported by actual evidence. In this case it is negated by a consideration of actual human starvation and the chemistry of the molecules involved.

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  • $\begingroup$ Body builders have to take up a high protein diet. Because of exercise the excess nitrogen is used up in muscle making. Won't it be advantageous if it was done without exercising considering our body would rather die of obesity than excrete carbs or fats.? $\endgroup$
    – Aurelius
    Sep 6, 2023 at 18:02
  • $\begingroup$ While we are at it, why is the body so crazy about carbs and fat storage? Why does it store more fat than would be required to survive without food for months? And when it has all that extra fat why make us hungry for more. $\endgroup$
    – Aurelius
    Sep 6, 2023 at 18:12
  • $\begingroup$ @Aurelius my answer is to a specific question. The way that SE works is that if you have a different question or questions — which you clearly have — you should post it as an independent question (after careful preparation to provide clarity and context). This will enable others — who may know more than me about the topic — to contribute, and, above all, allow indexed access to those who share your concerns. $\endgroup$
    – David
    Sep 6, 2023 at 18:57

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