Mavr Sports Nutrition Team
November 6, 2025. 8 min read.
Hydration for endurance athletes means balancing 400-800ml fluid with 400-800mg sodium per hour, customizing to sweat rate and conditions, and using Mavr to automate calculations around your training calendar.
You can train perfectly, fuel correctly, and still fall apart on race day — all because of hydration.
Most endurance athletes focus on carbs, but hydration and electrolytes are the foundation that keeps your engine running. Get them wrong, and no amount of gels will save you.
Let's break down the real science of hydration for endurance athletes, how electrolytes work, and how to avoid the mistakes that wreck performance.
Even a 2% drop in body weight from fluid loss can decrease performance, increase heart rate, and raise your perceived effort. By 4–5%, you're flirting with dizziness, cramps, and heat exhaustion.
Hydration isn't just about drinking more — it's about balancing water with electrolytes, especially sodium.
Electrolytes are minerals that conduct electrical signals in your body. They control nerve function, muscle contraction, and fluid balance.
Sodium (Na+)
Maintains fluid balance and muscle function
Typical Loss per Liter of Sweat400–1,500 mg
Potassium (K+)
Muscle and nerve function
Typical Loss per Liter of Sweat150–250 mg
Magnesium (Mg2+)
Energy metabolism, prevents fatigue
Typical Loss per Liter of Sweat10–20 mg
| Feature | Role | Typical Loss per Liter of Sweat |
|---|---|---|
| Sodium (Na+) | Maintains fluid balance and muscle function | 400–1,500 mg |
| Potassium (K+) | Muscle and nerve function | 150–250 mg |
| Magnesium (Mg2+) | Energy metabolism, prevents fatigue | 10–20 mg |
Sodium is the MVP here. It helps your body retain fluid and transport glucose into muscles during long efforts. If you drink much more than you sweat, especially plain water, you dilute your blood sodium. That is hyponatremia, a dangerous condition that can cause confusion, nausea, or worse. Drink to thirst and never force fluids.
Hydration isn't one-size-fits-all. Your needs depend on body size, weather, and sweat rate.
A simple rule: If your body weight drops more than 2% after training, you're underhydrating. Weigh yourself before and after long workouts to get your personal sweat rate.
If two or more of these show up — it's time to fix your hydration plan.
You don't need to guzzle water all morning. Instead, sip steadily and include electrolytes.
Pro tip: Avoid starting overhydrated — clear urine before a race is not the goal. Pale yellow means perfect balance.
Your aim during training is to replace what you lose, not to overcompensate.
<1 hour
FluidOptional
SodiumMinimal
NotesSip if needed
1–2 hours
Fluid400–600 ml/hr
Sodium400–600 mg/hr
NotesCombine with carbs
2–4 hours
Fluid600–800 ml/hr
Sodium600–800 mg/hr
NotesAlternate gels + electrolyte drink
4+ hours
Fluid700–900 ml/hr
Sodium800–1,000 mg/hr
NotesAdd solids or salt tabs
| Duration | Fluid | Sodium | Notes |
|---|---|---|---|
| <1 hour | Optional | Minimal | Sip if needed |
| 1–2 hours | 400–600 ml/hr | 400–600 mg/hr | Combine with carbs |
| 2–4 hours | 600–800 ml/hr | 600–800 mg/hr | Alternate gels + electrolyte drink |
| 4+ hours | 700–900 ml/hr | 800–1,000 mg/hr | Add solids or salt tabs |
If you train in hot or humid conditions, you may need 20–30% more.
After training, replace 150% of the fluid you lost within a few hours. If you lost 1 lb (0.45 kg), drink about 700 ml (24 oz) to restore balance.
Pair this with electrolytes + carbs to enhance glycogen replenishment. Skip the plain water — your kidneys will just flush it out.
The single highest-value action in this entire guide is to actually measure your own sweat rate rather than copying the ranges above. Pick a representative hour-long session, weigh yourself naked before and after, track any fluid you drank during it, and the math is simple: every kilogram lost (plus whatever you drank) is roughly a liter of sweat per hour. Do this in cool and hot conditions and you will have two numbers that beat any generic chart. Pair that with a rough read on whether you are a salty sweater — gritty white marks on your kit and cap are the tell — and you have a hydration profile built around your body, not an average one.
Carrying that sweat-rate number into a per-session fluid and sodium target, and scaling it up automatically for long or hot days on your calendar, is the part Mavr handles so you are not redoing the arithmetic each week. But the test itself — and the ranges in this guide — are yours to use today with nothing more than a bathroom scale.
Aim for 35–45 ml per kg of body weight daily, plus what you lose in workouts.
If you sweat heavily, get salty skin or clothes, or cramp often — you need more sodium.
Yes. Mix water, fruit juice, a pinch of salt (~400 mg sodium), and a bit of sugar for quick DIY hydration.
Look for a mix of carbs (6–8%), sodium (500–800 mg/L), and other electrolytes like potassium and magnesium.
Moderate caffeine (1–2 cups of coffee) doesn't dehydrate you — but excess can increase fluid loss slightly.
General guidance for healthy adults, not medical advice. Talk to a doctor or registered dietitian if you have a health condition, take medicines or have special dietary needs. Caffeine advice is for healthy adults only. EFSA's safe limit is 200 mg in one dose and 400 mg in a day.
Ask Mavr: "How should I hydrate for my next long workout?" then use the guidance below as a starting point.
Ask Kai, the coach in the app