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The Science Of Hydration

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The science of hydration for exercise is complex and controversial. While dehydration can cause problems, the real risk is [[Hyponatremia]] where the level of sodium in the blood is too low. The science shows that sodium losses increase exponentially with sweat rate, so exercise in the heat can result in extreme levels of sodium loss. For a simpler look at hydration, see [[Practical Hydration]].= 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==
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"
! colspan="3"|'''Heat Acclimated'''
! colspan="3"|'''Heat Non-Acclimated'''
|<center></center>'''Sweat Rate'''|'''Sweat Rate'''<center></center>| colspan="3" |<center>'''Heat AcclimatedSweat Concentration'''</center>| colspan="3" [[Sodium Loss]]|[[Sodium Loss]]<center>| '''Heat Non-AcclimatedSweat Concentration'''</center>| [[Sodium Loss]]| [[Sodium Loss]]
|<center>'''Sweat Rate(l/hr)'''</center>|<center>'''Sweat Rate(mg/cm2/min)'''</center>|<center>'''Sweat Concentration(mmol/l)'''</center>|<center>'''[[Sodium Loss]](mg/hr)'''</center>|<center>'''[[Sodium Loss]](tsp/hr)'''</center>|<center>'''Sweat Concentration(mmol/l)'''</center>|<center>'''[[Sodium Loss]](mg/hr)'''</center>|<center>'''[[Sodium Loss]](tsp/hr)'''</center>
|<center>'''0.51'''</center>|<center>'''0.4998'''</center>|<div align="right">22</div>32|<div align="right">249</div>732|<div align="right">0.1</div>3|<div align="right">31</div>46|<div align="right">355</div>1044|<div align="right">0.2</div>4
|<center>'''1.5'''</center>|<center>'''01.9847'''</center>|<div align="right">32</div>43|<div align="right">732</div>1450|<div align="right">0.3</div>6|<div align="right">46</div>61|<div align="right">1044</div>2066|<div align="right">0.4</div>9
|<center>'''1.52'''</center>|<center>'''1.4796'''</center>|<div align="right">43</div>53|<div align="right">1450</div>2402|<div align="right">0.6</div>1|<div align="right">61</div>75|<div align="right">2066</div>3423|<div align="right">01.9</div>5
|<center>'''2'''</center>|<center>'''1.96'''</center>|<div align="right">53</div>|<div align="right">2402</div>|<div align="right">1</div>|<div align="right">75</div>|<div align="right">3423</div>|<div align="right">1.5</div>|-|<center>'''2.5'''</center>|<center>'''2.45'''</center>|<div align="right">63</div>|<div align="right">3589</div>|<div align="right">1.5</div>|<div align="right">90</div>|<div align="right">5113</div>|<div align="right">2.2</div>
This table is based on the research quoted below showing a linear relationship between sweat rate in and 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.
Below are some sample sources of Sodium, with the concentrations defined.
{| class="wikitable"
| align="center" style="background:#f0f0f0;"|'''! Source'''| align="center" style="background:#f0f0f0;"|'''! Sodium - mmol per liter'''| align="center" style="background:#f0f0f0;"|'''! Sodium - grams per liter'''| align="center" style="background:#f0f0f0;"|'''! Sodium - grams per pint'''| align="center" style="background:#f0f0f0;"|'''! Salt - grams per pint'''
|Water + 1/4 Teaspoon salt per quart||27||0.6||0.3||0.75
|Gatorade+ 1/4 Teaspoon salt per quart||45||1.0||0.5||1.2
|S-Caps + 8oz water*||65||1.4||0.7||1.7
|Salt Stick + 8oz Water||38||0.84||0.4||0.98
|Salt Stick + 16oz Water||19||0.42||0.2||0.49
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 assume 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
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 <ref name="CLINC"/>:
* Intracellular fluid (ICF) which is 40% of body weight
* Extracellular fluid (ECF) which is the other 20% of body weight
** plasma Plasma is 25% of ECF/5% body weight** interstitial 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=These symptoms are for the general public<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{| {{table}}| alignclass="centerwikitable" style="background:#f0f0f0;"|'''! symptom'''| align="center" style="background:#f0f0f0;"|'''! mild dehdration (3-5% body weight)'''| align="center" style="background:#f0f0f0;"|'''! Moderate dehdration (6-9% body weight)'''| align="center" style="background:#f0f0f0;"|'''! Severe dehdration (>10% body weight)'''
| Level of consciousness||Alert ||Lethargic ||Obtunded
| Capillary Refill||2 seconds||2-4 seconds||>4 seconds
| Blood Pressure||Normal||Normal supine, lower standing||lower
| Skin Turgor||Normal||Slow||Tenting
| Eyes||Normal||Sunken||Very Sunken
= References =
<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
<ref name="ref14">Metabolic and exercise endurance effects of coffee and caffeine ingestion

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