Electrolytes Explained — When You Actually Need Them
Electrolytes are the minerals that carry an electrical charge in the body — sodium, potassium, magnesium, calcium and chloride — and they are essential for fluid balance, nerve signals and muscle contraction. Sports-drink marketing has made them sound like something every exerciser needs to replace after every workout, and the shelves are full of tablets, powders and brightly coloured drinks promising to restore them.
The reality is narrower: most workouts do not need electrolyte replacement at all, and plain water is enough for anything under about an hour. This guide explains what electrolytes do, how much you actually lose in sweat, and when replacement genuinely matters — which is during long, hot, sweaty exercise, and in a few medical situations. For everyone else, the sports drink is mostly sugar and marketing.
Use the calculator
Start with the numbers: try our water intake calculator to work out your own figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculator uses your real weight, height, age and goal.
Related reading: water intake calculator, calorie calculator, How to Start Walking for Fitness, Multivitamins for Athletes, full guides section.
The formula behind electrolytes
The numbers start with sweat: an average person sweats 0.5–2 litres an hour during exercise, depending on intensity, temperature and fitness, and sweat contains about 0.9 g of sodium per litre (roughly 20–80 mmol/L, with wide individual variation — 'salty sweaters' lose far more). A two-hour run in the heat can therefore lose 2–4 litres of sweat and 2–4 g of sodium, which is a meaningful loss; a 30-minute gym session loses about 0.5 litres and 0.5 g of sodium, which a normal meal replaces easily. The daily needs are: sodium about 2.3 g a day (the recommended upper limit, with most adults eating 3–4 g), potassium about 3.5 g a day, magnesium about 400 mg a day for men and 310 mg for women, and calcium about 1,000 mg a day. The energy maths is background: the Mifflin-St Jeor equation (Mifflin et al., American Journal of Clinical Nutrition, 1990) — 10 × weight (kg) + 6.25 × height (cm) − 5 × age, plus 5 for men or minus 161 for women, times an activity factor — estimates maintenance calories, and a sports drink adds about 100–150 kcal per bottle that most exercisers do not need. The safety floors (1,200 kcal/day women, 1,500 kcal/day men) are about the whole diet, not the drink.
The key points
Sodium is the electrolyte you actually lose. Sweat is mostly water and sodium, with much smaller amounts of potassium, magnesium and calcium. That means sodium is the electrolyte that matters for replacement — and the amount lost depends on how much you sweat and how salty your sweat is, which varies enormously between people. A 'salty sweater' (white marks on clothing, stinging eyes) can lose twice as much sodium as someone else doing the same workout. Potassium, magnesium and calcium losses in sweat are small, and a normal diet replaces them easily — which is why most electrolyte products are mostly sodium, and why the other minerals on the label are largely marketing.
Under an hour, water is enough. For exercise lasting less than about 60 minutes, in normal conditions, plain water is all you need. The sodium and fluid lost in that time are small, and your next meal replaces the sodium. Sports drinks add sugar (about 25–35 g per bottle) and calories that most exercisers do not need, and they do not improve performance in short sessions. The exception is a very hot, very sweaty hour — a hard session in 35°C heat — where some sodium may help, but even then water plus a salty meal afterwards is usually enough.
Over two hours, or very hot, electrolytes matter. For exercise lasting more than two hours, or in very hot conditions, sodium replacement becomes important: the American College of Sports Medicine recommends 300–600 mg of sodium per hour during prolonged exercise, and up to 1,000 mg an hour for very heavy sweaters. The reason is hyponatraemia — low blood sodium from drinking too much plain water while losing sodium in sweat — which is rare but dangerous, and has killed marathon runners who drank excessively. The practical rule for long events: drink to thirst, take some sodium (a sports drink, salt capsules, or salty food), and do not force yourself to drink large volumes of plain water.
Potassium comes from food, not from a tablet. Potassium is abundant in food — bananas, potatoes, beans, leafy greens, yoghurt — and a normal diet provides 3–4 g a day, far more than is lost in sweat. Potassium supplements are rarely needed for exercisers, and they can be dangerous in excess (high blood potassium affects the heart), which is why over-the-counter potassium supplements are limited to small doses in many countries. Eat a banana and a potato and you have replaced a day's sweat losses several times over; a potassium tablet is solving a problem you do not have.
Magnesium and cramps — the evidence is weak. Magnesium is widely sold for muscle cramps, but the evidence that magnesium supplements prevent exercise-associated cramps is weak: a Cochrane review found little evidence of benefit in the general population. Exercise cramps are more often caused by fatigue, unfamiliar effort, or dehydration than by magnesium deficiency, and true magnesium deficiency is uncommon in people who eat adequately. If you are deficient (a blood test can tell), supplementation helps; if you are not, it does not. The food sources — nuts, seeds, whole grains, leafy greens — are the better first step.
Over-drinking is more dangerous than under-drinking. The most dangerous hydration mistake in endurance exercise is not drinking too little but drinking too much plain water, which dilutes blood sodium and causes exercise-associated hyponatraemia — confusion, swelling, seizures, and in rare cases death. The practical rule is to drink to thirst, not to a schedule, and to include sodium during long events. Weighing yourself before and after a long session tells you how much fluid you actually lost, and replacing about 1–1.5 litres per kilogram lost, over the following hours, is a sensible guide — with some salt in the food or drink.
At a glance
| Situation | Duration / conditions | What you need | Why |
|---|---|---|---|
| Gym session, normal temperature | Under 60 min | Water | Losses are small; food replaces sodium |
| Run or ride, mild weather | 60–90 min | Water, maybe a little salt after | Still small losses |
| Long run or ride | 2+ hours | Water + 300–600 mg sodium/hour | Sodium loss becomes meaningful |
| Very hot, heavy sweating | 1+ hour in heat | Water + sodium | High sweat rate, high sodium loss |
| Marathon / ultra / Ironman | 3+ hours | Water + sodium, drink to thirst | Hyponatraemia risk if plain water only |
| Hot yoga / sauna | 60–90 min | Water + salty meal after | Sweat loss high, but short duration |
| Daily life, no exercise | — | Normal food | Diet provides all electrolytes |
| Cramps | — | Check fatigue, hydration, fitness | Magnesium rarely the cause |
A worked example: a marathon versus a gym session
Take a 70 kg runner doing a marathon in warm weather, sweating about 1 litre an hour for four hours — 4 litres of sweat, containing about 0.9 g of sodium per litre, or about 3.6 g of sodium lost. Her normal daily sodium intake is about 3 g, so the run alone loses more than a day's worth. If she drinks 4 litres of plain water and replaces none of the sodium, her blood sodium falls — the mechanism of exercise-associated hyponatraemia. The correct approach: drink to thirst (about 0.5–0.8 litres an hour for most runners, not a litre or more), take 300–600 mg of sodium an hour (a sports drink, salt capsules, or salty food at aid stations), and eat a salty meal afterwards. Now compare a 70 kg lifter doing a 45-minute gym session: he sweats about 0.5 litres, losing about 0.5 g of sodium, and his next meal contains 1–2 g — the loss is replaced before he notices it, and a sports drink would add about 130 kcal of sugar he does not need. His maintenance calories by Mifflin-St Jeor are 10 × 70 + 6.25 × 178 − 5 × 30 + 5 = 700 + 1,112 − 150 + 5 = 1,667 kcal/day, times 1.55 for moderately active = about 2,584 kcal/day — and a daily sports drink would add about 900 kcal a week, or about 0.1 kg of fat, for no benefit. The difference between the two athletes is not the product; it is the duration and the sweat rate. Match the replacement to the loss, and most of the time the loss is too small to need replacing.
How to use this guide
For sessions under an hour, drink water and eat normally — that is the whole plan. For sessions over two hours, or in very hot conditions, plan sodium: 300–600 mg an hour, from a sports drink, salt capsules, or salty food, and drink to thirst rather than to a schedule. After a long, sweaty session, weigh yourself: replace about 1–1.5 litres of fluid per kilogram lost over the following hours, with some salt in the food or drink. Eat potassium-rich foods (bananas, potatoes, beans, leafy greens) as part of a normal diet, and do not take potassium supplements unless a clinician advises it. If you get cramps, look at fatigue, unfamiliar effort and hydration first — magnesium is rarely the cause, and a blood test settles the question if you suspect deficiency. And remember that sports drinks are mostly sugar water: if you drink one after a 30-minute workout, you have probably consumed more calories than the workout burned.
What this guide does NOT tell you
This guide covers healthy adults doing normal exercise; it does not cover medical conditions that affect fluid and electrolyte balance — kidney disease, heart failure, diabetes, adrenal disorders, and many medicines (diuretics, blood-pressure drugs, lithium) all change the answer, and anyone with those conditions should follow their clinician's advice, not a general guide. The sweat rates and sodium concentrations quoted here (0.5–2 litres an hour, about 0.9 g of sodium per litre) are population averages with wide individual variation — salty sweaters lose far more, and the only way to know your own rate is to weigh yourself before and after exercise. Exercise-associated hyponatraemia is rare but serious, and this guide's advice (drink to thirst, include sodium during long events) is general guidance, not medical advice for an individual event. If you experience confusion, severe headache, swelling, vomiting or seizures during or after prolonged exercise, seek emergency medical care. And the safety floors apply to the whole diet: never go below 1,200 kcal/day for women or 1,500 kcal/day for men without medical supervision.
Frequently asked questions
Do I need electrolytes after a workout?
Usually not. For exercise under about an hour, plain water and your next meal are enough — the sodium lost is small and food replaces it easily. Electrolytes matter for exercise lasting more than two hours, or in very hot conditions with heavy sweating, where sodium loss becomes meaningful. A sports drink after a 30-minute gym session mostly adds sugar and calories you do not need.
What is the most important electrolyte to replace?
Sodium. Sweat is mostly water and sodium, with much smaller amounts of potassium, magnesium and calcium. A two-hour run in the heat can lose 2–4 g of sodium, which matters; the potassium, magnesium and calcium lost in sweat are small and a normal diet replaces them easily. That is why effective electrolyte products are mostly sodium.
Can drinking too much water be dangerous?
Yes — during prolonged exercise, drinking large volumes of plain water while losing sodium in sweat dilutes blood sodium and causes exercise-associated hyponatraemia, which can cause confusion, swelling, seizures and, rarely, death. Drink to thirst during long events, include sodium, and do not force yourself to drink to a schedule.
Does magnesium prevent muscle cramps?
Probably not, in most cases. A Cochrane review found little evidence that magnesium supplements prevent exercise-associated cramps in the general population — cramps are more often caused by fatigue, unfamiliar effort or dehydration. If a blood test shows you are deficient, supplementation helps; if you are not deficient, it does not. Food sources (nuts, seeds, whole grains, leafy greens) are the better first step.
How much sodium should I take during a long event?
The American College of Sports Medicine recommends 300–600 mg of sodium per hour during prolonged exercise, and up to 1,000 mg an hour for very heavy sweaters. That can come from a sports drink, salt capsules, or salty food. Drink to thirst, and eat a salty meal afterwards to replace the rest.
Are sports drinks worth it?
For long, hot, sweaty exercise — yes, because they provide water, sodium and carbohydrate together. For a normal gym session or a run under an hour — no, because they add about 100–150 kcal of sugar that most exercisers do not need. Water plus a normal meal is the better choice for short sessions.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.