How to hydrate during training: the NATA position, no myths

Lose just 2% of your body weight to sweat and you're already slower, even if you subjectively feel fine. That's exactly why the question of "how much and when to drink" is worth settling in advance rather than on the run. The updated position of the U.S. National Athletic Trainers' Association (NATA, 2017) comes down to a single idea: there is no universal requirement. Everyone loses fluid differently, and your drinking strategy has to be tailored to you. Let's look at which numbers to rely on and how to calculate your personal "sweat loss."

Why 2% is the red line

Normally the body keeps water within a narrow band: from +1% (a slight surplus) to −3% (a slight deficit). But losing more than 2% of body weight to sweat consistently lowers endurance — even if you drink to thirst, you won't squeeze your maximum out of yourself.

What happens inside during dehydration:

Heart rate climbs by roughly 3–5 beats per minute for every 1% of body weight lost.
Core temperature rises by roughly 0,15–0,20 °C for every 1% of weight lost.
Thermoregulation suffers once the deficit exceeds 1%, and the risk of heatstroke rises noticeably once you lose more than 3%.

Put simply, a dehydrated body runs at higher revs: the heart works harder and cooling gets tougher. The goal is to keep losses within 2%.

Calculate your sweat rate

The main practical tool in the document is weighing yourself before and after exercise. The NATA formula:

Sweat loss (l) = weight before (kg) − weight after (kg) + fluid consumed (l) − urine volume (l)

And the rate: sweat loss ÷ training time (h).

How to do it right:

Weigh yourself before training with no clothes (or minimal, dry ones), and towel off before weighing afterward.
Count how much you drank during the session.
Train in typical conditions (same weather, same intensity).

Example: before — 70,0 kg, after — 69,0 kg, drank 0,5 l in an hour. Sweat loss = 70,0 − 69,0 + 0,5 = 1,5 l/h. Now you know roughly how much to replace.

Important: sweat rate varies widely among adults — from 0,5 to 1,8 l/h — and sodium concentration in sweat ranges from 10 to 100 mEq/l. That's why other people's "norms" don't work; only your own measurements do.

Drink to thirst or by plan?

NATA offers two approaches, and both are valid — it depends on the goal:

You don't know your sweat rate or you're training for fun → drink to thirst. This is a safe strategy that protects against the main mistake — overdrinking.
You're chasing a result and you know your rate → drink by plan, replacing losses without gaining weight.

The key safety rule: you must not gain weight during exercise. Coming in heavier at the finish means you drank more than you lost — and that's the road to dangerous hyponatremia (EAH), when blood sodium drops below 135 mmol/l. In marathoners and long-race participants it's recorded in 10–20% of cases. One of the most notorious cases was the death of a footballer who drank about 4 gallons (roughly 15 l) of water and sports drinks trying to relieve cramps.

How to put it into practice

A scheme by duration:

Up to 1 hour: water is usually enough, drink to thirst.
1–3 hours: aim for about 200 ml every 15–20 minutes, a drink with carbohydrates (ideally 3–8%) and sodium.
Longer than 3 hours / ultra: calculate by your own sweat rate, weigh yourself periodically, don't gain weight. If your sweat is very salty (sodium above 60 mEq/l) and your sweat rate is above 2,5 l/h, add sodium.

Before the start: arrive already hydrated (in a state of normal hydration). An ordinary amateur doesn't need to "stock up" on water separately.

Afterward: replace up to 150% of the deficit in the first hours (up to 4 h) and eat within 2 hours to restore fluid, electrolytes, and carbohydrates.

Common mistakes:

Drinking extra "just in case" — a hyponatremia risk.
Weighing yourself in wet clothes — the number lies.
Relying on salt tablets "for prevention" — there's little evidence of benefit beyond your personal losses.
Judging hydration by a single sign. Three at once are more reliable: the color of your morning urine, thirst, and weight.

The bottom line

Keep fluid losses within 2% of body weight — beyond that, endurance drops and the load on the heart grows.
Every 1% of deficit = +3–5 beats of heart rate and +0,15–0,20 °C of core temperature.
Learn your sweat rate by weighing before/after: loss = weight before − weight after + what you drank.
If you don't know your rate or you train for yourself — drink to thirst; for a result — by plan.
Never gain weight on the course: it's a hyponatremia risk.
After exercise, replace up to 150% of the deficit and eat within 2 hours.

Source: NATA — Fluid Replacement for the Physically Active. https://www.nata.org/sites/default/files/2025-08/fluid_replacement_for_the_physically_active.pdf