The Science Of Hydration

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==Sodium Loss and Sweat Rate==
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- adjusted.jpg|none|thumb|300px400px|Sweat rate and sodium concentration<ref name="sweatrate"/>, adjusted using the formula for regional patch collection<ref name="WBWandPatch"/>.]]===Converting per-area sweat rates to whole body sweat rates===
We can convert from per-area sweat rates to whole body sweat rates by using a [ 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.
[[File:Sodium in sweat trained and untrained.jpg|none|thumb|300px400px|Sweat sodium concentration against sweating rate, showing for three different work intensities and for trained and untrained individuals. Note that this data is not adjusted for regional patch collection, so the rates are too high and should be scaled by 0.67.]]
==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- adjusted.jpg|none|thumb|300px400px|Sweat sodium concentration against sweating rate, before and after 10 days of heat acclimation training, adjusted using the formula for regional patch collection<ref name="WBWandPatch"/>.]]
==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. Unfortunately the units used in this study are not comparable with other studies. The mechanism behind this is unclear, but the implication is that the sodium concentration of sweat in cooler weather may be higher than expected from the above studies.
[[File:Sweat Rate Sodium Concentration for skin temperatures.jpg|none|thumb|300px400px|Sweat sodium concentration against sweating great, shown for two different skin temperatures.]]
=Sodium Retention=
<ref name="sweatheat">Sodium ion concentration vs. sweat rate relationship in humans</ref>
<ref name="WBWandPatch">Comparison of regional patch collection vs. w... [J Appl Physiol. 2009] - PubMed - NCBI</ref>