Salt really does get under your skin

Salt. It — or rather its sodium ions — are essential for animal life. The Romans used salt to pay their solders, salarium argentums. Ants and termites in tropical forests in the Amazon >100 km inland from the sea have smaller populations than those nearer the coast because they get less wind carried sea salt. Adding salt and sodium can increase their numbers seven-fold. Evolution has given us a taste for sodium ions. Cooks flavor our diet  with salt and we add our own.

But salt is also toxic in excess–ask any bacteria or fungi that seeks to grow and multiple on salt persevered meat or pickle. Brine is usually fatal to life unless cells can find away to protect themselves from its osmotic pressure that "drinks" water out of them. The average diet gives us a massive overdose–15 grams rather the 6 grams we need. How do we get rid of the unnecessary and dangerous sodium ions?

Urine and sweat was the traditional answer. Sweat can remove large amounts. In hot climates hard work can cause the body to bucket out sweat–up to 12 liters a day (though 8 to 10 litres are more normal). With that goes salt –  estimated depending on researchers to be 12–15 grams or even 25 grams (nearly an ounce). But this salt and with it sodium removal by sweat is a byproduct. Traditionally the answer was that the real heavy removal work was done by the kidneys and that was then flushed down the drain.

Now that story has changed. Our bodies also have a previously hidden way of removing excess sodium ions; storing them under our skin. Or rather into our subcutaneous lymphatic system its interstitium fluid (extracellular fluid between tissues).

The lymphatic system is where our body collects the fluid that bathe our tissues so it can be put back into circulation. The lymphatic system also works as the eyes and ears of the immune system. And now it turns out it is the body's salt salter. Excess sodium ions get temporarily stored by being bound to proteins called proteoglycans and glycosaminoglycans. This makes them osmotically inactive.

Not that hiding salt away is what this biosalt salter was evolved to do. Our ancestors faced the opposite problem to us of excess salt–deficiency. Animal bodies that could create a backup buffer had an advantage over those that could not store sodium ions. Animals face the same problem with energy–we are advantaged if we can store it from the times when it is available for when it cannot get it. The answer for energy storage is expanding adipose fat cells. With salt the body faced a similar storage problem and evolved salt salter proteoglycans in the subcutaneous lymphatic system.

Benvenuto Cellini's Saliera (saliera is Italian for salt cellar). This is sometimes called the "Mona Lisa of Sculpture". It shows masses of skin thus suggesting that Benvenuto "appreciated" 470 years before modern science the key link between skin and salt.

That we can biologically squirrel away sodium in this way is no small discovery. The kidney's regulation of sodium links to its regulation of blood volume and through this blood pressure. We are diet aliens to early humans. They eat  with  no salt on the table or in the kitchen a sodium-poor but potassium-rich diet of wild not factory processed meat and veg. (Potassium is an element next down in the periodic chemistry table and in some ways very like sodium–the chemist Dalton gave them very similar symbols as shown left and right to reflect this fact. However in the body they have very different roles its biochemistry). Our kidneys evolved to keep sodium and remove potassium ions. They now face the problem that we eat lots of hidden and not so hidden sodium. How does the system of balance sodium and potassium ions work in us with our evolution alien diet that is sodium rich and potassium poor? It seems that a physiological "bug" is exposed that results in our body's getting rid of too many potassium ions. This causes increased contraction of vascular smooth muscles and changes to the cerebral control of blood volume that ups blood pressure.

Knowing that sodium gets buffered under our skin presents us with a more complex story. For a start it suggests that our kidneys do not face the fall impact of our salt rich food–our skin helps us stop poisoning ourselves with the stuff. The storing of excess sodium is also odd. If experimental animals eat a high salt diet the lymphatic capillaries expand (see the green thicker lines on the right) due to the activation of macrophages (a type of immune cell). Stop those cells getting active and with that this lymphatic capillary expansion and blood pressure goes through the roof.

So the newly discovered salt salter under our skin really matters to us modern people. Thanks to our salt buffering subcutaneous lymphatic system, salt is changed from being a toxic substance that would kill us by dramatically raising blood pressure into a moderately toxic one with which we can live. 

References

Marvar PJ, Gordon FJ, Harrison DG 2009 .Blood pressure control: salt gets under your skin. Nat Med. May;15(5):487-8.

Machnik A, Neuhofer W, Jantsch J, Dahlmann A, Tammela T, Machura K, Park JK, Beck FX, Müller DN, Derer W, Goss J, Ziomber A, Dietsch P, Wagner H, van Rooijen N, Kurtz A, Hilgers KF, Alitalo K, Eckardt KU, Luft FC, Kerjaschki D, Titze J. 2009 Macrophages regulate salt-dependent volume and blood pressure by a vascular endothelial growth factor-C-dependent buffering mechanism. Nat Med. May;15(5):545-52.

Rabelink TJ, Rotmans JI. 2009 Salt is getting under our skin. Nephrol Dial Transplant. Nov;24(11):3282-3.

Kaspari M, Yanoviak SP, Dudley R, Yuan M, Clay NA. 2009. Sodium shortage as a constraint on the carbon cycle in an inland tropical rainforest. Proc Natl Acad Sci U S A. 106(46):19405-9.

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