Short answer
For a run of an hour or two in mild conditions, a normal diet covers your sodium and you do not need electrolytes. They earn their place beyond roughly two hours, particularly in heat and particularly if you are a salty sweater, because that is when absolute losses become large enough to matter. Two things sodium does not do: it does not prevent exercise-associated muscle cramps, which the evidence links to neuromuscular fatigue rather than salt depletion, and it does not protect you from hyponatraemia, which is caused by drinking beyond thirst. Sodium’s honest job is replacing large losses on long hot days and making fluid palatable enough that you drink to thirst.
The sports nutrition aisle has convinced a lot of runners that every run needs electrolytes. The physiology is more specific than the marketing, and it is worth getting the specifics right, because the wrong belief here does not just waste money — it can push people into the one hydration error that has actually killed marathoners.
What you actually lose in sweat
Sweat is mostly water with sodium as its dominant electrolyte. Baker’s review of the measurement literature puts the population range of sweat sodium concentration at roughly 10 to 90 mmol/L — about 230 to 2,000 milligrams per litre — with most athletes somewhere near the middle. Potassium, magnesium and calcium are present in far smaller amounts, and losses of those through sweat are trivial next to what food supplies.
Two things about that range matter. First, it is wide: one runner loses four times the sodium of another from the same litre of sweat. Second, it is largely fixed. Your sweat rate moves day to day with dew point and intensity; your sweat sodium concentration is a trait, lowered a little by heat acclimation and otherwise stable. If you finish long runs with salt crust on your face and white rings on your cap, you are at the high end, and you will stay there.
| Sweat sodium | mg per litre | Lost in 2 hours at 1.2 L/hr | Read |
|---|---|---|---|
| Low (~20 mmol/L) | ~460mg | ~1,100mg | A normal meal replaces it. No supplement needed. |
| Typical (~45 mmol/L) | ~1,000mg | ~2,500mg | Matters past two hours in heat; food covers shorter runs. |
| High (~70 mmol/L) | ~1,600mg | ~3,900mg | Replace some during long hot efforts; expect the salt crust. |
The two jobs sodium actually does
Replacing large absolute losses. On a two-hour easy run in mild weather, a typical sweater loses a couple of grams of sodium and gets it back at lunch. On a four-hour marathon in heat, a salty sweater can lose six grams or more, and there is a case for replacing part of it during the event rather than all of it afterwards. That is the scenario supplements were designed for: long, hot, high-loss. Sports Dietitians Australia’s position statement on exercise in the heat lands in the same place — sodium matters when losses are large and events are long.
Making you drink to thirst. Sodium in a drink makes it more palatable and helps the fluid stay in circulation rather than passing straight through. That is a real and underrated benefit: on a hot day, a lightly salted drink helps you follow thirst, which is the strategy the evidence supports. It is not a licence to drink more than thirst asks for.
Sodium replaces what long, hot runs take out. It does not fix cramps, and it does not rescue you from a bottle you should not have finished.
What sodium does not do
It does not prevent cramps. The dehydration-and-electrolyte theory of exercise cramps is one of the most durable ideas in running, and it has not survived the evidence. Schwellnus’s work, and the studies that followed, found no difference in hydration or serum electrolytes between runners who cramped and those who did not; what predicted cramping was muscle fatigue — running faster than trained for, and running longer. Cramps cluster in the muscles doing the work, not across the body, which is what you would expect from a neuromuscular cause and not from a blood chemistry one. Salt tablets do not stop them. Pacing to your training does.
It does not protect against overdrinking. Exercise-associated hyponatraemia — blood sodium falling below 135 mmol/L — happens when runners drink more than they lose, diluting the blood. The Third International Consensus Conference on the condition was explicit: it is caused by excessive fluid intake, sports drinks are too dilute to prevent it, and sodium supplementation does not reliably protect a runner who keeps drinking beyond thirst. The defence is drinking to thirst, not adding salt to a schedule that was already too much.
When electrolytes earn their place
Under 90 minutes, any weather
Water to thirst. Your next meal replaces the sodium. A sports drink is fine if you want the carbohydrate; the electrolytes are incidental.
Two to three hours in mild conditions
Optional. A typical sweater is fine on food afterwards; a heavy, salty sweater may feel better with some sodium in the bottle, mainly because it helps them drink to thirst.
Past two hours in heat, or any ultra
Yes. Roughly 300–600mg of sodium per hour from drinks, gels or food is the range most guidance settles on for high-loss conditions, scaled up if you know you are a salty sweater. Still drink to thirst.
The days around a hot race
Salt your food normally in the loading days; there is no benefit to pre-loading beyond that for most runners, and it can raise blood pressure for those sensitive to it.
Notice that the deciding variable is duration and heat, not the fact of sweating. Every runner sweats. The question is whether the total loss is large enough that waiting for lunch is a mistake, and for most training runs it is not.
How to know your own number
You can have sweat sodium measured — sports science labs and some running stores offer patch tests — but the practical signs are usually enough. Visible salt on skin and kit after long runs, sweat that stings the eyes, and a strong craving for salty food after hot sessions all point to the high end. Then pair it with your sweat rate: the sweat rate calculator tells you the litres, and the table above turns litres into milligrams. A 1.5 L/hr salty sweater on a three-hour hot run is losing around seven grams of sodium, and that is a run where the bottle should have some in it.
For everything shorter, the honest advice is the boring one: drink to thirst, eat properly afterwards, and spend the money on carbohydrate, which is the thing that actually decides how the last hour of a long run goes.
Kovr logs what you drink next to what the run cost you.
Every session comes in from Apple Health with the temperature and dew point it happened in, so a hot three-hour run and a mild one stop looking the same. Kovr estimates the fluid the session cost, logs what you drank beside it, and flags the days where conditions pushed you somewhere unusual — the days sodium actually matters.
Download Kovr on the App StoreFree with a 14-day trial · Garmin, Apple Watch & Oura
Sources cited
- Baker, L.B. (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine — the 10–90 mmol/L range of sweat sodium and its stability within individuals
- Hew-Butler, T. et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine — overdrinking as the cause of hyponatraemia; sodium supplementation does not protect against it
- Schwellnus, M.P. (2009). Cause of exercise associated muscle cramps (EAMC) — altered neuromuscular control, dehydration or electrolyte depletion?. British Journal of Sports Medicine — the neuromuscular-fatigue model of cramping versus the dehydration-electrolyte model
- McCubbin, A.J. et al. (2020). Sports Dietitians Australia Position Statement: Nutrition for Exercise in Hot Environments. International Journal of Sport Nutrition and Exercise Metabolism — sodium replacement guidance for long, hot events
Frequently asked questions
Is it normal for HRV to drop during taper?
Yes, particularly in the first taper week. Heavy training defers repair, and reducing volume is when the body finally carries it out — muscle repair, glycogen restoration and plasma volume shifts are all metabolically demanding. The pattern typically recovers toward baseline during the second week.
Why do I feel terrible during taper when I'm running less?
Because the taper is when recovery gets paid for, not when it pays out. Heavy legs, sluggishness and phantom niggles are the repair process being felt from the inside. In one study of 11 runners, 5km performance was at its worst at the end of the heavy block and improved through the taper — fitness was already built, fatigue was still masking it.
When is a taper HRV drop actually a problem?
Three cases. If the trend is still falling five or six days out, the taper likely started too late or is too shallow. If HRV is down and resting heart rate is up with no session to explain it, treat it as a possible early illness signal. And if a niggle has a specific location and reproduces on a specific movement, it is an injury, not a taper phantom.
Should I do a hard session to check my fitness during taper?
No. It cannot answer the question — you cannot outrun taper fatigue in a week — and it re-adds the exact load you are trying to shed. Keep some short race-pace work to hold the neuromuscular pattern, but cut overall volume substantially.
Why did my weight go up during race week?
Glycogen and water. Every gram of stored glycogen brings water with it, so 1–2kg of gain across the final loading days is the carb load working rather than a nutrition error. Do not try to optimise HRV during a carb load.
How long does the taper dip last?
Most commonly the first week, with metrics climbing back toward baseline through the second week and sitting at or above baseline by race week. Individual variation is large, so the shape matters more than the schedule — what you are looking for is direction across several days, not any single morning.
Frequently asked questions
Do I need electrolytes for a one-hour run?
No. For runs under about 90 minutes in ordinary weather, water to thirst is enough and your next meal replaces the sodium. A sports drink is fine if you want the carbohydrate, but the electrolytes are incidental at that duration.
Do salt tablets stop running cramps?
The evidence says no. Studies comparing runners who cramped with those who did not found no difference in hydration or blood electrolytes; what predicted cramping was muscle fatigue from running faster or longer than trained for. Cramps cluster in the working muscles, which points to a neuromuscular cause. Pacing to your training is the fix.
How much sodium should I take during a marathon?
For efforts past two hours in heat, most guidance settles on roughly 300–600mg of sodium per hour from drinks, gels or food, scaled up for known salty sweaters. In mild conditions a typical sweater can rely on food afterwards. Either way, drink to thirst rather than to a schedule.
Can electrolytes prevent hyponatraemia?
No. Exercise-associated hyponatraemia is caused by drinking more fluid than you lose. Sports drinks are too dilute to prevent it and sodium supplementation does not reliably protect a runner who keeps drinking beyond thirst. Drinking to thirst is the defence.
How do I know if I am a salty sweater?
Visible white salt on your skin or kit after long runs, sweat that stings your eyes, and strong salt cravings after hot sessions all point to the high end of the range. Sweat sodium concentration is largely a fixed trait, so once you know, it stays known.
Is sweat sodium the same every day?
Roughly, yes. Unlike sweat rate, which moves with dew point and intensity, sweat sodium concentration is mostly a personal trait. Heat acclimation lowers it somewhat over one to two weeks, which is why adapted runners lose less salt per litre.