Most hydration advice stops at "drink more water". But water is only half the story. The fluid you lose through sweat is not pure water — it carries dissolved electrolytes: primarily sodium, plus potassium and a little magnesium. Replace the water but not the electrolytes, and you can still end up flat, crampy and foggy.
This matters most for the people who sweat the most: athletes, anyone active in the heat, sauna-goers, fasters, and people on low-carb or ketogenic diets. For them, "just drink water" is incomplete — and sometimes counterproductive.
What Electrolytes Actually Do
Electrolytes are minerals that carry an electrical charge in body fluids. They run some of the body's most basic machinery:
- ⚡ Sodium — governs fluid balance and is the main driver of how much water your body holds. It's the electrolyte you lose most in sweat.
- 🍌 Potassium — works with sodium to control nerve signals and muscle contraction, including your heartbeat.
- 🌙 Magnesium — involved in hundreds of enzymatic reactions, muscle relaxation and sleep quality.
When these run low after heavy sweating, the symptoms are familiar: muscle cramps, fatigue, headache, lightheadedness and poor concentration — even when you've been drinking plenty of water.
What You Lose in Sweat
Sweat composition varies between people (some are "salty sweaters"), but the typical ranges are well documented:
| Electrolyte | Typical loss per liter of sweat | Notes |
|---|---|---|
| Sodium (Na) | 460–1840 mg (≈1000 mg avg) | By far the largest loss |
| Potassium (K) | ≈200 mg | Smaller but meaningful over long sessions |
| Magnesium (Mg) | ≈10 mg | Small; matters more for daily baseline |
An hour of intense exercise in the heat can cost 0.5 to 2 liters of sweat. At the higher end, that's well over a gram of sodium gone in a single session — on top of whatever you lose through a normal hot day.
How Much Do You Actually Need?
A sensible target is a baseline plus a sweat bonus. For the baseline, the European Food Safety Authority (EFSA) publishes adequate-intake reference values for adults:
| Electrolyte | EFSA baseline (adults) | Source |
|---|---|---|
| Sodium | ≈2000 mg/day (safe & adequate) | EFSA 2019 |
| Potassium | 3500 mg/day | EFSA 2016 |
| Magnesium | 350 mg/day (men) · 300 mg (women) | EFSA 2015 |
On top of the baseline, add roughly what you sweated out: about 1000 mg sodium, 200 mg potassium and 10 mg magnesium per liter of sweat. So a hot training day can easily push your sodium need up by a gram or more compared to a rest day.
🥑 Keto, Low-Carb and the "Keto Flu"
If you eat low-carb or ketogenic, electrolytes deserve extra attention. Lower carbohydrate intake means lower insulin, and lower insulin tells the kidneys to excrete more sodium and water. That's a major reason behind the headaches, fatigue and cramps of the first low-carb weeks — the so-called "keto flu".
The fix usually isn't more water — it's adequate sodium (within safe limits) plus potassium and magnesium. This is exactly the crowd that existing sodium trackers ignore, because those apps are built to help people cut salt for blood pressure, not to help active or low-carb people replace it.
How to Replace Electrolytes
- 🧂 Salt your food — the simplest sodium source. A pinch of table salt is roughly 580 mg sodium.
- 🥥 Coconut water — naturally high in potassium.
- 🥤 Electrolyte sachets / sports drinks — convenient and easy to dose, especially around training.
- 🍌 Potassium-rich foods — bananas, potatoes, leafy greens, beans.
- 💧 Mineral water — some brands carry meaningful magnesium and sodium; check the label (per liter).
Where Brim Comes In
The hard part of electrolytes isn't drinking the sachet — it's knowing how much you actually need today. That number changes with the weather and your training, and guessing it from generic tables is tedious.
Brim already does the hard math for hydration: it calculates how much you've sweated today from your workouts (Apple Health) and the heat (Apple WeatherKit). The new Electrolytes module reuses exactly that signal — it adds the corresponding sodium, potassium and magnesium losses on top of the EFSA baseline, so your target rises automatically on a hot or active day, and explains why.
⚡ Brim estimates your electrolyte need from your sweat
On a hot training day, Brim shows a higher sodium, potassium and magnesium target — derived from the same workout and weather data that drives your water goal. Log your sources in a tap, sync with Apple Health, and see exactly where you stand. Electrolytes are a Brim Pro feature.
Free on the App StoreFrequently Asked Questions
Does plain water contain electrolytes?
Pure or filtered water has almost none. Tap and mineral water contain some — mostly magnesium and calcium, with variable sodium and very little potassium. For replacing sweat losses (sodium and potassium), drinking water contributes little, so you generally need food, salt, electrolyte mixes — or, for mineral-rich water, check the label.
How much electrolytes do you lose when sweating?
Per liter of sweat: roughly 460–1840 mg sodium (≈1000 mg average), about 200 mg potassium and ≈10 mg magnesium. An hour of hard exercise in the heat can cost 0.5–2 liters of sweat — so 1–2 g of sodium in a single session is common for athletes.
Why do you need more electrolytes on keto or low-carb?
Low-carb lowers insulin, which makes the kidneys excrete more sodium and water — a key driver of "keto flu". People on low-carb diets often need more sodium, within safe limits, plus enough potassium and magnesium, to feel well.
What are the adequate daily intakes of sodium, potassium and magnesium?
EFSA sets a safe and adequate sodium intake of ≈2000 mg/day for adults, adequate potassium of 3500 mg/day, and adequate magnesium of ≈350 mg/day (men) and 300 mg/day (women). Sweat losses add to this on hot or active days.
How does Brim estimate my electrolyte need?
Brim calculates how much you've sweated today from your workouts (Apple Health) and the heat (Apple WeatherKit), then adds the matching sodium, potassium and magnesium on top of the EFSA baseline. Brim is not a medical app and does not replace medical advice.