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.

1 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.

2 Effects of dehydration

While it's commonly believed that even small levels of dehydration impacts performance, the research indicates that real world performance is not impacted by dehydration up to 4% of body weight[1][2]. Greater levels of dehydration will impact performance[3]. While authorities have recommended aggressive drinking[4], these guidelines are now recognized as erroneous[5], though they are still promoted by the beverage industry[6].

3 Dehydration and Body Weight

A common method of calculating dehydration is simply from body weight. While it's true that an athlete's weight loss during exercise will be predominantly from water, this is not the same as dehydration, nor does it necessarily imply this weight needs to 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[7]. 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). 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 from Glycogen depletion.

4 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)[8]
  • 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[9]. (These cases are roughly equal to water, 1/4, 1/2, 3/4 teaspoon of salt per quart/liter.)
  • Drinks containing 0.45g salt/100ml reduced urine output and improved hydration[10]. (0.45g/100ml is about 1/10 teaspoon of salt per quart/liter.)

5 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.

5.1 Anatomy of Sweating

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)[11]. Sweat glands are most numerous on the soles of the feet. Sweat is produced in two steps[11]:

  1. A coil generates an ultra-filtrated fluid. This fluid has the same sodium concentration (isotonic) as the blood.
  2. 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[11].

5.2 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.)

Heat Acclimated Heat Non-Acclimated
Sweat Rate Sweat Rate Sweat Concentration Sodium Loss Sodium Loss Sweat Concentration Sodium Loss Sodium Loss
(l/hr) (mg/cm2/min) (mmol/l) (mg/hr) (tsp/hr) (mmol/l) (mg/hr) (tsp/hr)
0.5 0.49 22 249 0.1 31 355 0.2
1 0.98 32 732 0.3 46 1044 0.4
1.5 1.47 43 1450 0.6 61 2066 0.9
2 1.96 53 2402 1 75 3423 1.5
2.5 2.45 63 3589 1.5 90 5113 2.2

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

5.3 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.

Sweat rate and sodium concentration[12], adjusted using the formula for regional patch collection[13].

5.3.1 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 m2, which is 17,400 cm2. Therefore 1 mg/cm2/min is 17,400 mg/min, or 17.4 g/min or 1,044 g/hour, or 1 liter/hour.

5.4 Sodium Loss and Fitness

While some sources suggest that increased fitness reduces the sodium concentration in sweat research[14] 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 V̇O2max) trained individuals will be performing a greater absolute work rate and therefore have a greater sweat rate and sodium concentration.

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.

5.5 Sodium Loss and Heat Acclimation

A study[15] 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.

Sweat sodium concentration against sweating rate, before and after 10 days of heat acclimation training, adjusted using the formula for regional patch collection[13].

5.6 Sodium Loss and Skin Temperature

A study[16] 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.

Sweat sodium concentration against sweating great, shown for two different skin temperatures.

6 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[17]. This reduction in the sodium concentration occurs at all sweating rates, but the relationship between sodium concentration and sweat rate remains a straight line[18]. 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%[18].

The effect of high (20,000mg/day NaCL) and low (500mg/day NaCL) on the sodium concentration of sweat at various sweat rates[18]. 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.

7 Sodium Intake

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
Gatorade 18 0.4 0.2 0.5
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 assume a constant ratio of one capsule per 8oz of water. See also Comparison of Gels.

7.1 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.

8 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)[19]. 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.

9 Hyponatremia

Hyponatremia is where the sodium (salt) levels in the blood become too dilute. The symptoms of mild hyponatremia tend to be a gain in weight and a general swelling and 'puffiness', most noticeable in the hands. However, mild Hyponatremia may have no clinical symptoms, or just weakness, dizziness, headache, nausea/vomiting, but more severe Hyponatremia is likely to have symptoms of cerebral edema, including altered mental status, seizures, pulmonary edema, coma, and death[20].

10 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[21]. 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.

11 Salt and High Blood Pressure

There is evidence that increased salt intake can increase blood pressure[22], and the common recommendation is to restrict your salt intake if you have high blood pressure. However, a recent study[23] 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.

12 Caffeine

The scientific evidence shows that Caffeine is generally not a diuretic[24][25]. 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[26]. (If you urinate more because you drink a 20oz Latte, it is because of the 20oz of fluid, not the Caffeine.)

13 Alcohol

Alcohol is a diuretic due to the suppression of vasopressin secretion[27][28], and the volume of urine produced is proportional to the alcohol consumed[29]. However, the diuretic effect of alcohol may be reduced 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[30]. 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.

14 Cramps

The evidence for hydration and electrolyte status causing Cramps is somewhat ambiguous, but supplementing your electrolyte intake may help.

15 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.

16 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[31]:

  • 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.

17 Symptoms of Dehydration

These symptoms are for the general public[31], and there is evidence[32] that they may not apply to athletes suffering from dehydration.

symptom mild dehydration (3-5% body weight) Moderate dehydration (6-9% body weight) Severe dehydration (>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

18 References

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