The Precision Hydration Pilocarpine Sweat Test was progress, but a resting sweat test is not the finish line. Precision hydration deserves credit: they were the first ones that pushed athletes from generic advice toward personalization.

The problem is what happens next. Many athletes take a pilocarpine sweat sodium test at rest and then turn that single number into a race-day sodium plan, as if it is fixed across intensities and conditions. The new evidence says it is not, and that is all what science is about, an evolving pursuit for truth.

Sweat testing after the January 2026 study: why “resting pilocarpine sodium” is a shaky foundation for a race plan

In January 2026, a peer reviewed study in Physiological Reports (Harris et al.) quietly changed the way athletes should interpret sweat testing. The authors compared pilocarpine induced sweat sodium (a resting, localized test) with exercise induced sweat sodium across low, moderate, and high cycling intensities [1]. Their results matter because they show something athletes rarely hear out loud: a single resting sweat sodium number does not stay “true” across intensities [1].

This post breaks down what they did, what they found, and then the math-driven reasons that pilocarpine is not a strong tool for crafting hydration and sodium strategies for marathons, Ironman, and other long events.

A simple starting point: you do not lose “mmol per liter,” you lose “mg per hour”

A hydration plan should be built around losses per hour, because that is what you must replace during a long event.

Sodium loss (mg per hour) = sweat rate (L per hour) × sweat sodium concentration (mmol per liter) × 23

That 23 is just a unit conversion: 1 mmol of sodium equals 23 mg.

This equation is the core issue with pilocarpine testing. It gives you a concentration number, but it does not capture the exercise conditions that drive sweat rate, and it does not reliably match exercise sweat sodium at all intensities [1].

What the January 2026 study did

Who was tested

Fifteen well-trained cyclists and triathletes (10 male, 5 female) participated [1].

The two methods compared

1. Pilocarpine testing at rest, which stimulates sweat locally on the forearm using iontophoresis and then measures sweat sodium concentration. Local sweat rate during pilocarpine collections was not quantified [1].
2. Exercise induced sweat collected from the forearm during cycling intervals, with sweat rate calculated from body mass changes (the typical whole-body approach) [1].

The intensity design

They used three exercise conditions: low, moderate, and high intensity, each lasting 20 minutes. The workload targets were based on a threshold estimate derived from a ramp test, then set at 50% (low), 75% (moderate), and 100% (high) of that estimated anaerobic threshold power [1].

The environment

Testing was performed in a temperate, low humidity setting: about 20°C and 35% relative humidity. They also used a commercial dehumidifier and directed strong airflow (about 4 m/s) at the face and torso [1].

That environmental setup matters a lot. It makes cooling easier and tends to reduce thermal strain compared with many race scenarios, especially running, where airflow is often lower and sun load can be high.

What they found

As exercise intensity increased, both sweat rate and sweat sodium concentration increased [1]:

• Sweat rate increased from 0.62 L/h (low) to 1.26 L/h (moderate) to 1.92 L/h (high).
• Exercise sweat sodium increased from 44.5 mmol/L (low) to 54.9 mmol/L (moderate) to 61.3 mmol/L (high).

Then the key statement for athletes building a plan:

Pilocarpine sodium overestimated exercise sodium at low intensity, was close at moderate, and underestimated at high intensity [1].

This is a critical result because it means the bias is not random. It changes with intensity in a way that can systematically mislead your sodium plan.

Why this matters: “precision” can still be predictably wrong

If a test were merely noisy, athletes could average multiple results and reduce error. But this paper suggests a different problem: the pilocarpine-to-exercise relationship changes with the context you actually care about (how hard you go) [1]. That means you can get a clean, repeatable number and still be off-target when you race.

The authors also report that pilocarpine results were stable across four visits, with a mean coefficient of variation of about 5.5% [1]. Stability is good, but stability is not the same as representativeness.

Pilocarpine testing cannot directly produce the number you need for planning

Hydration strategy needs sodium loss per hour, which requires sweat rate. Pilocarpine testing does not measure local sweat rate, and it is performed at rest, independent of whole-body thermoregulatory drive [1]. That makes it structurally mismatched to the way athletes lose sodium in real training and racing.

Their “high intensity” is hard, but not a worst-case sweating scenario

Their high condition was 20 minutes at 100% of estimated anaerobic threshold power. The reported average power and RPE indicate a very hard effort, but it is still a controlled, short interval in a cool, dry, high-airflow lab [1]. Many of the biggest sweating exposures in endurance sport happen with:

• longer duration (hours, not minutes)
• higher ambient temperatures
• higher humidity
• lower effective airflow (especially on the run)
• sun load

So even if the intensity is hard, the protocol is not designed to reproduce the hottest, most sweat-driven race conditions where plans tend to fail.

The lab environment likely compressed sweat rate and therefore compressed real-world error

The study was done at about 20°C and 35% relative humidity with strong airflow and dehumidification [1]. Sweat rate is highly sensitive to environmental heat load. A controlled endurance study reported that, during fixed-intensity exercise, sweat rate increased by about 0.04 L/h per 1°C increase in air temperature [2]. Heat-stress reviews also explain how exercise in hotter conditions increases thermal strain and drives greater sweating and fluid loss [3].

This matters because sweat rate is the multiplier in the sodium loss equation. If real race sweat rates are higher than in the lab, the practical error from using the wrong sodium concentration increases proportionally.

The direction of bias creates two different real-world failures

Because pilocarpine overestimates at low intensity and underestimates at high intensity, it can push athletes in opposite directions depending on how hard they are working [1]. This is not just academic. It changes how much sodium you target per hour.

The math: how big can the planning error be?

To make this concrete, let’s do the calculation using their reported means.

Assume an athlete uses a typical mid-50s pilocarpine result as their plan number. The study reported pilocarpine means around 54 to 57 mmol/L across visits [1]. Use 55 mmol/L as a representative planning value.

Compute sodium loss per hour from the study’s exercise values

Low intensity (study means)

• Sweat rate: 0.62 L/h
• Exercise sodium: 44.5 mmol/L
  True loss = 0.62 × 44.5 × 23 = about 635 mg/h

Pilocarpine-based plan (55 mmol/L):
Planned loss = 0.62 × 55 × 23 = about 784 mg/h

Error = +150 mg/h (over-planning sodium)

Over a 6-hour steady ride: ~900 mg more sodium than the exercise value would suggest.

High intensity (study means)

• Sweat rate: 1.92 L/h
• Exercise sodium: 61.3 mmol/L
  True loss = 1.92 × 61.3 × 23 = about 2707 mg/h

Pilocarpine-based plan (55 mmol/L):
Planned loss = 1.92 × 55 × 23 = about 2429 mg/h

Error = -278 mg/h (under-planning sodium)

Over a 4-hour marathon effort: ~1110 mg less sodium than the exercise value would suggest.

These are not hypothetical. They follow directly from the means in the paper [1].

Now stress-test the results for real racing heat

The paper itself flags a major limitation: exercise bouts were short and temperate, so extrapolation to longer sessions or hotter and humid conditions warrants confirmation [1]. That limitation is especially relevant because many key races occur in warm or hot conditions.

Using the temperature sensitivity reported in Jenkins et al. (sweat rate +0.04 L/h per 1°C during fixed-intensity exercise) [2], you can estimate how a hotter race day magnifies sodium planning error.

Example: scaling the high-intensity mismatch from 20°C to 30°C

In the study’s high condition:

• Exercise sodium minus plan sodium = 61.3 − 55 = 6.3 mmol/L
• Sweat rate = 1.92 L/h

If air temperature rises from 20°C to 30°C:

• Sweat rate increase estimate = 10 × 0.04 = +0.4 L/h
• New sweat rate estimate = 2.32 L/h

New hourly error estimate:
Error (mg/h) = 2.32 × 6.3 × 23 = about 336 mg/h

Over 4 hours: ~1344 mg sodium underestimated

At 35°C (15°C hotter than the lab):

• Sweat rate increase estimate = 15 × 0.04 = +0.6 L/h
• New sweat rate estimate = 2.52 L/h
• Hourly error = 2.52 × 6.3 × 23 = about 365 mg/h
• Over 4 hours: ~1460 mg sodium underestimated

This is the core practical critique: the study’s cool, dry, high-airflow setup likely limited sweat rate, and sweat rate is the multiplier that converts a concentration bias into a large multi-hour plan error [1], [2].

These errors are associated with just 4h of exercise. Well, multiply that by 4 times for a 16h event race.

What about humidity?

Humidity reduces evaporative cooling potential and increases thermal strain during exercise in the heat. That generally increases the difficulty of maintaining heat balance and can raise cardiovascular and perceptual strain, which is why heat-stress literature treats humidity as a major factor in real-world performance and thermoregulation [3]. A methodology and variability review also emphasizes that sweat rate and sweat sodium can vary substantially with environmental conditions, collection method, and timing [4].

The important athlete takeaway is not a single humidity formula. It is that the study did not test the kind of hot, humid race environment where sweating behavior and hydration needs are most likely to diverge from resting lab measures [1], [3].

What the study is useful for, and what it is not

Useful for

• Demonstrating that exercise sweat sodium rises with intensity alongside sweat rate [1].
• Showing that pilocarpine results can be repeatable under standardized conditions [1].
• Revealing intensity-dependent bias, which is valuable because it tells you when a resting number is most likely to mislead [1].

Not a strong foundation for

• Crafting a race hydration plan for long events in variable conditions.
• Predicting sodium losses per hour, because pilocarpine does not provide sweat rate and does not consistently reflect exercise sodium across all intensities [1].

The practical conclusion for athletes

If you want a hydration plan that works in racing, you need to respect three realities:

1. Sweat sodium concentration is not a constant across intensities [1].
2. Sodium loss is a rate problem multiplied by time [1].
3. Heat and humidity can push sweat rates higher, making any mismatch in sodium concentration more costly per hour [2], [3].

A pilocarpine test can give you a baseline number. This Precision Hydration study from January 2026 paper shows that treating that baseline as your “race truth” is where the plan becomes fragile.

Where hDrop fits

The direction of travel is clear. Real personalization requires measuring sweat behavior during actual training across representative conditions, not relying on a single resting, localized stimulus.

hDrop is built for that next step: helping athletes quantify sweat rate and sodium loss during real sessions so the plan reflects what your body does when you are actually training and racing. For more accurate hydration planning, wearable sweat sensors like the hDrop offer real-time monitoring of sweat composition during actual workouts. These sensors provide more precise measurements of sweat sodium and can help athletes avoid dehydration, overhydration, and electrolyte imbalance.

Sources

Baker LB. “Sweating rate and sweat sodium concentration in athletes: A review of methodology and intra/interindividual variability.” Discusses how methodology and context affect sweat rate and sweat sodium measurements. (PubMed)

Harris CT, Hunt L, Shepherd SO, Hew-Butler TD, Blow AV. “Comparison of pilocarpine- versus exercise-induced sweat sodium concentration across exercise intensities in trained athletes.” Physiological Reports (Jan 2026).

Jenkins EJ et al. “Delineating the impacts of air temperature and humidity for endurance exercise.” Reported sweat rate increase of about 0.04 L/h per 1°C warmer air during fixed-intensity exercise. (PubMed)

Périard JD et al. “Exercise under heat stress: thermoregulation, hydration, performance implications and mitigation strategies.” Comprehensive review of physiological responses and hydration issues during exercise in the heat. (PubMed)