Why does the body need a constant supply of new essential minerals and trace elements, like magnesium or selenium, and not simply recycle what it already has?

When I think about it, elements are not like glucose or fat, the body doesn't burn them in a way that it would become not an element (that would require a nuclear reaction), it can only lose them through stool, urine or sweat, and perhaps breathing, shedding dead skin, hair, fingernails or toenails being cut, saliva, ejaculate, is there any other way I forgot the body can lose elements?

Anyway, for most elements, I guess the majority is lost through defecation, but why does the body dump the element there, if it will need it again for enzymes and other processes? Couldn't it recycle them back from the colon, so the human doesn't need to consume for example a new 400 mg of magnesium every day to replenish what is thrown away with feces?

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    Probably because evolution is not perfect. Did you try researching this on your own? – Thomas Aug 23 at 11:15
  • Your body is a living system where things are constantly being built, torn down, reshaped. The rates of change themselves change over time, and the only way to adjust the amount available to what is needed is to have constant in- and outflow. If you look into some examples, you'll find that systems with very little change in requirements (like haemopoeisis) do recycle their minerals extensively and have minimal in- and outflow (like iron). Magnesium is regulated through urinary excretion by the way ncbi.nlm.nih.gov/pmc/articles/PMC3894481 – Armatus Aug 23 at 11:40

Because it is not usually thermodynamically possible to do so.

For example Iron (Fe) is an essential mineral for the oxygen carrying molecule haemoglobin. This iron is only useful in Fe+ and Fe2+ forms and it switches between these in order to adsorb and release oxygen. If the iron molecule becomes further oxidised to the Fe3+ form it can no longer be turned back to Fe2+ because Fe3+ is a more stable ionic form and hence the amount of energy required to change it back is too great to be done physiologically and so it is excreted.

Another example is vitamin C which is an antioxidant (reducing agent) which works by donating electrons to certain enzymes. This changes the form of vitamin C to an unusable form. In this case it is possible to reverse the reaction however if the rate of change exceeds the bodies ability to convert it back and so it is excreted via the kidneys and hence needs to be replaced by diet.

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    This seems to be a good answer. Welcome to the site. The only problem is we require answers to provide supporting references for their main claims of fact. References don't have to be extensive, just enough to demonstrate that your claims are factual. For example, your explanation of how iron works would benefit from a reference. – Carey Gregory Aug 27 at 23:29

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