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The Science Of Hydration

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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 [7]<ref name="ref7"/>. (Dehydration of 5% does impact performance [11]<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 [8]<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 [8]<ref name="ref8"/>.
== Salt 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 [9]<ref name="ref9"/>. This means that you may have to experiment with your salt intake, both during and after exercise. Anecdotal tip: If your skin is crusty with salt after a run, you are probably someone who sweats out a lot of salt.
== More on Hyponatremia ==
Hyponatremia is where the sodium (salt) levels becomes too dilute. Initial symptoms tend to be a gain in weight and a general swelling and 'puffiness', most noticeable in the hands. More severe symptoms are caused by a swelling of the brain (cerebral edema) including nausea, vomiting, headache and malaise [10]<ref name="ref10"/>.
The cause of Hyponatremia is poorly understood, but believed to be related to excessive water intake [1]<ref name="ref1"/>. (I believe that this is excessive fluid intake in the absence of sufficient electrolytes.) Hyponatremia can be common in endurance athletes. In a 1997 Ironman triathlon, almost 4% of competitors received attention for Hyponatremia [4]<ref name="ref4"/>. In a study of the 2002 Boston Marathon, 13% of finishers had some level of Hyponatremia, and 0.6% had critical Hyponatremia [2]<ref name="ref2"/>. The study revealed that the risk factors for Hyponatremia include a slow finish time (&gt;4 hour) and consumption of &gt;6 pints (3 liters) of water during the race; BAA suggests a 'slight build' is also a risk factor[12]<ref name="ref12"/>. Healthy kidneys can excrete about 2 pints (1 liter) of fluid per hour, but this may be reduced by exertion or illness [3]<ref name="ref3"/>. So drinking &gt;6 pints in 4 hours could easily exceed the kidneys capacity to cope.
The recent rise in Hyponatremia may be due to earlier advice to athletes to "drink as much as possible" [13]<ref name="ref13"/>, combined with a general concern about salt intake.
== 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 [17]<ref name="ref17"/>. 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 ==
If you have high blood pressure, you may need to be careful with your salt intake. There is evidence that increased salt intake can increase blood pressure [4]<ref name="ref4"/>. If you have high blood pressure, discuss these issues with your doctor. If your doctor is not an athlete, I would highly recommend changing to one that is. If you don't know what your blood pressure is, get it checked. (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 [5, 14, 15]<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 [16]<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 [5]<ref name="ref5"/>.
== Muscle Cramps ==
The common wisdom that muscle cramps are caused by lack of electrolytes or dehydration does not appear to be supported by science [6]<ref name="ref6"/>.
== Blisters and black toe nails ==
== References ==
[1]<references><ref name="ref1">Exercise-Associated Hyponatremia
[http://cjasn.asnjournals.org/cgi/content/abstract/2/1/151 http://cjasn.asnjournals.org/cgi/content/abstract/2/1/151]
</ref>
[2]<ref name="ref2">Hyponatremia among Runners in the Boston Marathon
[http://content.nejm.org/cgi/content/abstract/352/15/1550 http://content.nejm.org/cgi/content/abstract/352/15/1550]
</ref>
[3]<ref name="ref3">Water Intoxication
[http://en.wikipedia.org/wiki/Water_intoxication http://en.wikipedia.org/wiki/Water_intoxication]
</ref>
[4]<ref name="ref4">Micronutrient Information Center - Sodium
[http://lpi.oregonstate.edu/infocenter/minerals/sodium/ http://lpi.oregonstate.edu/infocenter/minerals/sodium/]
</ref>
[5]<ref name="ref5">Caffeine dehydration : Caffeine and alcohol - just how dehydrating are they?
[http://www.pponline.co.uk/encyc/caffeine-dehydration.htm http://www.pponline.co.uk/encyc/caffeine-dehydration.htm]
</ref>
[6]<ref name="ref6">Muscle Cramps : No link between hydration and cramps
[http://www.pponline.co.uk/encyc/muscle-cramps.htm http://www.pponline.co.uk/encyc/muscle-cramps.htm]
</ref>
[7]<ref name="ref7">Hydration - fluid intake advice and tips
[http://www.pponline.co.uk/encyc/hydration-fluid-intake-advice-and-tips-40789 http://www.pponline.co.uk/encyc/hydration-fluid-intake-advice-and-tips-40789]
</ref>
[8]<ref name="ref8">The Relation Of Glycogen To Water Storage In The Liver
[http://www.jbc.org/cgi/reprint/96/2/367.pdf http://www.jbc.org/cgi/reprint/96/2/367.pdf]
</ref>
[9]<ref name="ref9">Cracking the Code on Hydration
[http://www.active.com/cycling/Articles/Cracking-the-Code-on-Hydration.htm http://www.active.com/cycling/Articles/Cracking-the-Code-on-Hydration.htm]
</ref>
[10]<ref name="ref10">Hyponatremia
[http://en.wikipedia.org/wiki/Hyponatremia http://en.wikipedia.org/wiki/Hyponatremia]
</ref>
[11]<ref name="ref11">Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise
[http://www.edb.utexas.edu/coyle/pdf%20library/%286329%20Dehydration%20reduces%20cardiac%20output%20&%20increases%20systemic%20&%20cutaneous%20vascular%20resistance%20during%20exercise,%20J%20Appl%20Physiol%2079,%201487-96,%201995.pdf http://www.edb.utexas.edu/coyle/pdf%20library/%2863%29%20Dehydration%20reduces%20cardiac%20output%20&%20increases%20systemic%20&%20cutaneous%20vascular%20resistance%20during%20exercise,%20J%20Appl%20Physiol%2079,%201487-96,%201995.pdf]
</ref>
[12]<ref name="ref12">BAA - HYDRATION, DEHYDRATION AND HYPONATREMIA
[http://www.bostonmarathon.org/BostonMarathon/WelcomeBooklet.asp#hydration http://www.bostonmarathon.org/BostonMarathon/WelcomeBooklet.asp#hydration]
</ref>
[13]<ref name="ref13">USATF Announces Major Changes in Hydration Guidelines for Long Distance Runners
[http://www.rrm.com/archive03/042803n2.htm http://www.rrm.com/archive03/042803n2.htm]
</ref>
[14]<ref name="ref14">Metabolic and exercise endurance effects of coffee and caffeine ingestion
[http://jap.physiology.org/cgi/content/full/85/3/883 http://jap.physiology.org/cgi/content/full/85/3/883]
</ref>
[15]<ref name="ref15">Effects of caffeine ingestion on body fluid balance and thermoregulation during exercise
[http://www.ncbi.nlm.nih.gov/pubmed/2383801 http://www.ncbi.nlm.nih.gov/pubmed/2383801]
</ref>
[16]<ref name="ref16">Caffeine ingestion and fluid balance: a review.
[http://pt.wkhealth.com/pt/re/jhnd/abstract.00009862-200312000-00004.htm;jsessionid=KNhWhGQSZnXhY11p2f7qnnmn1Q7z376shvhsK7hTWDLVGQhWpGGJ%21811725889%21181195628%218091%21-1 http://pt.wkhealth.com/pt/re/jhnd/abstract.00009862-200312000-00004.htm;jsessionid=KNhWhGQSZnXhY11p2f7qnnmn1Q7z376shvhsK7hTWDLVGQhWpGGJ!811725889!181195628!8091!-1]
</ref>
[17]<ref name="ref17">Sodium Status of Collapsed Marathon Runners
[http://arpa.allenpress.com/arpaonline/?request=get-document&doi=10.1043%2F1543-2165%282005%29129%3C227:SSOCMR%3E2.0.CO%3B2 http://arpa.allenpress.com/arpaonline/?request=get-document&doi=10.1043%2F1543-2165%282005%29129%3C227:SSOCMR%3E2.0.CO%3B2]
</ref>
 
</references>

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