From Fellrnr.com, Running tips
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= Effects of dehydration =
Everyone knows that dehydration is bad. But how bad? Current research indicates that some level of dehydration (up to 3%) does not impact performance, or impacts performance much less than expected<ref name="ref7"/>. (Dehydration of 5% does impact performance<ref name="ref11"/>.) This may be due to the fact that carbohydrate ([[Glycogen]]) is stored with water, in the ratio of about 1g
glycogen to 2.5g water<ref name="ref8"/>. This means that 2000 calories of glycogen depletion that are likely to occur in marathon distance runs would result in about 4lb weight loss with no reduction in hydration (2000Kcal/4=500g glycogen + 1250g water = 1750g). In practice moving from a high carbohydrate to high fat diet can see 6lb weight loss, believed to be glycogen + water depletion<ref name="ref8"/>.
= Sodium loss through sweat =
Humans have 2-4 million sweat glands over nearly their whole body surface, and though each is tiny, together they weigh as much as a kidney (~100g)<ref name="Fitzpatrick"/>. Sweat glands are most numerous on the soles of the feet. Sweat is produced in two steps<ref name="Fitzpatrick"/>:
# A coil generates an ultra-filtrated fluid. This fluid has the same sodium concentration (isotonic) as the blood.
# The fluid travels from the coil up the sweat duct which reabsorbs sodium and chloride (but not potassium). This reabsorption is via active transportation (i.e. it requires energy from ATP). The
glycogen stored in the sweat glands will only support the reabsorption for less than 10 min., so the energy is predominantly supplied by the blood. Glucose is the preferred energy source, though lactate and pyruvate can also be used. Fatty acids, ketones, and amino acids are very poorly used. The reabsorption process also acidifies the final sweat.
The rate of sweat production depends on the local skin temperature and core body temperature. A rise in the localized skin temperature will produce an increased sweat rate within 60 seconds<ref name="Fitzpatrick"/>.
==Sodium Loss Table==
[[File:Sodium in sweat trained and untrained.jpg|none|thumb|400px|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|400px|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==
As an aside, if you have low blood pressure, which I do, increasing your salt intake can really help.
= Caffeine and Alcohol =
The scientific evidence shows that [[Caffeine]] is generally not a diuretic<ref name="ref5"/><ref name="ref14"/><ref name="ref15"/>. Previous studies have shown that if you don't normally take
caffeine and then get a large dose, there is some diuretic effect. However normal intakes of caffeine by non-users and use by regular users is not a diuretic<ref name="ref16"/>. (If you urinate more because you drink a 20oz Latte, it is because of the 20oz of fluid, not the caffeine.)
Alcohol is another story; drinking anything stronger than 2% will cause dehydration. Because alcohol takes 36 hours to clear the body, it should be avoided for 48 hours before you wish to avoid impaired performance<ref name="ref5"/>.
= Cramps =
The evidence for hydration and electrolyte status causing [[Cramps]] is somewhat ambiguous, but supplementing your electrolyte intake may help.
= Blisters and black toe nails =
Dehydration reduces body weight, which can reduce the size of your feet. This in turn changes the fit of your
shoes, causing blisters. [[Hyponatremia]] can cause swelling, which increases the size of your feet and can cause blisters. Both conditions can also increase the chance of black toe nails.
=Sodium and Water in the Body<ref name="CLINC"/>=
Approximately 60% of the human body weight is water, though this varies primarily with body fat as adipose (fat) tissue contains a lower percentage of water. Total Body Water (TBW) can be divided up into