What is a sweat test, and should you take one?

If you’ve had issues with hydration or cramp in the past and have gone online looking for a solution then the chances are that you’ve seen the term ‘sweat test’ mentioned.

Why do humans sweat so much?

Here’s a pub-quiz-worthy fact for you, humans sweat considerably more than any other animal on earth.

Our sweat rate can be up to 2x higher than camels and we can be as much as 5x sweatier than horses (in terms of sweat produced per hour relative to total body surface area).

It’s widely believed that the ability to sweat developed under evolutionary pressures when our ancestors were hunting and gathering on the hot, dry African Savannah.

Sweating allowed us to offload excess heat to the environment more easily than panting (the method used by many other animals) does, so early humans with higher sweat rates could move around during the hottest part of the day without overheating and exploited this competitive advantage when it came to hunting and gathering.

As a result, heavy sweating was promoted in the gene pool and we became better and better sweaters as time went on.

Although most of us no longer need to hunt and gather on the Savannah for survival, modern humans have retained the ability to sweat profusely for cooling purposes and this is one of the physical traits that makes us good at doing endurance exercise in the heat.

What’s the downside of sweating?

The downside to all that sweating is that it leads to relatively rapid fluid and electrolyte losses.

If these losses go uncorrected for long enough, a dip in performance naturally follows. That’s largely because your blood volume is reduced and cardiovascular strain increases when you’re significantly dehydrated.

So we need to consume fluids – and often electrolytes – in relatively large quantities (compared with other animals) when we’re perspiring a lot in order to replace what we’re losing.

In the last 50 years or so there’s been a lot of interest in finding out how much – and what – athletes need to drink in order to maximise performance, especially in hot and humid environments.

Sweat testing has been at the core of much of the research into the hydration requirements of athletes and it’s increasingly becoming something that athletes are turning to when looking for performance gains.

Why test your sweat?

Sweat testing is usually used with athletes to answer two key questions…

1) How much sweat are you losing during exercise?

2) What are you losing in that sweat? (So you can replace it to maintain your performance…)

The second question largely focuses on the electrolyte composition of your sweat and, from a hydration point of view at least, the most important electrolyte found in your sweat is sodium because it helps you absorb and retain fluid in your blood.

Sodium’s also the one that’s lost in vastly different quantities from person to person, from as little as 200mg of sodium per litre of sweat to as much as 2,000+mg/l!

Getting the answers to these two questions enables you to personalise your hydration strategy and this can help you maximise your performance, especially in hot/humid conditions.

One recent study saw athletes that replaced an adequate proportion of what they’d lost in their sweat finishing a 70.3 Ironman 26 minutes faster than those who didn’t!

Whilst there’s no guarantee that everyone will see similarly massive performance gains from taking in more sodium in races, it does point to the fact that sodium replenishment can be very important, in longer and hotter races in particular.

The different types of sweat test

There are a few different methods commonly used to collect and analyse an athlete’s sweat…

1) The Whole Body Wash down (WBW) technique

This is generally considered the ‘gold standard’ in exercise sweat collection and analysis.

As the name suggests, this method involves completely washing an athlete in de-ionised water and then getting them to exercise (presumably naked) in an entirely enclosed plastic environment. Every drop of sweat produced can then be painstakingly collected and analysed for very accurate analysis in a lab.

The main reason that this method is held in such high regard from a scientific standpoint is that it’s the only way to actually collect 100% of the sweat from an athlete’s entire body for analysis and therefore get a truly unambiguous assessment of their total body sweat and electrolyte losses.

The obvious downsides are largely practical; it’s time consuming and complex. So WBW is very rarely used in practice with athletes and is more often reserved for use during specific

research studies with small numbers of subjects when collecting 100% of sweat losses is a priority.

2) Patch testing

Exercise testing using adhesive, absorbent patches stuck to various sites on the body is probably the most common type of sweat testing used with athletes.

Small patches are stuck to the skin and left on during exercise, with sweat soaking into the material as the athlete perspires. These patches are then removed and the sweat extracted (usually using a syringe to ‘squeeze’ them out, or in a centrifuge) and the sweat composition analysed in the lab.

As 100% of the athlete’s sweat is not collected, total sweat volume or sweat rate is usually estimated using body weight changes (corrected for any fluid consumed) instead.

One advantage of this method is that sweat can potentially be collected during a wide range of highly sport-specific conditions. This means that results ought to be very representative of those that would occur during real world sporting scenarios, although it’s most often still done in a lab environment.

A disadvantage of patch testing is that evaporative loss of sweat from patches (or contamination of patches that are not handled correctly) can skew the results significantly.

Whilst this can be minimised by experienced practitioners in controlled conditions, it’s a tricky job to get right all of the time.

Patch testing is also still a relatively time consuming process, so the number of athletes that can be tested in a given period of time is limited and access to specialist facilities is still needed.

3) Non-exercise sweat testing

Cystic Fibrosis sufferers lose an extremely high amount of salt (sodium chloride) in their sweat, so sweat electrolyte levels are used as part of the diagnostic criteria for the disease.

Because the diagnosis of very young children is often required, sweat tests have been developed that don’t require the subject to exercise to produce sweat.

These methods use chemicals applied to the skin to mimic the effect of certain neurotransmitters that stimulate the sweat glands and allow sweat samples to be collected ‘at rest’.

In recent years this type of testing has been used increasingly frequently with athletes (including by my business, Precision Hydration) because of the many advantages that at-rest testing offers over exercise-based methods.

Non-exercise sweat testing has the advantage of being less time consuming and not requiring significant exertion. Also, as the collection and analysis process are very controlled and simple, the results are extremely reliable and reproducible with a low risk of ‘user error’ in administering the test.

Results are also usually available immediately in most cases, getting actionable data to athletes faster.

As this type of testing doesn’t specifically allow for sweat rate calculation at the same time as sweat composition analysis, further testing is needed to quantify sweat rate, but this can be done pretty simply with no more than a decent set of bathroom scales before or after an at-rest sweat test to give a more complete picture of an athlete’s likely net fluid and electrolyte losses.

What should you do if you can’t take one of these sweat tests?

If taking one of these sweat tests is not feasible for you, then our free online ‘Sweat Test’ has been designed to help you get an idea of your fluid/electrolyte requirements during training and racing so you can start refining your hydration plan through some trial and error. This blog highlights the signs to look out that might indicate that you’re losing a lot of salt in your sweat.

Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.

Top image by Precision Hydration