In previous blogs we’ve discussed what sodium is and how it might be helpful during exercise. We’ve also discussed that the amount of sodium lost in sweat varies significantly from person to person. In this article, we’ll take a look at how to determine your sweat sodium losses, and key considerations if you choose to do the testing.
What is a sweat sodium test?
A sweat sodium test is basically a way of collecting a sample of sweat and then analysing it to determine the sodium concentration (how many mmol or mg per L of sweat). There are a few different testing methods, and this is important because some tests, or even the way the test was conducted, will influence how well the sample collected reflects the sweat losses that will happen in a real training or competition scenario.
Sweat testing methods
In the perfect world, a sweat test would collect sweat from the entire body, because sweat rates and sodium losses differ from one body region to another. The perfect test would also not interfere with the normal function of sweat glands, by not covering them or preventing the natural air flow to the skin. Whilst a whole body washdown technique exists that meets these criteria, it’s very impractical, requires exercise conditions that don’t reflect real world sports, and can only be done on a stationary bike.
Instead, the preferred method in recent times for use in athletes has been sweat patches, applied to specific sites on the skin. These patches absorb sweat as it’s produced. It can then be extracted from the patch either by using a large syringe to plunger it out, or a special tube that can be centrifuged in a lab. The sweat sample is then analysed for the sodium concentration using specialised laboratory equipment. More recently, a pocket sized electrolyte meter for sodium has been found to be a valid substitute, and can be used in field settings. The sweat sodium concentration is then combined with the sweat rate to determine the hourly sodium losses.
Biometric sensors for sweat testing
Recent advances in biometric sensors and wearable devices have led to the development of patches that can self-analyse the sweat they collect. This saves on careful patch removal, sweat extraction and analysis, reducing the need for skilled technicians and expensive laboratory equipment. Disposable versions of these patches are now available commercially.
Regardless of the patch used, the skin site where the patch is applied must be thoroughly cleaned and dried prior to prevent contamination. The absorbent patches that are removed for analysis should only be applied to the skin for as long as needed to collect a viable sweat sample. Excessive sweat under the patch begins to block the sweat glands and prevents their normal function.
Sweat measurements sitting in a chair
Another method of sweat collection, originally developed for the diagnosis of Cystic Fibrosis, is sometimes used with athletes too. This technique uses a pharmaceutical agent (pilocarpine) applied to the skin to induce sweating. The test is usually performed with the athlete sitting in a chair. No exercise is involved. Unfortunately there are currently no studies that show whether or not this form of testing is an adequate substitute for sweat testing during exercise, and it is therefore not recommended for athletes.
Considerations when sweat testing
As mentioned in a previous blog post, sweat sodium concentration can vary with differences in sweat rate. For this reason, ideally sweat testing should be conducted at the same intensity and in the same conditions as those you want to apply the results to (training or competition day). If the training or competition that you want to apply the results to exceeds 2 hours, bear in mind that sweat rate will increase steadily in the first 30 minutes of exercise as core body temperature increases from resting. Ideally you would apply the patches after at least 30 minutes of exercise so the test result reflects the majority of your event, not the first half hour.
Interpreting sweat testing results
A few factors need to be understood when making sense of your sweat testing results. Athletes often simply trust numbers they are given, but if you understand the possible problems with these numbers it allows athletes to make better decisions.
Regional sweat collection versus whole body sweat
Firstly, sodium losses at any given part of the body do not reflect whole body losses. Equations can be used to provide whole body estimates, although they are not perfect. If you are having a commercial sweat test, ask the provider if they are reporting the actual concentration obtained from the sweat sample, or the estimated whole body value. If they don’t know what you’re talking about, this is a big red flag.
You sweat differently every day
Secondly, no two days of exercise outdoors are ever the same. Sweat rate, and sweat sodium concentration, will therefore vary to at least some degree from one day to the next. Few studies have investigated day-to-day variation in sweat sodium concentration, but those that have found the variation was around 15%. This may be due to many factors, including the sodium eaten in the days preceding the test (as discussed in a previous blog), small but possibly important differences in temperature, humidity or airflow, clothing, or simply natural day-to-day variation in the hormones that regulate sodium losses (this has been observed in urine losses even with identical sodium intakes).
Variations in conditions during longer events
Finally, consider for longer events that the environmental conditions will vary across even the same day. It might be cool at the beginning of a road cycling event in the morning, but the event finishes in the heat of the day. Even if the same pace was maintained throughout, sweat rate would vary due to the changing weather, as would the sweat sodium losses from hour to hour. So rather than taking the value obtained in testing as an exact target, think of it more as a ball-park figure, classifying your sodium concentration or losses as low, medium or high. Which brings us to how you take that result and apply it...
How to use sweat test results?
For all the research that’s gone into perfecting the sweat testing process, it’s rather surprising that there is almost no research at all into how to interpret and apply the results! Do you replace 100% of the losses? 50%? 10%? Currently there are no guidelines or studies to help answer that question. As mentioned in a previous post, sodium replacement does not seem to influence performance, so the question is, what benefit/s do we get from replacing sodium losses, and if we do, how much should be replaced? This will be explored in the next post.
Summary
There are different methods of sweat testing, which all have strengths and limitations. Currently the patch technique is the preferred method of obtaining sweat samples that reflect real-world exercise scenarios. Ensure the exercise done when the patch is applied is similar to competition conditions, that the patch is applied once you’ve been sweating for a considerable time, and the results are converted to estimate whole body losses. And remember that the values obtained are a ball-park figure, not a precise number.
References
Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 2017; 47(Suppl 1):111-128.
Baker LB et al. Exercise intensity effects on total sweat electrolyte losses and regional vs. whole-body sweat [Na(+)], [Cl(-)], and [K(+)]. Eur J Appl Physiol. 2019; 119(2):361-375.
Baker LB et al. Validity and reliability of a field technique for sweat Na+ and K+ analysis during exercise in a hot-humid environment. Physiol Rep. 2014; 2(5):e12007.
