Educational illustration of a trail runner linking heat, sweat and drinking during mountain exercise.

Trail hydration guide

Drink enough without overdrinking

Hydration is not a single ml/h number. It links what you lose, what your sweat contains, what you actually drink and what the course allows you to carry.

Scientific revision 1.0 · 31 July 2026

Understand before calculating

Four measurements answer four different questions

Mixing them creates a number that looks precise but is fragile in the field. Trailboard should keep them separate.

L/h

Sweat rate

Volume approximately lost in a specific context.

mg/L

Sweat sodium

Individual concentration, separate from sweat volume.

ml/h

Actual fluid intake

Amount consumed, which may differ from the plan.

mg/h

Actual sodium intake

Total from drinks, food and supplements actually consumed.

An estimated need is neither an instruction to drink mechanically nor the volume that must be carried from the start.

Trail runner surrounded by visual flows showing fluid entering, heat exposure and sweat leaving the body. — What goes in, what comes out and duration provide an estimate. No sensor turns it into a biological constant.

Experiment 1

Observe the flow instead of guessing a number

Weighing before and after a comparable run can approximate sweat rate. The result belongs to the test conditions: temperature, intensity, clothing, duration and altitude.

Practical estimate: Body-mass change + fluid consumed - any urine, divided by duration.
Compare like with like: Repeating across conditions is more useful than generalising one run.
Accept uncertainty: Wet clothing, scales, food and non-sweat losses limit precision.

Baker (2017) McDermott et al. (2017)

Transparent bottle separated into water, sodium particles and carbohydrate energy with three distinct flows. — An isotonic drink also contributes to the carbohydrate plan. Its composition and the amount actually consumed matter more than its marketing name.

Experiment 2

One bottle can serve three functions

A drink first provides fluid. It may also provide sodium and carbohydrates. Count all three contributions together without treating them as interchangeable.

Water: Volume contributes to hydration and depends on actual intake.
Sodium: A concentration in mg/L must be converted using the volume consumed.
Carbohydrates: They count towards g/h and change drink concentration.

Thomas, Erdman & Burke (2016) Tiller et al. (2019)

Two trail runners sweating a similar volume, one with few salt crystals and the other with many visible salt marks. — Volume lost × sodium concentration = estimated sodium loss. Without a reliable measurement, Trailboard keeps a range and states the assumption.

Experiment 3

Same sweat volume, different sodium loss

Two runners may lose the same amount of fluid while losing very different amounts of sodium. Sweat rate alone does not reveal its concentration.

Salt marks are a clue: They do not replace standardised sweat measurement.
Sodium is not a shield: It guarantees neither cramp prevention nor protection from hyponatraemia.
The total matters: Add drinks, salty foods, broth and supplements to avoid double-counting.

Baker (2017) Hew-Butler et al. (2015)

Illustrated mountain route with a start, carried flasks, aid stations and successive refills. — Total need, carrying capacity and refills are separate calculations. Missing aid-station data remains an unknown, not a reason to block the plan.

Experiment 4

Race need is not bag contents

The plan estimates intake for the whole race. The bag only needs enough to reach the next reliable water point, with a context-appropriate margin.

By segment: Distance, estimated time, heat and water access determine what to carry.
Verified aid station: Never assume that a point provides water, electrolytes or isotonic drink.
Fallback: Keep a reasonable margin without carrying the whole race requirement from the start.

Tiller et al. (2019) Racinais et al. (2023)

Adapt, do not copy

Heat changes the test, not just the slider

The same athlete can have several hydration profiles. A value tested in cool weather is not automatically valid for a hot race.

Unknown context

Sweat rate and sweat sodium are not measured.

Keep a cautious range and show assumptions.

Comparable session

Fluid intake, body-mass change and conditions are known.

Use the result as a local reference, then repeat it.

Heat or exposure

Thermal demand, water access and tolerance may change.

Retest progressively and recalculate carrying needs by segment.

Thirst remains a useful cue within an individual strategy. Forced drinking that produces body-mass gain during exercise signals excessive intake.

Racinais et al. (2023) McDermott et al. (2017)

Guardrails

Avoid both extremes

Insufficient hydration and excessive fluid intake can both impair performance and create risk. The plan seeks a testable balance, not full replacement of every loss.

Track actual intake, thirst, conditions, aid stations and body-mass change during comparable tests.

Do not mechanically drink beyond losses, treat sodium as insurance or improvise a large dose on race day.

Confusion, marked headache, repeated vomiting, faintness, unusual swelling or rapid deterioration during prolonged exercise require stopping and urgent medical assessment.

Hew-Butler et al. (2015) McDermott et al. (2017)

Scientific references

Guidance for individualisation, not a prescription

Consensus statements frame underhydration and overdrinking risk. Sweat research explains why one value cannot represent every athlete and condition.