From Fellrnr.com, Running tips
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[[File:Sweat Rate Sodium Concentration for skin temperatures.jpg|none|thumb|400px|Sweat sodium concentration against sweating great, shown for two different skin temperatures.]]
The human body
is very good at maintaining its sodium balance under most conditions. The higher your sodium intake the higher the sodium losses in sweat and urine. There is evidence<ref name="a1946"/> that under moderate sweat rates and a restricted sodium diet that the sodium concentration of sweat can drop to extremely low levels. However, I have found no evidence to suggest the sodium retention is effective at higher sweating rates.
Below are some sample sources of Sodium, with the concentrations defined.
<ref name="WBWandPatch">Comparison of regional patch collection vs. w... [J Appl Physiol. 2009] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/19541738</ref>
<ref name="Fitzpatrick">Fitzpatrick's Dermatology in General Medicine, Chapter 81. Biology of Eccrine, Apocrine, and Apoeccrine Sweat Glands</ref>