Electrolytes for Runners: When You Need Them and How Much

A sachet earns its place on a run that lasts beyond about 90 minutes, a run in heat, or for a runner who finishes with salt crusted on their kit; for a 5k or 10k in British weather, water to thirst and your next meal replace what you lost. The arithmetic: in 506 athletes, sweat carried about 0.8 g of sodium a litre (range 0.4 to 1.6 g) at an average 1.2 litres an hour, roughly 1 g of sodium an hour for an average runner. The documented marathon danger runs the other way: at Boston in 2002, 13% of finishers tested had low blood sodium, and gaining weight during the race, which means drinking more than was lost, was an independent risk factor.^{1, 2} It is the runners' spoke of the Wellgard electrolytes guide.

Sources checked on 30 October 2026.

At what distance do runners need electrolytes?

An hour's 10k at the average 1.2 litres an hour and 0.8 g a litre loses about 1 g of sodium, which is 2.5 g of salt, about two fifths of the NHS daily maximum and about what the next meal replaces.^{1, 3} A two-hour half marathon doubles that: cool, water to thirst and food afterwards; warm, or for a salty sweater, sodium in the bottle or a salted snack starts to matter. A four-hour marathon at average rates loses around 5 litres of sweat and 4 g of sodium; sweat rates in that dataset reached 5.7 litres an hour.¹

How much sodium do you lose when you run? The weigh-yourself method

The sports-medicine position stand that set the 2% rule says to weigh yourself before and after exercise, and a 2017 methods review calls body-mass change the simplest and most accurate measure and finds forearm sweat sodium about 25 to 100% higher than whole-body sodium.^{3, 4} Weigh yourself in minimal clothing before a timed run, towel off and weigh again, add what you drank and subtract any urine; a kilogram lost is a litre of sweat; divide by hours. Multiply litres by 0.8 g for an average sweater, 0.4 g if you never see salt on your kit and 1.6 g if you always do, and you have your sodium loss. Two targets: aim to finish no more than about 2% lighter (1.4 kg for a 70 kg runner), and never heavier.^{2, 3}

Averages; weighing yourself gives your own row, and heat or a heavy sweater can double every figure.¹

Am I a salty sweater?

Whole-body sweat sodium in those 506 athletes ranged from 18 to 71 mmol/L, which is 0.4 to 1.6 g a litre, so the saltiest lose four times the sodium per litre, and a salty sweater on a two-hour run at the average sweat rate loses close to 4 g.¹ A high-sodium sachet built for that run supplies around 2 g of sodium (5 g of salt), usually with potassium and magnesium, and where the magnesium reaches 15% of its reference intake (56.25 mg) it contributes to electrolyte balance; one such sachet is also about 80% of the NHS 6 g daily salt maximum, so it is sized for the long, hot or salt-crusted run rather than an ordinary day, and anyone with high blood pressure, heart failure or kidney disease, or on ramipril, losartan, spironolactone or amiloride, asks a pharmacist before any sodium-and-potassium sachet.^5-7 Sweat sodium trials: electrolytes for athletes.

Can you take too many electrolytes on a run?

Yes, and the better-documented way is too much fluid, which dilutes blood sodium (exercise-associated hyponatraemia). At Boston in 2002, 13% of 488 finishers tested had a sodium of 135 mmol/L or below and 0.6% a critical level; weight gain during the race multiplied the odds by about four, finishing in over four hours by about seven, and the type of drink made no difference.² In five years of a 161 km ultramarathon, 15% of finishers were hyponatraemic, more in hot years, a third of them dehydrated rather than over-hydrated, and the authors called excessive sodium supplementation and drinking beyond thirst ill advised.⁸ The 2015 international consensus statement, from a panel of 17 clinicians and scientists, advises drinking to thirst and treats over-drinking, not under-salting, as the cause to control; nausea, headache, confusion or tight rings late in a long event mean stop drinking and seek help.⁹ Dehydration vs hyponatraemia in long events sets out the risk.

What are salt tablets for, and when should you not take them?

Salt tablets are sodium on its own. Taken daily off the run they add to the NHS 6 g salt ceiling, and in the Cochrane review cutting sodium lowered systolic pressure by about 1 mmHg in white participants with normal blood pressure and about 6 mmHg in those with hypertension.^{5, 6} The cohorts did not find that they stop cramp. In a 161 km ultramarathon the 14% who cramped matched the rest on sodium supplement use and post-race blood sodium; cramp tracked a history of cramping and muscle damage, and a graded review found the altered-neuromuscular-control explanation better supported than lost salt or water.^{10, 11} Not with high blood pressure, heart failure or kidney disease, not during a stomach bug (a pharmacy rehydration sachet is the right product), and not to drink more than thirst asks.^{7, 9}

What is a good electrolyte drink for a long run?

The GB register's only water-absorption wording belongs to carbohydrate-electrolyte solutions: a drink with 80 to 350 kcal a litre from mostly high-glycaemic carbohydrate and 20 to 50 mmol/L of sodium (460 to 1,150 mg a litre) and an osmolality of 200 to 330 mOsm/kg may say it contributes to the maintenance of endurance performance during prolonged endurance exercise and enhances the absorption of water during physical exercise; sodium on its own has no authorised claim.¹² Carbohydrate does the energy job, sodium the salt job, and a sugar-free sachet supplies the sodium with no sugar, leaving the carbohydrate to gels or food (which drinks contain electrolytes has the per-100 ml figures). A homemade version sits inside that band, and the retention trial is small: in 12 men dehydrated by 2%, a drink with 61 mmol/L of sodium restored fluid balance where 23 mmol/L did not, and full rehydration needed about 150% of the volume lost; a quarter teaspoon of salt (1.5 g) in a litre of squash gives roughly 26 mmol/L, inside the band.¹³

What we don't know

• Whether sodium during a run prevents cramp or low blood sodium: in the 161 km cohorts, cramp did not differ by sodium supplement use, and low blood sodium tracked fluid and heat more than salt.^{8, 10}
• How well the averages fit you: the 506-athlete sweat sodium figures were predicted from forearm patches by an equation, not measured over the whole body.^{1, 4}

Frequently asked questions

Do runners actually need electrolytes?

Beyond about 90 minutes, in heat, or if you sweat salty; under that, water to thirst and a meal replace the roughly 1 g of sodium an hour.^{1, 3}

Should I take electrolytes before, during or after a run?

For runs that need them, during and after; in the 12-man trial the saltier drink left half the urine of the weaker one, so the sodium belongs with the meal afterwards.^{3, 13}

Is it OK to take salt tablets every day?

Only on days you have sweated the salt out; on rest days they add to the 6 g the NHS treats as a ceiling, and with high blood pressure, heart failure or kidney disease the answer belongs to a pharmacist.^{5, 6}

What changes when it is hot?

Sweat rate, which can reach several litres an hour, and with it sodium loss and the over-drinking risk: ultramarathon hyponatraemia rose in hot years. Drink to thirst and add sodium; heat-exhaustion signs (tiredness, dizziness, headache, nausea, cramps) mean cooling and fluids, and still being unwell after 30 minutes of rest and cooling is a 999 call.^{1, 8, 14}

Sources

1. Baker LB, et al. Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences, 2016. Retrospective analysis, 506 athletes; authors employed by a sports-drink company's research institute. PubMed 26070030.
2. Almond CS, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005. Cohort study, 488 finishers with blood samples; US National Institutes of Health funding. PubMed 15829535.
3. American College of Sports Medicine; Sawka MN, et al. Position stand: exercise and fluid replacement. Medicine and Science in Sports and Exercise, 2007. Position stand of a professional society. PubMed 17277604.
4. Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 2017. Methods review in a supplement supported by a sports-drink company's research institute. PubMed 28332116.
5. NHS. Salt in your diet. Page last reviewed 17 April 2023 (review overdue). nhs.uk/live-well/eat-well/food-types/salt-in-your-diet.
6. Graudal NA, Hubeck-Graudal T, Jurgens G. Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride. Cochrane Database of Systematic Reviews, 2020. Systematic review of randomised trials, 195 included references; authors at public institutions, no salt-industry funding. PubMed 33314019.
7. Sarnowski A, et al. Hyperkalemia in chronic kidney disease: links, risks and management. International Journal of Nephrology and Renovascular Disease, 2022. Narrative review; one author declares pharmaceutical grants and honoraria. PubMed 35942480.
8. Hoffman MD, et al. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine and Science in Sports and Exercise, 2013. Pooled cohort, 887 finishers over five years, sodium measured in 669; non-government funding listed on PubMed. PubMed 23135369.
9. Hew-Butler T, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. British Journal of Sports Medicine, 2015. Consensus statement, 17-member expert panel. PubMed 26227507.
10. Hoffman MD, Stuempfle KJ. Muscle cramping during a 161-km ultramarathon: comparison of characteristics of those with and without cramping. Sports Medicine Open, 2015. Observational study, 280 runners surveyed; no competing interests declared. PubMed 26284165.
11. Schwellnus MP. Cause of exercise associated muscle cramps (EAMC): altered neuromuscular control, dehydration or electrolyte depletion? British Journal of Sports Medicine, 2009. Narrative review graded by evidence level; non-government funding listed on PubMed. PubMed 18981039.
12. Great Britain nutrition and health claims (NHC) register. Updated 19 May 2026. gov.uk/government/publications/great-britain-nutrition-and-health-claims-nhc-register.
13. Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ. Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Medicine and Science in Sports and Exercise, 1996. Randomised trial, 12 men in two groups of six; funding not stated on PubMed. PubMed 8897383.
14. NHS. Heat exhaustion and heatstroke. Page last reviewed 28 May 2026. nhs.uk/conditions/heat-exhaustion-heatstroke.

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This article is general information, not medical advice. With high blood pressure, heart failure or kidney disease, or on a diuretic or blood-pressure medicine, ask a pharmacist before using a sodium-and-potassium sachet or salt tablets.