Electrolytes for Runners: Sodium, Sweat, and Strategy

This page is for runners who have finished a long run feeling wrecked in a way fitness does not explain — the headache, the salt crust on the shirt, the second-litre-of-water thirst that never resolves. Electrolytes for runners is really a story about one mineral, one variable (your sweat), and one threshold (about an hour). Get those three right and the product choice becomes the least important part. Get them wrong and no powder fixes it.

What do you actually lose in sweat?

Sweat is mostly water with a meaningful amount of sodium and a supporting cast in tiny quantities. The ranges below come from the sweat-composition literature — and the width of the sodium range is itself the lesson.

Per litre of sweat	Typical range	What it means for you
Water	1 litre (by definition)	Sweat rates run roughly 0.5–2.0 L per hour
Sodium	~460–1,840 mg (20–80 mmol/L)	The mineral that decides your hydration plan
Potassium	~100–300 mg	Real but small; a banana and normal meals cover it
Magnesium & calcium	A few mg each	Not a during-run concern; diet covers them

Whole-body sweat sodium concentration averages around 40–50 mmol/L but ranges from about 20 to 80 mmol/L between runners — a fourfold difference between two equally fit people at the same pace. A runner losing 1.5 L an hour at the salty end sheds close to 2.7 g of sodium an hour; a light sweater at the other end loses a quarter of that. Any dosing advice that ignores this spread — “one tablet per run” — is marketing, not physiology. The deep-dive maths lives on our sodium for runners page.

When does water stop being enough?

The sports-medicine consensus is consistent on the shape of the answer. For exercise under about an hour, the American College of Sports Medicine finds little difference between water and electrolyte drinks — your previous meal still has you covered. Past that point, three things change: total sodium loss becomes large enough to matter, thirst alone under-drives drinking for many people, and the risk profile flips from “slightly dehydrated” toward cramps, collapse of pace, and — with aggressive plain-water drinking over very long events — hyponatraemia.

Our when do you need electrolytes page works through the full decision table (duration, heat, sweat rate, session type). The short version: under 60 minutes, water; 60–90 minutes, water plus food sodium for most, electrolytes if it is hot or you are salty; beyond 90 minutes, electrolytes are part of the plan, and beyond two hours they share the plan with carbohydrate. Long-run specifics — marathon and ultra fuelling — are on the electrolytes for long runs page.

How much sodium should you take, and when?

During the run: the ACSM position stand recommends beverages containing 0.5–0.7 g of sodium per litre for exercise lasting more than an hour. That is a deliberately moderate target — below average sweat concentration — because you are topping up, not performing a chemistry transplant. Salty sweaters and very long efforts justify going above it toward 1 g per litre or more, ideally based on a sweat test rather than a guess.

Around the run: aim to start hydrated (pale-yellow urine, not clear), and afterwards drink roughly 1.25–1.5 litres for every kilogram of body weight lost, with sodium in the drink or the meal — plain water alone after a big loss mostly becomes urine. Keep body-mass loss during the run under about 2%; beyond that, performance and thermoregulation measurably suffer.

Finding your numbers: weigh yourself nude before and after a one-hour run, add what you drank, subtract any bathroom break. Weight lost in kilograms ≈ sweat lost in litres (1 kg ≈ 1 L). Repeat in race-like conditions and you own the only sweat-rate number that matters — yours.

Products are just delivery formats for that plan: sticks and tablets for convenience, capsules for runners whose stomachs reject sweet drinks, and salty food either side of the run. If cost or taste puts you off the commercial options, the homemade electrolyte drink ratios land in the same ranges — and if you train low-carb, read the honest trade-offs on sugar-free electrolytes before assuming zero-sugar is automatically better on a three-hour run.

What does a race-day plan look like, hour by hour?

Theory is tidy; a worked example is more useful. Take a 70 kg runner, marathon day, forecast warm, sweat-tested at 1.3 L per hour with moderately salty sweat (about 50 mmol/L, so roughly 1.5 g of sodium lost per hour at full sweat rate):

• Breakfast, 2–3 hours out: normal carbohydrate-rich meal, salted normally, with 500–750 ml of fluid. Some runners use a stronger electrolyte drink (around 1–1.5 g sodium per litre) in the final hours to arrive with fluid on board; salty sweaters benefit most from this.
• During, each hour: about 0.6–0.8 L of fluid — enough to finish roughly 1–1.4 kg down, within the 2% guidance — as a drink carrying 0.7–1 g of sodium per litre, plus gels or chews bringing carbohydrate to 45–60 g per hour. Sodium intake lands around 0.5–0.8 g per hour: roughly half of losses, replaced in real time, with the balance made up afterwards. That fraction is normal; full in-race replacement is neither achievable nor necessary for most marathoners.
• First two hours after: 1.25–1.5 L per kilogram lost, taken gradually with sodium — a recovery drink, then a properly salted meal. Weighing yourself at home closes the loop: if you finished 2 kg down, the target is roughly 2.5–3 L across the afternoon, not a single heroic litre at the finish line.
• That evening: normal eating, normally salted, plus drinking to thirst. Most runners are back to baseline by the next morning; lingering headache or unusual fatigue usually traces to an under-replaced total, not a mysterious deficit.

Now the same day for a light sweater (0.7 L/hour, 25 mmol/L): a standard sports drink at 0.5 g/L, drunk to thirst, with gels, covers nearly everything, and the recovery column needs no special attention. The plan is a formula, not a script — plug in your own two numbers and it rewrites itself. If your numbers come out extreme in either direction, that is your signal to rehearse in training rather than discover it at mile 22.

What about potassium, magnesium, and the rest of the label?

New runners often ask why these pages keep returning to sodium when product labels list five electrolytes in a friendly row. The answer is loss size, and it is not close. Potassium leaves in sweat at perhaps a tenth of sodium’s rate, and the body’s potassium stores are large and well defended — a day of normal eating replaces a long run’s losses without trying. Magnesium and calcium losses in sweat are measured in single milligrams per litre against daily needs in the hundreds of milligrams; no study of runners has ever found a during-run magnesium emergency in a fed athlete. Chloride follows sodium automatically (it is the other half of salt). So when a label advertises its five-electrolyte completeness, read it as decoration: the formulation question that decides whether the product works is always the same one — how much sodium per litre, and will you actually drink that litre? That is also why cramp research keeps disappointing people looking for a potassium or magnesium fix: for the classic late-run cramp, the levers with evidence behind them are sodium, pacing, and conditioning, in that order of convenience. Feed normally, salt deliberately, and let the supporting cast look after themselves.

What changes for two-a-day training and stage racing?

One scenario quietly breaks the standard advice: the second session. Runners in a marathon build with morning and evening sessions, and multi-day stage racers, do not get the luxury of “recover by tomorrow.” Between sessions there may be only six to eight hours, and the recovery column of this page becomes the performance column of the next start line. The protocol tightens: weigh out of session one, replace 125–150% of the loss with sodium included before session two, eat the carbohydrate the second session will need, and treat the between-session window as a scheduled task rather than grazing. Stage racers add one more habit — a deliberately salted evening meal and a strong electrolyte drink before bed on the biggest days, because overnight is the only long repair window the race gives you. None of this requires new products; it requires the same products, treated as non-negotiable appointments.

Who should not follow standard electrolyte advice?

• People with high blood pressure, heart failure, or kidney disease. Standard runner dosing deliberately adds sodium — potentially several grams on a long-run day. If you have been told to restrict sodium, your running plan needs your clinician’s input, not a blog’s.
• Anyone pregnant. Hydration and sodium needs shift in pregnancy and heat tolerance drops; long efforts in heat deserve a conversation with your care team first.
• Under-18 runners. Kids and teens thermoregulate differently and should be on water-and-food plans supervised by adults who know the signs of heat illness — not adult dosing schedules.
• People on diuretics, lithium, or blood-pressure medication. These medicines interact with fluid and sodium balance directly. Ask your doctor or pharmacist how event-day sodium fits your prescription before race day, not at the start line.
• Anyone with symptoms of hyponatraemia — confusion, bloating, vomiting, a headache that worsens while drinking — needs medical help, not more fluid of any kind.