Prepare for 90 Minute Races: Hydration for Athletes Using Wearables

Most athletes do best by a simple rule: stay euhydrated day to day, drink to thirst during typical sessions, and only build a personalised plan when the event is long, hot, or unusually sweaty. Exceptions matter: endurance efforts over 90 minutes, high heat, and heavy sweaters need scheduled fluid and sodium intake worked out in advance. Get this balance right and you protect both performance and recovery.

TL;DR:

• Athletes should build personalized hydration plans based on sweat rate testing rather than relying on fixed formulas or drink-to-thirst methods.
• Electrolytes are generally unnecessary for activities under 90 minutes in mild conditions but become important during longer or hotter sessions with substantial sodium losses.
• Overhydration leading to weight gain during exercise indicates a risk of hyponatraemia, while losing more than 2% of body mass signals dehydration and performance decline.
• Tracking hydration using wearable devices and food logs helps optimize fluid and sodium intake, especially in high-heat or extended effort scenarios.
• Rehydration should include 100 to 150% of fluid losses paired with carbohydrate and sodium to accelerate glycogen resynthesis and ensure next-day recovery.

Table of Contents

• Your pre, during and post-exercise hydration timetable
• When electrolytes matter and when water is enough
• How to measure your sweat rate and build a personal plan
• Practical tips to drink more during training and racing
• The real risks: overdrinking, hyponatraemia and heat illness
• Hydration’s role in glycogen recovery and next-day readiness
• How wearables and Heala fit into a hydration plan
• What matters most, in six points
• A practical way to apply this guide
• Sources
• FAQ

Your pre, during and post-exercise hydration timetable

Hydration works best as a timetable, not a single habit. The NATA position statement lays out a sequence that suits most training sessions and competitions, with room to adjust for sweat rate and how easily you can drink during the activity itself.

1. Drink around 500 to 600 millilitres two to three hours before exercise, then another 200 to 300 millilitres 10 to 20 minutes before you start.
2. During exercise, aim for 200 to 300 millilitres every 10 to 20 minutes where access allows, adjusting upward if you sweat heavily or the conditions are hot.
3. After finishing, replace 100 to 150% of your fluid losses if you need to recover quickly for a second session, and pair that fluid with carbohydrate and sodium.

Sports with limited access to fluids, think football, martial arts, or point-to-point races, need this plan built around natural breaks rather than a fixed clock. The goal stays the same: never leave the recovery window until you’re already several hours behind.

When electrolytes matter and when water is enough

Plain water covers most sessions. The ACSM’s hydration guidance notes that sessions under 60 to 90 minutes in mild conditions generally don’t need electrolyte replacement, since a normal diet keeps sodium topped up.

Heavy sweaters can lose a substantial amount of sodium per hour, with some exceptional cases losing very high amounts, according to the ACSM. That range explains why one athlete finishes a long run fine on water while another cramps badly on the same route.

Electrolytes become useful once you cross into longer or hotter territory:

• Sessions lasting more than 90 to 120 minutes, particularly in heat or humidity.
• Multi-day tournaments or training camps where sweat losses accumulate.
• Any activity where you notice salt crusting on your skin or clothing, a sign of a high sweat sodium concentration.

Consuming roughly 500 milligrams of sodium about 90 minutes before exercising in heat can help your body hold onto fluid, per the ACSM. In practice, that can mean dissolving a salt tablet in water beforehand, since dissolving it improves tolerability compared with swallowing it whole, or simply eating something salty. A pinch of table salt, a handful of pretzels, or a commercial electrolyte drink all work depending on what sits well with your stomach.

How to measure your sweat rate and build a personal plan

Fixed formulas rarely fit every athlete, which is why sweat-rate testing has become the standard recommended by NATA. The test itself is straightforward and something you can run in normal training.

1. Weigh yourself nude immediately before a standardised session, then again immediately after.
2. Record how much fluid you drank during the session and account for any urination.
3. Calculate your sweat rate in litres per hour: bodyweight lost, plus fluid drunk, minus urine output, divided by session length.
4. Repeat the test across different intensities and conditions, then average the results rather than relying on one session.

Pro Tip: If your sweat rate comes out at 1.0 litre per hour but you’re only managing 0.4 litres per hour during the session, plan to carry extra fluid or schedule additional stops rather than hoping thirst catches you up.

Averaging across sessions matters because a single hot day or an unusually intense workout can skew the number. Building a plan from several data points, alongside metrics like heart rate or training load from a resting heart rate check, gives a more reliable picture of what you actually need on race day.

Practical tips to drink more during training and racing

Getting the numbers right on paper is one thing. Actually drinking enough during a session is another, and small tweaks make a real difference to how much you take on voluntarily.

• Chilled fluids, roughly 10 to 15°C, and flavoured drinks encourage more voluntary sipping than warm, plain water.
• Mark bottles in 100 millilitre increments so you can track intake at a glance without stopping to think.
• Smaller containers, drunk more often, tend to improve compliance compared with one large bottle you keep putting off.
• For events with limited access, plan fluid stops around natural breaks and treat carbohydrate intake as part of the same routine on anything over 90 minutes.

None of this needs to be complicated. It just needs to be planned before you’re thirsty, tired, and losing focus mid-session.

The real risks: overdrinking, hyponatraemia and heat illness

Drinking too much carries its own dangers. Forced drinking beyond thirst raises the risk of exercise-associated hyponatraemia, a dangerous drop in blood sodium that can cause confusion even when body temperature is normal, according to the CDC Yellow Book.

Losing more than 2% of body mass during exercise is the threshold NATA flags as a point where performance and safety both start to suffer. Gaining weight during a session is the opposite warning sign, and it usually means you’ve overdrunk relative to your sweat rate.

• Watch for heat exhaustion signs: heavy sweating, weakness, nausea, and a rapid pulse.
• Move to shade or air conditioning, remove excess clothing, and start cooling immediately.
• Rehydrate with water plus salt or a commercial rehydration solution, as recommended in the CDC Yellow Book.
• Call emergency services if confusion, vomiting, or a very high body temperature develops.

Combined hyperthermia and dehydration during prolonged exercise in heat also raise biomarkers linked to acute kidney injury, and limiting either factor reduces that biomarker response, per CDC research. Staying cool matters just as much as staying hydrated.

Hydration’s role in glycogen recovery and next-day readiness

Rehydration isn’t just about how you feel in the next hour. Dehydration and hyperthermia can impair glycogen resynthesis in the short term, according to a systematic review in Nutrients, which means a poorly rehydrated athlete restocks fuel stores more slowly.

Pairing fluid with carbohydrate and sodium speeds that process up. Milk, a carbohydrate-electrolyte drink, or a carbohydrate meal alongside something salty all work as practical recovery pairings, echoing the point made in a piece on under-recovering. On multi-session days, treat the meal after training one as preparation for training two, not an afterthought.

How wearables and Heala fit into a hydration plan

Personalised hydration plans depend on consistent tracking, which is where wearable data earns its place. Heala pulls in metrics from devices like Fitbit and WHOOP alongside logged hydration and meals, then flags trends rather than leaving you to spot them yourself.

• Automated alerts highlight when hydration logs trend low relative to training load.
• Photo meal logging captures sodium intake from meals without manual entry, similar to the approach described in Heala vs WHOOP.
• Reminders tied to scheduled sessions prompt pre-exercise drinking at the right time, not after the fact.

The app supports tracking and behaviour change. It isn’t a substitute for a clinician if you’re dealing with cramping, fainting, or symptoms of heat illness.

What matters most, in six points

Hydration works best as a habit built into training, not a formula applied once on race day. Get the daily basics right and the harder decisions around heat, distance, and sodium become far easier to plan for.

• Start every session euhydrated rather than trying to catch up mid-workout.
• Test your sweat rate across a few sessions and average the results.
• Build a specific plan for heat, altitude, or events over 90 minutes.
• Add sodium through food or drinks once sessions run long or sweat losses are heavy.
• Avoid overdrinking: weight gain during exercise is a warning sign, not a good one.
• Use weigh-ins and urine colour as simple, free checks on your hydration status.

— Kerem

A practical way to apply this guide

Heala centralises the pieces this guide asks you to track: wearable data, hydration logs, and meal photos that capture sodium intake without manual entry. Adaptive plans adjust as your training load and sweat-rate data change, so the personalisation NATA recommends doesn’t rest entirely on memory or guesswork.

The free tier covers the basics, while Pro unlocks deeper analytics and unlimited history for athletes who want to track sweat rate and recovery trends over full seasons. Heala supports the tracking and behaviour side of hydration. It doesn’t replace a clinician’s advice if you’re managing a heat illness episode or a diagnosed condition affecting fluid balance.

This article is general information, not a substitute for advice from a qualified doctor. Consult a qualified healthcare professional about your own circumstances before acting on anything here.

Sources

• Fluid replacement for athletes — NATA position statement (2025)
• 9 facts about hydration & electrolytes — ACSM
• Heat and cold illness in travelers — CDC Yellow Book

FAQ

What is the best hydration for athletes?

Plain water is generally sufficient for sessions under 60 to 90 minutes in mild conditions, according to the ACSM. Longer, hotter, or heavy-sweat sessions benefit from electrolyte-containing fluids, since sodium losses can range widely between individuals.

What is the 4-2-1 rule for athletes?

The 4-2-1 rule is an older fixed-volume hydration formula, but expert consensus now favours personalised, sweat-rate-based plans or a pragmatic drink-to-thirst approach for most athletes, as discussed in expert commentary on hydration strategies. Rigid formulas tend to be less reliable than testing your own sweat rate across a few sessions.

What are the warning signs of dehydration?

Signs include thirst, dark urine, fatigue, dizziness, and a rapid pulse, with heat exhaustion adding heavy sweating and nausea, per the CDC Yellow Book. Losing more than 2% of body mass during exercise is a threshold worth avoiding, flagged in the NATA position statement.

How should athletes rehydrate after exercise?

Replace 100 to 150% of fluid losses when rapid recovery is needed, pairing fluids with carbohydrate and sodium, according to NATA guidance. Practical options include milk, a carbohydrate-electrolyte drink, or a meal alongside something salty.

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