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From Fellrnr.com, Running tips
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The concentration of sodium in sweat depends on the sweat rate. This is believed to be because the sweat is released with a high sodium concentration, then the sodium is reabsorbed before it reaches the surface. The faster the sweating, the less chance for reabsorption.
[[File:Sweat Rate Sodium Concentration.jpg|none|thumb|300px|Sweat rate and sodium concentration<ref name="sweatrate"/>.]]
We can convert from per-area sweat rates to whole body sweat rates by using a [http://www.halls.md/body-surface-area/bsa.htm Body Surface Area Calculator]. For example, a 135 pound, 70 inch high person has a skin surface area of 1.74 m<sup>2</sup>, which is 17,400 cm<sup>2</sup>. Therefore 1 mg/cm<sup>2</sup>/min is 17,400 mg/min, or 17.4 g/min or 1,044 g/hour, or 1 liter/hour.
==Sodium Loss and Fitness==
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.
==Sodium Loss and Heat Acclimation==
A study <ref name="sweatheat"/> shows that the sodium concentration of sweat is reduced by heat acclimation training. The study used three bouts of 30 min. of exercise in environmental chamber with 10 min. of rest between each bout.
[[File:Sweat Rate Sodium Concentration for heat adaptation.jpg|none|thumb|300px|Sweat sodium concentration against sweating great, before and after 10 days of heat acclimation training.]]
<ref name="skintemp">SpringerLink - European Journal of Applied Physiology, Volume 94, Number 4 http://www.springerlink.com/content/n569w2538g13516r/</ref>
<ref name="sweatheat">Sodium ion concentration vs. sweat rate relationship in humans http://jap.physiology.org/content/103/3/990.full</ref>
</references>
