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The Science Of Hydration

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The advice given to runners on hydration has changed over time and looks set to continue to change. There are competing forces at work - sports drink manufacturers, event organizers (often sponsored by the manufacturers) and scientists (some also sponsored by the manufacturers). One thing is clear about hydration - it is important. Incorrect hydration can lead to impaired performance, and in extreme cases, death. A condition related to dehydration is [[Hyponatremia]], which is where the sodium (salt) level in the blood becomes too dilute. This is a dangerous condition that has killed a number of runners. This entry is a follow on to [[Practical Hydration]] which should be read first.
=Effects of dehydration=
Everyone knows While it's commonly believed that even small levels of dehydration is bad. But how bad? Current impacts performance, the research indicates that some level of real world performance is not impacted by dehydration (up to 34%) does not impact performance, or impacts performance much less than expectedof body weight<ref name="Goulet2012"/><ref name="ref7Goulet2011"/>. (Dehydration Greater levels of 5% does dehydration will impact performance<ref name="ref11González-Alonso-1995"/>. While authorities have recommended aggressive drinking<ref name="Convertino-1996"/>, these guidelines are now recognized as erroneous<ref name="WallWatson2013"/>, though they are still promoted by the beverage industry<ref name="beverageWWW"/>. =Dehydration and Body Weight =A common method of calculating dehydration is simply from body weight.) This may While it's true that an athlete's weight loss during exercise will be due predominantly from water, this is not the same as dehydration, nor does it necessarily imply this weight needs to the fact that be restored quickly though drinking. This is because a carbohydrate ([[Glycogen]]) is stored with water, in the ratio of about 1g [[Glycogen]] to 2.5g water<ref name="ref8Nilsson-1973"/>. This means that 2000 calories of [[Glycogen]] depletion that are likely to occur in marathon distance runs would result in about 4lb /2Kg [[Weight Loss]] with no reduction in hydration (2000Kcal/4=500g [[Glycogen]] + 1250g water = 1750g). In practice To restore the weight lost in endurance exercise the Glycogen reserves must also be restored, something that may take days. Even moving from a high carbohydrate to high fat diet can see 6lb [[Weight Loss]], believed to be from [[Glycogen]] + depletion.=Sodium Intake and Rehydration=Several studies have shown that drinks containing sodium provide better rehydration by reducing urine output. * Consuming drinks with 61 mmol/l (~1/2 teaspoon salt per quart/liter) reduced urine output and improved hydration when compared with 23 mmol/l (~1/4 teaspoon salt per quart/liter)<ref name="Shirreffs-1996"/>* When comparing drinks with 2, 26, 52 and 100 mmol/l, urine output was inversely proportional to sodium concentration, and the 2 & 26 mmol/l cases resulting in dehydrated subjects after 5.5 hours, where the others were rehydrated<ref name="Maughan-1995"/>. (These cases are roughly equal to water depletion, 1/4, 1/2, 3/4 teaspoon of salt per quart/liter.)* Drinks containing 0.45g salt/100ml reduced urine output and improved hydration<ref name="ref8Nose-1988"/>.(0.45g/100ml is about 1/10 teaspoon of salt per quart/liter.)
=Sodium loss through sweat=
The amount of salt that is lost through sweating varies a lot. It varies from individual to individual, and for an individual it will vary depending on fitness and heat acclimation<ref name="ref9"/>. This means that you may have to experiment with your salt intake, both during and after exercise.
==Anatomy of Sweating==
[[File:Sweat Gland.jpg|right|thumb|500px|A drawing of the sweat gland. (C=clear cells, D=dark cells, IC=Intracellular canaliculi, M=Myoepitherial cell, Mc=Mitochondria. )]]
==Sodium Loss Table==
The table below is based on the research showing that sweat sodium concentration increases with sweat rate. The table below is for a runner who is 174cm/70inches high and weighs 60Kg/132lbs, but you can create a customized chart at [[Sodium Loss]]. To check your sweat rate, simply weigh yourself before and after a run. Dropping 1 Kg or 2.2 pounds equates to 1 liter of sweating. (Obviously you need to adjust for any fluid intake and avoid urination.)
{| class="wikitable"
==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|400px|Sweat rate and sodium concentration<ref name="sweatrateBuono-2008"/>, adjusted using the formula for regional patch collection<ref name="WBWandPatchBaker-2009"/>.]]
===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="trainingHamouti-2011"/> 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|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="sweatheatBuono-2007"/> 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="WBWandPatchBaker-2009"/>.]]
==Sodium Loss and Skin Temperature==
A study<ref name="skintempShamsuddin-2005"/> 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|400px|Sweat sodium concentration against sweating great, shown for two different skin temperatures.]]
=Sodium Retention=
The human body has mechanisms to try to maintaining its sodium balance. Greater sodium intake results in the excess being excreted in the sweat and urine. Conversly, restricted sodium intake will result in a reduction of the sodium concentration of the sweat<ref name="a1946CONN-1946"/>. This reduction in the sodium concentration occurs at all sweating rates, but the relationship between sodium concentration and sweat rate remains a straight line<ref name="SweatSaltIntakeSigal-1968"/>. So at any given sweat rate, a restricted sodium intake will result in less sodium in the sweat. However, even on a restricted sodium intake, the more you sweat, the greater the sodium concentration. Comparing the maximum sodium concentration of sweat between a salt intake of 500mg/day and 20,000mg/day, the low salt intake reduced the sodium concentration by 30-48%<ref name="SweatSaltIntakeSigal-1968"/>. [[File:SodiumIntakeSweatConcentration.jpg|none|thumb|500px|The effect of high (20,000mg/day NaCL) and low (500mg/day NaCL) on the sodium concentration of sweat at various sweat rates<ref name="SweatSaltIntakeSigal-1968"/>. Three subjects were put on the diet for a week, then tested for five days while remaining on that diet. The two tests were separated by a month.]]
=Sodium Intake=
Below are some sample sources of Sodium, with the concentrations defined.
{| class="wikitable"
! Source
! Sodium - mmol per liter
There is a table showing a range of values at [[Sodium Loss]].
[[Hyponatremia]] is where the sodium (salt) levels becomes in the blood become too dilute. Initial The symptoms of mild hyponatremia tend to be a gain in weight and a general swelling and 'puffiness', most noticeable in the hands. More However, mild Hyponatremia may have no clinical symptoms, or just weakness, dizziness, headache, nausea/vomiting, but more severe Hyponatremia is likely to have symptoms are caused by a swelling of the brain (cerebral edema) , including nauseaaltered mental status, seizures, pulmonary edema, vomitingcoma, headache and malaisedeath<ref name="ref10RosnerKirven2006"/>.
=HypERnatremia - the opposite of HypOnatremia=
Generally, Hypernatremia (too much sodium in the blood) seems to be a result of dehydration rather than excessive salt intake<ref name="ref17Kratz-2005"/>. It should be noted that taking [[Electrolyte Capsules]] bypasses the body's taste. This sense of taste seems to reflect our body's internal sensors; our desire for salty foods reflects our salt requirements.
=Salt and High Blood Pressure=
There is evidence that increased salt intake can increase blood pressure<ref name="ref4"/>, and the common recommendation is to restrict your salt intake if you have high blood pressure. However, a recent study<ref name="JAMAStolarz-Skrzypek-2011"/> has shown that reducing your salt intake may increase your risk of a heart attack rather than lower it. For more on the health risks of low salt diets see
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="ref5Graham-1998"/><ref name="ref14"/><ref name="ref15Falk-1990"/>. 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="ref16Maughan-2003"/>. (If you urinate more because you drink a 20oz Latte, it is because of the 20oz of fluid, not the [[Caffeine]].)=Alcohol=Alcohol is another story; drinking anything stronger than 2% will cause dehydration. Because alcohol takes 36 hours a diuretic due to the suppression of vasopressin secretion<ref name="ROBERTS-1963"/><ref name="RubiniKleeman1955"/>, and the volume of urine produced is proportional to clear the bodyalcohol consumed<ref name="Eggleton-1942"/>. However, it should the diuretic effect of alcohol may be avoided for 48 hours before you wish to avoid impaired performancereduced when consumed when suffering from post-exercise dehydration. A study looked at consuming water or an alcoholic beverage of 1%, 2%, and 4% after 1 hour of moderate exercise<ref name="ref5Shirreffs-1997"/>. The study found that urine production tended to increase with alcohol content, but the difference was not statistically significant. The alcohol tended to reduce the rate of recovery for both blood and plasma volume, though tendency was only statistically significant at 4%. In summary, alcoholic beverages of up to 2% appear to have little impact on rehydration rate compared with the equivalent volume of water, but 4% beverages tend to delay recovery.= 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=
The volume of extracellular fluid is typically 15 liters in a 70 kg human, and the 50 grams of sodium it contains is about 90% of the body's total sodium content.
=Symptoms of Dehydration=
These symptoms are for the general public<ref name="CLINC"/>, and there is evidence<ref name="SYMPT"/> that they may not apply to athletes suffering from mild dehydration.{| class="wikitable"
! symptom
! mild dehdration dehydration (3-5% body weight) ! Moderate dehdration dehydration (6-9% body weight) ! Severe dehdration dehydration (>10% body weight)
| Level of consciousness
=References =
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<ref name="CLINC">Clinical Studies in Fluid and Electrolyte Balance</ref>
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