There are a few asking a good question on the net: how well can nutrients be absorbed transdermally when the skin is meant to keep things out and oral ingestion meant to absorb?
Since no one is answering that question on the net, I thought I'd give it a try.
First off, I agree that skin appears to be excretory in its main function. It supports the kidneys in that is a route to throw out toxins like excess sodium/salt and urea and 'xenobiotics.' But mostly it seems to absorb and retain the toxins removing them from the inner organs. Sweating, scaling, inflammation, hardening and other attempts by the skin to slough off the toxic load isn't efficient. So unless the liver's bile does the detoxification or the kidneys do it, a dog's skin can only store the worst of it.
I think the skin is meant to 'keep things out', but really keep toxins out from damaging the inner, more important tissues.
Osmosis is absorptive and the skin does take in liquids by osmosis.It also seems very receptive to fats since the liver coats our skin with cholesterol to make it absorptive of sunlight and produce Vitamin D. So there's a good link between the skin and the liver that seems to make it a digestive aid.
That's my argument for transdermal use.:)
In my experience I've found that the healthier street dogs absorb Magnesium chloride and Zinc sulphate within 24 hours through the skin, and it leaves the fur shiny and clean. This is so astonishing to me that I've tried putting Mag chlor on my hair and leaving it on. Even 3 days later my hair is a sticky salty mess. My skin too absorbs Mag chlor very slowly, feeling damp for 2-3 days before it sloughs off leaving fresh, cleaner skin under.
So what is it that makes street dogs such efficient users of transdermal absorption? My initial theory was that they were in greater need of the nutrient than the dogs at home or myself, but now I don't think that's right. Perhaps it is the exposure to the sun that makes them better at processing transdermally.
But the dogs at home are getting better at it. Initially they would take 3-5 days and would need me to wipe off the remaining salt, but now it takes them a day or two and the salts are completely absorbed. So maybe as the liver improves its function, so does the skin.
As of now it appears to be a good test of the condition of the liver (or kidneys?). Doesn't say much for my own then!:)