Hydration and Sweat Composition

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Of the relatively small number of mammals who perspire as a mechanism to lose heat, humans are by far and away the sweatiest!

Horses, donkeys, camels and baboons all have maximum sweat rates (when expressed relative to body surface area) that are less than half that of an active person, and many other mammals don’t even sweat at all.

The theory goes that the ability to lose significant amounts of heat through the evaporation of sweat from our relatively hairless skin (along with the fact we walk upright, presenting less surface area to the sun) gave our ancestors a competitive advantage on the African savannah. It meant we could be out during the heat of the day and through the practice of ‘Persistence hunting’ catch antelope despite their superior sprinting speeds; harassing them into a state of heat exhaustion by following steadily behind for several hours at a time, never letting them rest long enough to fully cool down before having to sprint again until they collapsed. If this is correct it goes a long way towards explaining why human beings are very good at exercising for prolonged periods in hot, dry environments (where sweat can evaporate easily from the skins surface) but start to struggle in very humid conditions where sweating is far less effective.

Of course despite being good for temperature regulation losing large amounts of water through sweating does mean that there becomes a point at which dehydration occurs and both performance and health start to suffer due to reduced body fluid levels. As a result humans need to ingest fluids reasonably frequently to stop levels dropping too low. This ultimately means that unlike many other non-sweating animals a human being in a hot environment will start to struggle after just a few hours and most likely die of dehydration within 2 to 3 days if he or she cannot drink. However that is concerning survival; as far as optimizing sporting performance goes there is furious debate around exactly how much fluid athletes should drink to in relation to what they lose in sweat:

Prior to the 1980s most athletes were advised to drink little or nothing during competition (regardless of the length, intensity or temperature) as it was thought that it would impair performance. Athletes such as the cycling world champion Tommy Simpson were quoted as saying “Drink as little as possible, it is only a question of will power!”

Then, all of a sudden, with research predominantly funded by sports drink manufacturers to back it up, the pendulum swung the other way and the message changed to ‘drink as much as you can’ and that ‘if you’re thirsty it is too late and your performance is already suffering’. This lead to an increasing number of athletes overdrinking to the extent they were hospitalized or dying of a condition called hyponatreamia (water intoxication or lowering of the body’s sodium levels), something that was so rare as to be unheard of before the 80s.

Recently a more moderate approach has been coming to the fore with ‘learn to listen to your body’ being the message promoted. Not everyone us yet on board but highly regarded figures in sport and exercise science such as Dr Tim Noakes are busily proving that athletes tend to perform best when they choose how much to drink based on thirst, even if this means they don’t end up replacing 100% of their sweat losses during exercise as a moderate fluid deficit can be replenished in recovery. In other words, the pendulum looks like it may be returning to the middle as it is recognized that neither deliberately restricting fluid intake nor forcing large quantities of fluid in regardless of thirst are beneficial for both athletic performance and health.

To add another layer of complexity to the issue, human sweat contains far more than just water. Electrolytes (electrically charged ions such as sodium, potassium, calcium and magnesium) are also lost and so these ultimately need to be replaced as well as the fluid itself. We get a lot of electrolytes from the food we eat however many sports drinks also have them on their ingredient lists.

Again, as with how much fluid to drink, the level of electrolyte replenishment needed to optimize sports performance is a source of disagreement between scientists. What does seem clear is that for relatively short duration activities (less than about 60 minutes) little fluid should be required to sustain performance and water alone (taken to thirst) should be sufficient, assuming the athlete starts the session appropriately fuelled and watered as any electrolytes lost will be replaced in subsequent meals.

However, when considering longer, hotter and more intense events the picture starts to change. There is a lot of evidence that sweat composition (in terms of the sodium content in particular) varies significantly from person to person (See box 1). Some athletes will excrete as little as 200 mg/l sodium where others 1700mg/l. Coupled with variations in overall sweat rate it means some can lose over 10 times more sodium than others during the same time period, which then becomes highly significant over several hours or anytime when sweat output is very high. This may be one reason why some athletes seem to respond very positively to consuming extra sodium before and during exercise and for others it seems to make little or no real difference.

Sports scientists have been measuring the levels of sodium in athlete’s sweat for many years but this used to be only really available to professionals. However, in recent years with advances in technology and far more people taking part in endurance events in particular sweat testing is slowly becoming more widely available (see box 2 ‘About sweat testing’). This gives more athletes the opportunity to understand their individual physiology in more detail, and the implications it may have for hydration and electrolyte replenishment.

Overall what is clear in terms of how athletes should approach re-hydration and electrolyte replacement is that optimizing performance is centered on individualization based on your physiology, the demands of your sport and the environmental conditions. The human body does not tend to respond well to a being faced with a large deficiency of anything (water and electrolytes especially) but neither does it have an infinite capacity to deal with excess (e.g. issues with hyponatramia when too much water is consumed), so just throwing more and more in is definitely not the answer. Therefore learning what works for your own particular exercise habits and body, using a combination of scientific knowledge and good old fashioned trial and error is the best way to discover what works for you.

The constituent parts of sweat and how these differ from athlete to athlete:

  • Average electrolyte content of human sweat:
  • Sodium (0.9 gram/litre)
  • Potassium (0.2 g/l)
  • Calcium (0.015 g/l)
  • Magnesium (0.0013 g/l)

(Data from Int J Sport Nutr Exerc Metab. 2007 Dec;17(6):574-82.

Sweat mineral-element responses during 7h of exercise-heat stress. Montain SJ, Cheuvront SN, Lukaski HC.)

However, the amount of sodium can vary greatly from person to person (Range approximately 200mg/l to 1700mg/l) whilst the variation in the other electrolytes is far less pronounced or significant for exercise performance.

Different theories exist as to why there is such a wide range of sodium loss between people and acclimatization effects as well as the amount of sodium in a person’s diet are implicated. However certain genetic factors that influence the amount of sodium re-absorbed by the sweat glands as perspiration leaves the body look likely to play a key role. At the extreme people who suffer with the genetic disorder Cystic Fibrosis (CF) lose huge amounts of sodium in their sweat, however some healthy individuals can lose amounts not too dissimilar to those found in the CF patients. In other words whilst sweat sodium composition can alter slightly with acclimatization or diet your genetic pre-disposition is instrumental in how much sodium you basically excrete in your sweat.

About sweat testing

A sweat test involves taking a sweat sample from an athlete and analyzing it for sodium concentration. Various methods can be used including attaching absorbent patches to the skin that become soaked in sweat during exercise, then sent to a lab for analysis, however exercise free testing is also possible.

Exercise free sweat testing makes the process much simpler and quicker for an athlete and a result can be obtained in about 25 minutes with the following steps:

1)    Firstly a chemical called Pilocarpine is applied to the forearm using a technique called iontophoresis (using a small electrical current to ‘push’ the chemical into the skin). Pilocarpine causes the sweat glands in a small area of the arm to start contracting.

2)     A specialist capillary tube in a collection device called a ‘Macroduct’ is then applied to the arm and sweat is collected into the tube.

3)    Once a sufficient amount of sweat is collected in the tube it is removed from the athlete’s arm and the sample passed through a machine that measures the conductivity of the solution.

4)    From the conductivity reading the amount of sodium in the sample can be calculated and the information fed back to the athlete straight away.

Sweat sodium levels typically fall between 200mg/l and 1700mg/l in healthy athletes and this information is used to advise them on how much sodium they should consume before and during exercise to meet their losses.

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