From Fellrnr.com, Running tips
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While some sources suggest that increased fitness reduces the sodium concentration in sweat research<ref name="training"/> shows this is not the case. For both trained and untrained individuals sodium concentration depends mainly on sweat rate. In fact, for a given relative intensity (% of [[VO2max|V̇O<sub>2</sub>max]]) trained individuals will be performing a greater absolute work rate and therefore have a greater sweat rate and sodium concentration.
[[File:Sodium in sweat trained and untrained.jpg|none|thumb|300px|Sweat sodium concentration against sweating great, showing for three different work intensities and for trained and untrained individuals.]]
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="
1946"/> 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 but significant switching rates.
Below are some sample sources of Sodium, with the concentrations defined.
<ref name="training">SpringerLink - European Journal of Applied Physiology, Volume 111, Number 11 http://www.springerlink.com/content/mxj56211q04v4455/</ref>
1946">Relationship between salt intake and sweat salt concentration under conditions of hard work in humid heat