The Science Of Hydration
- 1 Introduction
- 2 Effects of dehydration
- 3 Sodium loss through sweat
- 4 Sodium Loss While Running
- 5 Hyponatremia
- 6 HypERnatremia - the opposite of HypOnatremia
- 7 Salt and High Blood Pressure
- 8 Caffeine and Alcohol
- 9 Cramps
- 10 Blisters and black toe nails
- 11 Sodium and Water in the Body
- 12 Symptoms of Dehydration
- 13 References
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.
2 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 . (Dehydration of 5% does impact performance .) 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 . 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 .
3 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 . 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.
3.1 Sodium Loss Table
|Source||Sodium - mmol per liter||Sodium - grams per liter||Sodium - grams per pint||Salt - grams per pint|
|Sweat of un-acclimated, unfit||80||1.8||0.9||2.2|
|Sweat of un-acclimated, fit||60||1.4||0.7||1.7|
|Sweat of acclimated, fit||40||0.9||0.4||1.1|
3.2 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.
3.3 Sodium Source Table
Below are some sample sources of Sodium, with the concentrations defined.
|Source||Sodium - mmol per liter||Sodium - grams per liter||Sodium - grams per pint||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 a constant ratio of one capsule per 8oz of water.
See also Comparison of Gels.
Here are some hypothetical examples
- Adam, a fit, heat acclimatized runner, weighs himself before and after his run and the difference is 8 pounds, which is roughly equivalent to 8 pints of sweat. We estimate that Adam has lost 0.4 grams of sodium per pint, for a total of 3.2 grams of sodium, which is 8 grams of salt or about 1.3 teaspoons.
- Bob, a fit, but not heat acclimatized runner, weighs himself before and after his run and the difference is 10 pounds, which is roughly equivalent to 10 pints of sweat. We estimate that Bob has lost 0.7 grams of sodium per pint, for a total of 7 grams of sodium, which is 17 grams of salt or about 3 teaspoons.
- Charlie, a fit, but not heat acclimatized runner, weighs himself before and after his run and the difference is 8 pounds, which is roughly equivalent to 8 pints of sweat. Charlie had also consumed 80 ounces of Gatorade on the run. This gives an estimate of 8+5=13 pints of sweat. At 0.7 grams of sodium per pint, that’s a total of 9 grams of sodium, which is 23 grams of salt or about 4 teaspoons. The Gatorade provided approximately 1 gram of sodium, or a 1/2 teaspoon.
- For the next run, Charlie changes his drink to add 1/4 teaspoon of extra salt to his Gatorade. He sweats and drinks the same amounts as the previous run. This time, his drink provides him with 4.7 grams of salt, or 3/4 teaspoon of salt.
4 Sodium Loss 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). 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 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 .
6 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 . 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.
7 Salt and High Blood Pressure
There is evidence that increased salt intake can increase blood pressure , and the common recommendation is to restrict your salt intake if you have high blood pressure. However, a recent study 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.
8 Caffeine and Alcohol
The scientific evidence shows that caffeine is generally not a diuretic . 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 . (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.
The evidence for hydration and electrolyte status causing Cramps is somewhat ambiguous, but supplementing your electrolyte intake may help.
10 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.
11 Sodium and Water in the Body
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
- Intracellular fluid (ICF) which is 40% of body weight
- Extracellular fluid (ECF) which is the other 20% of body weight
- plasma is 25% of ECF/5% body weight
- 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.
12 Symptoms of Dehydration
These symptoms are for the general public, and there is evidence that they may not apply to athletes suffering from mild dehydration
|symptom||mild dehdration (3-5% body weight)||Moderate dehdration (6-9% body weight)||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|
- Hydration - fluid intake advice and tips http://www.pponline.co.uk/encyc/hydration-fluid-intake-advice-and-tips-40789
- Dehydration reduces cardiac output and increases systemic and cutaneous vascular resistance during exercise 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
- The Relation Of Glycogen To Water Storage In The Liver http://www.jbc.org/cgi/reprint/96/2/367.pdf
- Cracking the Code on Hydration http://www.active.com/cycling/Articles/Cracking-the-Code-on-Hydration.htm
- 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
- Hyponatremia http://en.wikipedia.org/wiki/Hyponatremia
- 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
- Micronutrient Information Center - Sodium http://lpi.oregonstate.edu/infocenter/minerals/sodium/
- 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
- Caffeine dehydration : Caffeine and alcohol - just how dehydrating are they? http://www.pponline.co.uk/encyc/caffeine-dehydration.htm
- Metabolic and exercise endurance effects of coffee and caffeine ingestion http://jap.physiology.org/cgi/content/full/85/3/883
- Effects of caffeine ingestion on body fluid balance and thermoregulation during exercise http://www.ncbi.nlm.nih.gov/pubmed/2383801
- Caffeine ingestion and fluid balance: a review. http://pt.wkhealth.com/pt/re/jhnd/abstract.00009862-200312000-00004.htm;jsessionid=KNhWhGQSZnXhY11p2f7qnnmn1Q7z376shvhsK7hTWDLVGQhWpGGJ!811725889!181195628!8091!-1
- Clinical Studies in Fluid and Electrolyte Balance
- 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