From Fellrnr.com, Running tips
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
but significant switching rates.
Below are some sample sources of Sodium, with the concentrations defined.