Product images: hDrop Technologies (hdroptech.com) and Precision Fuel & Hydration (precisionhydration.com)
About the Author
Albertus Roux is a sports scientist and ultra-trail runner specialising in endurance performance and human physiology. Drawing on scientific research, coaching experience, and years of ultra-distance mountain running, his work focuses on resilience, discipline, and the mental and physical demands of endurance sport. He helps athletes improve performance and unlock long-term potential through evidence-based insight and real-world experience.
The summer is fast approaching, and with that, the trail and ultrarunning season in South Africa. Our flagship races typically happen in the dead of summer, which calls for specific hydration solutions. The first step is testing both sweat rate and sweat-sodium concentration to prescribe water and sodium intake. Many companies claim their testing methods are accurate, but decades of research have helped prove or disprove those claims. Luckily, Baker (2017) conveniently lays out each method and their accuracy.
Part 1: Measuring Sweat Rate
Whole-body sweat rate (click here to find your spreadsheet)
This is the gold standard method and also the simplest. Weigh yourself before and after exercise, correct for what you drank and passed:
WBSR = [pre-exercise mass − (post-exercise mass − fluid intake + urine output)] ÷ exercise duration
Baker calls change in nude body mass “the simplest and most accurate method” for whole-body sweat rate. Accuracy depends on how precise you are. Sweat trapped in clothing caused an 8–10% underestimation, so weigh nude and dry off all sweat.
Then there are more complex variables: you lose body mass through metabolism, literally breathing it out. Combined respiratory water loss plus metabolic mass loss causes a further 9% (warm environment) to 20% (cool environment) overestimation. These variables don’t all warrant correction. Yes, there are published formulas if you want to work these out, but you’re crossing the line of paralysis by analysis. In a sport as complex as trail running, you very seldom get to stick to a specific intake.
Log this on a spreadsheet over time against average temperature to get a global view of your sweat rates. Start by replacing 75% of your WBSR in subsequent exercise, then adjust to your preference. It costs nothing, and it is more accurate than anything you can buy.
Accuracy verdict: it’s the gold standard, but only when done properly. Nude (or minimal clothing) weigh-ins, duplicate measurements, logged fluid intake, and urination corrections are essential. Expect roughly 5% day-to-day variation even when you do everything right.
Local sweat rate: ventilated capsule (hygrometry)
Dry air of a known temperature is pumped through a small capsule stuck to the skin, and sweat rate is calculated from how much moisture the air picks up on the way out. It is the reference technique for local sweat rate, with a coefficient of variation around 2% or extremely reliable.
Accuracy verdict: superb precision, but limited relevance for extrapolating whole-body sweat rate. Forced ventilation keeps the skin artificially dry, which encourages sweating, so it can overestimate what really happens under a sweaty shirt in still, humid air.
Local sweat rate: gravimetry (absorbent patches)
Weigh an absorbent patch before and after exercise, divide by surface area and time. Portable, cheap, and the best for field testing. Compared with ventilated capsules, it reads 6–37% lower in the first 10–30 minutes, but the two converge once steady-state sweating is achieved, after which gravimetry is a reliable, practical substitute.
The catch is that an occlusive patch traps moisture and the sweat rate under the patch falls. This means a saturated patch underreports both sweat rate and sodium concentration.
Accuracy verdict: good for local sweat rate after 20–30 minutes of exercise, provided the patch never saturates. Local sweat rate does correlate strongly with whole-body sweat rate across several sites.
Part 2: Measuring Sweat-Sodium Concentration
Whole-body wash-down, the gold standard
The athlete exercises in a controlled space, and every drop of sweat runoff is collected for analysis. Recovery is 102 ± 2% for water and 99 ± 2% for sodium, essentially complete. Crucially, it doesn’t interfere with normal evaporative sweating.
Accuracy verdict: both the true gold standard for sodium loss and completely impractical. It requires a lab and is mostly limited to stationary cycling. Nobody is doing this anymore. But it matters, because every other method in this list is judged against it.
Regional absorbent patch during exercise
This is the most common commercial method. Skin is cleaned, a patch is applied, sweat is collected and analysed via a medical-grade electrode analyser like the Medica Easylyte. Its strength is specificity because it captures sweat under exercise conditions.
Its weakness is that a local number is not a whole-body number. Compared to whole-body washdown, forearm, scapula, chest, and forehead sites are 25–100% higher; foot, thigh, and lower back are about the same. Reputable providers apply regression equations to convert local sweat [Na⁺] into an estimated whole-body value. If your test result was never corrected, it is almost certainly too high.
Other ways the patch method can be wrong include electrolyte leaching from the skin into the patch (falsely high), sweat evaporating from the patch (falsely high), patch saturation (falsely low), and contamination from careless handling. Storage matters too: freezing at −80 °C dropped sweat [Na⁺] by about 7%, while a week of refrigeration at 7 °C raised it by about 14%.
Accuracy verdict: the best practical option, if the protocol is right and the result is regression-corrected. Expect around 15% day-to-day variation even so.
Arm bag / rubber glove
Sweat pools inside a sealed bag or glove over the limb.
Accuracy verdict: worst of the local methods. It overestimates sweat electrolyte concentration more than any other technique, largely because of the extreme microclimate it creates.
Pilocarpine iontophoresis, the resting test (PF&H)
A test developed for diagnosing cystic fibrosis, and adapted by Precision Fuel and Hydration. A small electrical current drives pilocarpine into the skin, chemically stimulating sweat glands while you sit still. It takes minutes, it needs no exercise, and is repeatable. Harris et al. (2026) tested 15 well-trained cyclists and triathletes four times each and found an excellent within-athlete coefficient of variation of just 5.5%.
Reproducible is not the same as correct. Pilocarpine sweating comes from local cholinergic stimulation only. Exercise sweating is influenced by core temperature, skin temperature, skin blood flow, central command and the exercise pressor reflex.
Harris and colleagues had those same athletes cycle at low, moderate and high intensity. Exercise sweat sodium climbed with intensity, tracking sweat rate:
Intensity | Sweat rate (L/h) | Exercise sweat [Na⁺] (mmol/L) | Pilocarpine [Na⁺] (mmol/L) |
Low (2.1 W/kg) | 0.62 | 44.5 | ~57 |
Moderate (3.1 W/kg) | 1.26 | 54.9 | ~55 |
High (4.2 W/kg) | 1.92 | 61.3 | ~54 |
The resting test overestimated at low intensity, matched at moderate, and underestimated at high. Agreement with exercise sweat was strongest at moderate intensity (ICC 0.94) and weakest at low (ICC 0.73, effectively no reliable agreement).
The physiology behind this is well established: as sweat flows faster through the duct, there’s less time to reabsorb sodium, so the sweat leaving your skin gets saltier. A resting test can’t capture this, because it reproduces only one flow rate.
Accuracy verdict: highly reliable, only conditionally valid. It’s a reasonable proxy at moderate workloads under standardised conditions, but it offers little significance at high or low intensities. Since most ultra-endurance events happen at low intensity, you might be misled into taking in more sodium than needed. Any company selling a resting test as a direct measure of your race-day sweat sodium is overselling it.
Wearable sweat sensors (hDrop and similar)
Wearable sweat sensors have been around for a few years; the only reusable option to date is the hDrop Gen 2. hDrop describes the Gen 2 as a reusable sweat sensor platform built for repeated testing during training and racing, rather than a one-off lab visit. Worn on the upper arm, it reports sweat rate, sweat sodium concentration, total sodium loss, total potassium loss and near-skin temperature. It presents results through “Sweat Zones”: concentrated, target, or dilute, benchmarked against your own rolling average. The advantage is repeatability across many sessions rather than the precision of one.
Their first independent validation has now been published. Sellner et al. (2026) tested the hDrop Gen 2 for sweat-sodium against the Medica Easylyte, a medical-grade electrode analyser, and the WBSR method for sweat rate.
For sweat rate, the device performed well: 1.0 ± 0.3 L versus 1.1 ± 0.4 L, with no statistically significant difference (p = 0.1) and excellent day-to-day repeatability. For sweat sodium, it read consistently high: 52 ± 13 mmol/L versus 40 ± 10 mmol/L, a mean difference of +12 mmol/L (p < 0.001) or ~276 mg/L. A systematic overestimate of this size could lead athletes to take on more sodium than they’re losing.
Accuracy verdict: very useful for fluid loss, and reliable enough for day-to-day use. For sodium concentrations and sodium replacement recommendations, don’t treat it as an exact number. Note also that this was one hour of moderate exercise in a temperate chamber; the authors flag that other climates and longer durations are still untested, which is the realm that trail running lives in. Still, a reusable device that you can wear session after session is extremely useful, because what you actually want is a range across conditions.
Why This Matters for Trail Runners
All but one of these methods give you a number that is valid only for the conditions in which it was collected. That’s the single most important take-away from this blog, and it’s easily lost in marketing. A test done at rest on a bike in a 20 °C lab does not describe your sweat-loss fluctuations during a mountain ultra. Temperature and humidity can vastly change within 24 hours.
Long ultra efforts sit well below the moderate intensity where the resting test is accurate, so it likely overestimates what you’re losing over a long day out. On the other hand, it may underestimate your losses when in hot weather. The truth about the resting test is that you’ll never really know which one of the two is happening. On shorter, hotter, and harder races, a resting test underestimates sodium loss, which could lead to hyponatremia if you drink too much.
As for wearable technology, it’s constantly evolving and improving. It’s worthwhile having some idea of what your body is doing in response to different stimuli. Exercise intensity and environmental conditions clearly play a massive role in sweating mechanics, and it becomes even more important over longer efforts, such as an ultra. It’s definitely much more useful than any other test for sweat rate and sweat-sodium concentration.
My Takeaway
Do this, in order.
- Weigh yourself before and after long runs. Nude, bladder empty, log your fluid intake, in several different conditions across the year. This is the reference method, and it’s free. Build a spreadsheet: cool morning, hot afternoon, race pace, easy long run.
- Only then think about sodium concentration, and only if you’re regularly racing long in the heat. For most trail runners it doesn’t matter.
- If you test, test during exercise, in conditions like your goal race, with a patch protocol that meets best practice and ask whether the result has been regression-corrected to a whole-body value. If they can’t answer that question, be sceptical of the number. The second-best option is to buy and use the hDrop Gen 2 over a long time.
- Treat any single result as a range, not a value. Even flawless methodology carries ~5% day-to-day variation in sweat rate and ~15% in sweat sodium. Baker recommends categorising athletes as low, moderate, or high and giving a range of options rather than chasing a decimal point.
- Practice water and sodium intake ranges. Start by replacing 75% of water loss, and 50% of sodium loss. Pay attention to how you feel and adjust accordingly. If you feel watery and bloated, you’re likely drinking too much or not consuming enough salt. If you’re feeling dry and thirsty, you’re likely not drinking enough or consuming too much salt.
References
- Baker, L. B. (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 47(Suppl 1), 111–128.
- Harris, C. T., Hunt, L., Shepherd, S. O., Hew-Butler, T. D., & Blow, A. V. (2026). Comparison of pilocarpine- versus exercise-induced sweat sodium concentration across exercise intensities in trained athletes. Physiological Reports, 14(1), e70724.
- Sellner, R., Perez, R., Allen, B., Butts, C., McKenna, Z., Wierick, S., Zhao, X., & McDermott, B. (2026). Validity and reliability of a reusable hydration wearable during exercise in the heat. Physiology, 41(S1), 2301191. (American Physiology Summit 2026 abstract.)
- hDrop Technologies. The Science of Sweat and Hydration. hdroptech.com.
FAQs
What is the most accurate way to measure sweat rate for trail running?
The most accurate and practical way to measure sweat rate is whole-body sweat rate testing. Weigh yourself before and after a run, track fluid intake and urine output, and divide the net body-mass change by exercise duration. For trail runners, repeating this in different temperatures and intensities gives the best real-world hydration data.
Are sweat sodium tests accurate?
Sweat sodium tests can be useful, but their accuracy depends on the method. Whole-body washdown is the gold standard, while exercise-based absorbent patches are the best practical option if they follow a strict protocol and convert local sweat sodium into an estimated whole-body value. Resting tests and single measurements should be treated with caution.
How much sodium should trail runners take during long races?
Sodium intake for trail running should be based on your sweat rate, estimated sweat sodium concentration, race duration, temperature, and personal tolerance. A single sweat test should not dictate an exact number. A practical starting point is to replace part of your estimated sodium loss, then refine your intake during training and long runs.
Can wearable sweat sensors help with hydration planning?
Wearable sweat sensors can help trail runners understand sweat-rate trends across repeated sessions, especially in different weather conditions. They may be useful for estimating fluid loss over time, but sodium readings should be interpreted as a guide rather than an exact prescription for race-day sodium replacement.
Why does sweat sodium concentration change during exercise?
Sweat sodium concentration changes with exercise intensity, sweat rate, temperature, humidity, and individual physiology. As sweat rate increases, there is less time for sodium to be reabsorbed in the sweat duct, so sweat often becomes saltier. This is why testing conditions should match the demands of your target trail race as closely as possible.