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The Science Of Hydration

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= Introduction =

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 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 =

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. )]]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==

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.)

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<center>'''Heat Acclimated'''</center>

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<center>'''Heat Non-Acclimated'''</center>

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<center>'''Sweat Rate'''</center>

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<center>'''Sweat Rate'''</center>

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<center>'''Sweat Concentration'''</center>

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<center>'''[[Sodium Loss]]'''</center>

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<center>'''[[Sodium Loss]]'''</center>

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<center>'''Sweat Concentration'''</center>

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<center>'''[[Sodium Loss]]'''</center>

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<center>'''[[Sodium Loss]]'''</center>

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This table is based on the research quoted below showing a linear relationship between sweat rate in sweat sodium concentration.

==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.

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|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==

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|400px|Sweat sodium concentration against sweating great, shown for two different skin temperatures.]]

=Sodium Retention=

The human body is very good at maintaining its sodium balance under most conditions. The higher your sodium intake the higher the sodium losses in sweat and urine. There is evidence<ref name="a1946"/> that under moderate sweat rates and a restricted sodium diet that the sodium concentration of sweat can drop to extremely low levels. However, I have found no evidence to suggest the sodium retention is effective at higher sweating rates.

=Sodium Intake=

Below are some sample sources of Sodium, with the concentrations defined.

|Salt Stick + 16oz Water||19||0.42||0.2||0.49

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Note: S-Caps does not specify the amount of fluid to take with each capsule, but does mention 'at least one cup', so this ratio is used. The per-pint and per-liter equivalents assumes a constant ratio of one capsule per 8oz of water.

See also [[Comparison of Gels]].

==Example Sodium Losses==

Here are some hypothetical examples

* Adam, a heat acclimatized runner, weighs himself before and after his four hour run and the difference is 8 pounds, which is roughly equivalent to 8 pints/4 liters of sweat. Based on 1 liter/hour of sweating we estimate Adam lost 4 grams of sodium, which is about 2 teaspoons.

* Bob is not heat acclimatized runner, and losses 9 pounds in three hours (9 pints/4.5 liters). From the sweat rate we estimate that Bob has lost 7.5 grams of sodium, which is about 3.3 teaspoons.

=Sweat Rates While Running=

Sweat rates in male runners have been measured in the range from 0.75-2.23 in winter to 0.99-2.55 in the summer (Liters per hour)<ref name="acsm"/>. At the low end, we can imagine a fit runner finishing a 3-hour marathon in winter and sweating only 2.25 Liters. Assuming they are also heat acclimated, they would only lose 2 grams of sodium, which is 5 grams of salt, less than a teaspoon. On the other end of the scale, a fit, but unacclimatized runner completing a 5 hour marathon in summer would sweat out nearly 13 Liters, 18 grams of sodium, which is 45 grams of salt or more than 7 teaspoons.

There is a table showing a range of values at [[Sodium Loss]].

= 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 <ref name="ref10"/>.

= 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="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 =

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="JAMA"/> 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 http://www.drmirkin.com/public/ezine050811.html

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 =

** interstitial fluid which is 75% of ECF/15% of body weight, typically 11 Liters/22 pints.

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<ref name="CLINC"/>=

These symptoms are for the general public, and there is evidence<ref name="SYMPT"/> that they may not apply to athletes suffering from mild dehydration

<ref name="CLINC">Clinical Studies in Fluid and Electrolyte Balance</ref>

<ref name="SYMPT">Sensitivity and specificity of clinical signs for assessment of dehydration in endurance athletes

http://bjsm.bmj.com/content/early/2010/04/22/bjsm.2008.053249.abstract

</ref>

<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>

<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>

<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>

<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>

<ref name="ref10">Hyponatremia

[http://en.wikipedia.org/wiki/Hyponatremia http://en.wikipedia.org/wiki/Hyponatremia]

</ref>

<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>

<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>

<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>

<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>

<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>

<ref name="acsm">http://journals.lww.com/acsm-msse/_layouts/oaks.journals/ImageView.aspx?k=acsm-msse:2007:02000:00022&i=TT2

</ref>

<ref name="ref4">Micronutrient Information Center - Sodium

http://lpi.oregonstate.edu/infocenter/minerals/sodium/

</ref>

<ref name="JAMA">Fatal and Nonfatal Outcomes, Incidence of Hypertension, and Blood Pressure Changes in Relation to Urinary Sodium Excretion

http://jama.ama-assn.org/content/305/17/1777

</ref>

Na+ secretion rate increases proportionally more than the Na+ reabsorption rate with increases in sweat rate

http://jap.physiology.org/content/105/4/1044.full

</ref>

<ref name="training">SpringerLink - European Journal of Applied Physiology, Volume 111, Number 11 http://www.springerlink.com/content/mxj56211q04v4455/</ref>

<ref name="a1946">Relationship between salt intake and sweat salt concentration under conditions of hard work in humid heat

http://www.ncbi.nlm.nih.gov/pubmed/20984571</ref>

<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>

<ref name="WBWandPatch">Comparison of regional patch collection vs. w... [J Appl Physiol. 2009] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/19541738</ref>

<ref name="Fitzpatrick">Fitzpatrick's Dermatology in General Medicine, Chapter 81. Biology of Eccrine, Apocrine, and Apoeccrine Sweat Glands</ref>

</references>

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