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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.]]
==Sodium Loss and Skin Temperature==
A study<ref name="skintemp"/> of sweating great sodium concentration for different temperatures has shown that sodium reabsorption is greater at high temperatures. The mechanism behind this is unclear.
[[File:Sweat Rate Sodium Concentration for skin temperatures.jpg|none|thumb|300px|Sweat sodium concentration against sweating great, shown for two different skin temperatures.]]
=Sodium Retention=
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="1946a1946"/> 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.
=Sodium Intake=
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>
<ref name="1946a1946">Relationship between salt intake and sweat salt concentration under conditions of hard work in humid heat
http://www.ncbi.nlm.nih.gov/pubmed/20984571</ref>
<ref name="skintemp">SpringerLink - European Journal of Applied Physiology, Volume 94, Number 4 http://www.springerlink.com/content/n569w2538g13516r/</ref>
</references>
