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Electrolyte Sweat Loss Guide

What sweat actually takes out and how to replace it before hyponatremia hits.

Per litre of sweat you lose ~800 mg Na, 200 mg K, 1000 mg Cl, 20 mg Mg — with 2-3× individual variation that matters for endurance events.

Plain-water hydration breaks down past 90 minutes of sustained work in heat. The reason is ion math: sweat carries roughly 800 mg sodium per litre (ACSM/Maughan reference values), and replacing fluid-only dilutes serum sodium toward the hyponatremic threshold. Heavy salt-sweaters lose 1500+ mg per litre and cramp faster. Heat and effort both raise sweat rate. The replacement rule: 125-150% of fluid loss, with ion matching once sessions exceed 90 minutes.

Part of: Bio-Chemical Logistics

Quick answer

Per litre of sweat you lose ~800 mg Na, 200 mg K, 1000 mg Cl, 20 mg Mg — with 2-3× individual variation that matters for endurance events.

What you are trying to do
What sweat actually takes out and how to replace it before hyponatremia hits.
Best next step
Ion Balance · Electrolyte Loss
Limit to remember
Treat this as a practical aid for the task, not a replacement for professional judgment.

Key points

  • ▸ Baseline sweat ion content per litre: Na+ ~800 mg · K+ ~200 mg · Cl- ~1000 mg · Mg2+ ~20 mg.
  • ▸ Individual variation 2-3× — salty sweaters can lose 1500+ mg Na+/L (visible salt streaks on clothing are the giveaway).
  • ▸ Sweat rate scales with heat and intensity: 0.5 L/h in cool conditions, 1.0-1.5 L/h in hot or hard training, 2-3 L/h for acclimatised athletes in extreme heat.
  • ▸ Fluid replacement target: 125-150% of sweat loss (over-replace because kidney excretion also runs during and after).
  • ▸ Under 90 minutes: plain water is fine for most people. Over 90 minutes or in heavy heat: electrolyte mix becomes essential to avoid hyponatremia.
  • ▸ Hyponatremia threshold: serum Na+ < 135 mmol/L. Symptoms start with confusion and nausea; severe cases seize and can fatally swell the brain.
  • ▸ Calibrate individual sweat rate by weighing before and after a training block (1 kg lost ≈ 1 L of sweat).

How to

  1. Weigh before and after a 60-min block in typical training conditions to calibrate sweat rate (1 kg ≈ 1 L).
  2. Enter duration, intensity, and sweat rate into the calculator.
  3. Read total fluid + ion losses (Na, K, Cl, Mg).
  4. For sessions under 90 min in mild conditions: plain water, replace 100% of fluid.
  5. For 90 min+ or hot conditions: replace 125-150% fluid and match Na+ loss with an electrolyte mix (300-800 mg Na+ per litre of replacement drink).
  6. For 4h+ events: salt capsules + electrolyte drink; target Na+ intake 500-1000 mg/h depending on sweat profile.

Examples

  • 2 h hard · 1.0 L/h sweat rate
    Fluid 2.0 L lost. Na+ 1600 mg · K+ 400 mg · Cl- 2000 mg · Mg2+ 40 mg. Replace with 2.5-3 L electrolyte mix at ~500 mg Na+/L.
  • 90 min moderate · cool · 0.5 L/h
    Fluid 0.75 L lost. Na+ 600 mg — plain water with post-workout meal is sufficient. Electrolyte mix is optional at this load.
  • 5 h ultra · salty sweater · 1.2 L/h · 1500 mg Na+/L
    Fluid 6 L · Na+ 9000 mg. Needs aggressive replacement: 800-1000 mg Na+/h via drink plus salt capsules. Plain water at this duration is a hyponatremia recipe.

When to use which tool

▸ Operational Thresholds
  • CYAN · STABLE — Under 90 min in mild conditions — plain water replacement is adequate; ion loss negligible.
  • GOLD · GUARDED — 90 min to 4 h or hot conditions — electrolyte mix at 300-800 mg Na+/L, 125-150% fluid replacement.
  • MAGENTA · CRITICAL — 4 h+ events or salty-sweater phenotype — salt capsules plus electrolyte drink, 500-1000 mg Na+/h, monitor for hyponatremia symptoms.
▸ Pivot
Electrolytes sized. Confirm total hydration holds through the session.
Hydraulic Integrity · Coolant Load →

Related

Frequently asked questions

› Is plain water enough for a 1-hour workout? Trust & accuracy

For most people in mild conditions, yes. Sub-90-minute sessions rarely deplete ion reserves to problematic levels, and a normal post-workout meal restores the small amounts lost. Ion-aware hydration matters above 90 min, in heavy heat, or for known salty sweaters.

› Are salt tablets necessary?

For 4-hour-plus endurance events or salty-sweater phenotypes, yes — drink-only replacement cannot deliver enough Na+ fast enough. For typical gym training or sub-2-hour runs, balanced food with electrolyte mix during the session is sufficient.

› What is hyponatremia and how do I avoid it? Definition

Hyponatremia is serum Na+ below 135 mmol/L — severe cases cause seizures and fatal cerebral edema. It happens when endurance athletes over-drink plain water during long events, diluting serum sodium. Avoidance: match fluid replacement to sweat loss rather than drinking on a fixed schedule, and use electrolyte mix over plain water beyond 90 minutes.

› How should I use this guide with a Kefiw tool? How-to

Use the guide as the plan and the linked Kefiw tool as the check. Read the steps first, try the move manually, then use the tool to compare outputs, catch edge cases, and decide whether the result actually fits your task.

› What mistake do tool guides help avoid? Troubleshooting

Tool guides help avoid using a utility mechanically without understanding what you are trying to accomplish. Most word, writing, and text utilities are fast, but speed can hide context mistakes. Know whether you are solving a puzzle, cleaning copy, drafting a line, or checking a rule.