Replacing fluid after a long training day is mostly about sodium, not volume. Drink a litre of plain water after heavy sweating and much of it leaves again within the hour. Put the same fluid alongside a salted meal and it stays on board. Sweat rates vary enormously between people, so the plan that works is the one that replaces what you actually lost.
What sweat actually removes
Sweat is not just water. It carries sodium, a little potassium, and trace amounts of magnesium and calcium. Sodium is by far the largest of these, and it is the one that determines whether the fluid you drink stays with you. That is why the standard advice has shifted from "drink more" to "drink with some salt in it".
How much you lose is personal. Two runners on the same route on the same day can differ by a factor of three depending on body size, heat acclimatisation and genetics. Some people finish a session with visible salt marks on their kit; others never do. The salty sweater needs a genuinely different plan, and no generic guideline will tell them that.
Weigh yourself: the only measurement worth doing
The simplest reliable method is a before-and-after weigh-in. Weigh yourself before the session, train, towel off, weigh again. Each kilogram lost is roughly a litre of fluid. Do not drink during the session for a truer reading, or record what you drank and add it back.
Replace around 1.25 to 1.5 litres per kilogram lost, because losses continue for a few hours afterwards through urine and ongoing sweating. Spread that over two or three hours alongside food rather than drinking it in one go — a large volume at once is mostly excreted.
Why food beats a drink most of the time
For anything under about 90 minutes, a normal meal afterwards covers the job. The sodium in an ordinary plate of food, plus the fluid you drink with it, replaces what a moderate session removed. Electrolyte mixes earn their place in longer sessions, in real heat, or on back-to-back hard days — not as a daily habit after an hour in the gym.
Useful food-first options: a properly salted meal, potatoes for potassium, yoghurt or milk for fluid plus protein and minerals, nuts and seeds for magnesium, and a pinch of salt in the water bottle if you want the cheapest electrolyte drink available.
Long days, hot days and two-a-days
Heat changes the arithmetic. Sweat rates climb, sodium losses climb with them, and what you drink during the session becomes part of the plan rather than an afterthought. Sessions above roughly 30°C, or two hard days in a row, are where a deliberate sodium strategy starts paying for itself.
Back-to-back days have a specific problem: you start the second session before the first one's replacement has finished. If you finish long, replace poorly and train hard again the next morning, the second session happens at a deficit that has nothing to do with fitness. Start those days by drinking something with sodium before you train, not after.
Reading the signals
Urine colour is crude but serviceable day to day: pale straw is the target, consistently dark means you are starting behind, and very clear urine all day with a headache can point the other way — too much plain fluid with too little sodium. Thirst is a lagging signal; by the time you feel it during a long session you are already behind.
Two other markers are worth knowing. A session that felt far harder than the pace justifies is often a fluid problem rather than a fitness one. And a heart rate that drifts upward during a steady effort in the heat — cardiac drift — is partly a fluid and sodium issue.
What to carry, what to skip
Most people need very little: a bottle, a salty snack for long days, and a plan for the meal afterwards. Commercial mixes are a convenience, not a requirement, and the useful comparison between them is sodium content per serving rather than the number of minerals on the label. A mix with a meaningful amount of sodium beats a long list of trace ingredients in tiny quantities.
Skip the very sweet ones for long sessions if your stomach is sensitive — high concentrations of simple sugar slow gastric emptying precisely when you want fluid on board quickly.
Key takeaways
- Sodium, not volume, decides whether the fluid you drink stays with you.
- Weigh before and after; replace about 1.25 to 1.5 litres per kilogram lost.
- Spread replacement over two or three hours alongside food.
- A salted meal covers most sessions under 90 minutes.
- Mixes matter for long sessions, heat, and back-to-back hard days.
Frequently asked questions
Do I need an electrolyte drink after a normal one-hour session?
Usually not. A normally salted meal and the fluid you drink with it covers most sessions of that length. Mixes become relevant beyond about 90 minutes, in heat, or on consecutive hard days.
How can I tell whether I am a salty sweater?
White salt marks on kit, a salty taste on the lips and stinging eyes during a session are the three commonly used markers. If you have all three, plan for more sodium than average.
Can I drink too much water after training?
Yes. Large volumes of plain water after heavy sweating, without sodium, tends to leave you feeling worse rather than better. Spread intake and include food or salt.
Is coconut water enough on its own?
It is a good potassium source but comparatively low in sodium, so it is not a complete match for sweat losses. It works well as part of a meal, less well on its own after a long session.
Related supplements
If you want to explore this area, the Magnesium and Energy Support collections cover the relevant supplements and formats.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Talk to a healthcare professional before starting any new supplement, especially if you are pregnant, nursing, taking medication, or managing a health condition.