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Electrolyte Loss in Sweat

Sodium, potassium, chloride, and magnesium — how much each litre of sweat costs you.

Every litre of sweat carries about 800 mg sodium and a smaller dose of three other ions — replace by volume and composition, not just water.

Sweat is mostly water, but the small ion load matters out of proportion to its volume. Sodium drives nerve conduction and fluid balance; potassium handles membrane potential; chloride shadows sodium. Over a long event the ion loss compounds into real performance and safety consequences. The numbers below come from ACSM and Maughan consensus data.

Quick answer

Every litre of sweat carries about 800 mg sodium and a smaller dose of three other ions — replace by volume and composition, not just water.

What you are trying to do
Sodium, potassium, chloride, and magnesium — how much each litre of sweat costs you.
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

  • ▸ Per litre of sweat (average): 800 mg sodium, 200 mg potassium, 1000 mg chloride, 20 mg magnesium.
  • ▸ Salty sweater variant: sodium up to 1500-2000 mg/L. Tell-tale: visible white salt rings on clothing after training.
  • ▸ Sweat rate: 0.5-2.5 L/h depending on fitness, heat, and acclimatisation. Measure by weighing before and after a session.
  • ▸ Plain water replacement past 90 minutes in heat risks exercise-associated hyponatremia — low serum sodium.
  • ▸ Replacement target: 125-150% of fluid lost, sodium-matched to concentration, over the hours following the session.

Examples

  • 2 hours hard · 1.0 L/h sweat rate
    Fluid lost ~2 L · sodium 1600 mg · chloride 2000 mg. Replace with an electrolyte drink at roughly 500-700 mg Na per litre.
  • Marathon in 80°F
    Sweat 1.3 L/h × 4 h = 5.2 L · sodium 4160 mg. Without salt tablets or sports drink, hyponatremia risk is real.
  • Heavy lifter in a hot gym
    60 min × 0.8 L/h = 0.8 L · sodium 640 mg. Below the threshold where replacement beyond post-session food matters.

When to use which tool

Related

Frequently asked questions

› How do I measure my actual sweat rate? How-to

Weigh naked before a session, weigh again after towelling dry. Account for fluid drunk during the session. (weight_before − weight_after + drunk_volume) / duration = sweat rate.

› Does sweat composition change with acclimatisation?

Yes. Heat-acclimated individuals produce higher sweat volumes but lower sodium concentration — the body conserves salt under repeated heat exposure. 10-14 days of heat training is enough to shift the balance.

› 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.

› Can a tool guide help me learn the skill? How-to

A tool guide can help you learn if you pause before accepting the output and ask why it worked. Compare your first guess with the tool result, look for the rule or pattern, and repeat that review. Passive copying solves one task; active review builds the skill.