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I discussed a leather store about the white border aka salt border on leather shoes. They said that it is due to inner features of leather and you could try to add some fat/cream to push it back inside the shoes. I haven't yet understood physiology of skin here, "why does it act like that?" is the first doubt and then the second "does human skin act the same way?" -- I can see this kind of salt borders in gym clothes when they cry uncleaned but haven't seen them yet on my skin at least. The extra salty water from legs causes real damage, such as getting the inner fluids out, to the leather. When you do heavy training, the salty sweat in the human skin acts apparently the same way. So:

how to manage the skin in living human skin and how does it differ from leather maintenance? What is the common physiological background with the different kinds of skins?

I try to answer it myself:

  1. leather: you can add the fat/cream/etc only externally
  2. living human ski: you can add the fat/cream-etc both internally (fatty food) and externally (lotions)

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The two processes are quite different: On leather shoes no active transport takes place. The salt either comes with the snow (most likely most of it) when the streets get salted to avoid ice. Your shoe soaks up the salt-water mixture and when the water dries up, the salt stays behind. The other possibility (more for new leather shoes) is that water goes into the leather and dissolves salts, which remain from the tanning process of the leather. With older shoes I have never observed this "snow marks" in the summer when the shoes got wet in rain. Using a wet cloth removes the salt, subsequent treatment of the shoe prevents that water can enter the leather.

The skin is quite different: Here we have living cells and an active transport of water through pores when you body get warm. This is to cool you and prevent overheating and since the body fluids contain salt, so does sweat.

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Why is salt used in taining process? To kill bacteria? To force other ions out or some other reason? –  hhh Feb 3 at 17:15
    
Its needed to preserve the leather. Salts are remnants of the process. –  Chris Feb 3 at 17:35
    
Salt is used in many points of tanning such as curing, liming and pickling (Wikipedia). So yes the salt coming from leather seems to be totally different thing to the salt coming from the living organism. So the leather maintenance only requires some fats to keep it nice, not salt additions? Then again an athletic person requires all kind of inputs such as salt, hydrocarbons, protein, oils, etc after training. –  hhh Feb 4 at 9:46
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Yes, leather needs to get some fats to keep it nice. As athletes are living organisms, they need much more stuff for proper "maintenance", as they loose or metabolize the stuff they take up. –  Chris Feb 4 at 9:54
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Leather is dead material. So we need to protect it from too much moisture so it will keep its current state. If its permanently wet, it will finally get colonized by funghi and bacteria which will break it down. –  Chris Feb 4 at 10:31

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