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, mostly from drinking more than they lost.^{1, 2} Below: the thresholds, the weigh-yourself method, salt tablets, and the sachet sized for the long hot run. It is the runners' spoke of the Wellgard electrolytes guide.

At what distance do runners need electrolytes?

The forums have the threshold about right, nothing under about 10 km or 90 minutes unless it is hot, and work it out in training; they lack the arithmetic. 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, and heat multiplies everything: sweat rates in that dataset reached 5.7 litres an hour.¹ The "over 4k you need electrolytes" line has no arithmetic behind it: 4 km loses under half a gram.

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 accurate measure; forearm patches overestimate 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.³

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

Am I a salty sweater?

Probably, if your cap dries white, your kit shows tide marks and your eyes sting. 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.¹ It is a feature of your sweat glands, not a blood problem, and a salty sweater on a two-hour hot run can lose 3 to 4 g of sodium. A high-sodium sachet built for that run supplies around 2 g of sodium (5 g of salt), the sodium in about two litres of average sweat, usually with potassium and magnesium, and magnesium 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 Signs you might be low on electrolytes covers what a measurably low sodium looks like.

Can you take too many electrolytes on a run?

Yes, and the better-documented way is too much fluid: exercise-associated hyponatraemia is blood sodium diluted by drinking beyond what you lose. 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 sports physicians and physiologists meeting in California, advises drinking to thirst and treats over-drinking, not under-salting, as the cause to control.⁹ An electrolyte drink is not a licence to drink more: a sachet's sodium does not undo two litres you did not need, and nausea, headache, confusion or tight rings late in a long event mean stop drinking and seek help.

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

Salt tablets are sodium on its own, used by heavy sweaters in heat and in hot occupations. Side effects are nausea, stomach upset and thirst, and taken daily off the run they add to the NHS 6 g salt ceiling; across 195 trials, cutting sodium lowered systolic pressure by about 1 mmHg in normotension and 6 mmHg in hypertension.^{5, 6} They do not 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 on the blood-pressure medicines above, not during a stomach bug (a pharmacy rehydration sachet is the right product), and not to drink more than thirst asks. Sodium chloride tablets are also prescribed for some salt-losing conditions under blood-test monitoring; that is a prescriber's decision, not a running aid.

What is a good electrolyte drink for a long run?

The GB register of health claims authorises exactly one hydration wording, and it 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) may say it contributes to the maintenance of endurance performance during prolonged endurance exercise and enhances the absorption of water during exercise; sodium on its own has no authorised claim.¹² That is sugar plus salt in a dilute drink, and it is where "fuelling" and "electrolytes" get muddled: carbohydrate (gels, jelly babies) 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). The homemade version works: 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; a "pinch per pint" falls well below it.¹³ Creatine does a different job again and can go in the same bottle.

Frequently asked questions

Do runners actually need electrolytes?

On runs beyond about 90 minutes, in heat, or if you sweat salty, replacing some of the roughly 1 g of sodium an hour an average runner loses is sensible; under that, water to thirst and a meal do it.^{1, 3}

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

For runs that need them, during and after; a salted meal afterwards holds fluid better than plain water.^{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, add sodium, and treat heat-exhaustion signs (dizziness, headache, nausea, cramps, a fast pulse) that do not settle in 30 minutes as 111.^{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. PubMed 26070030.
2. Almond CS, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005. 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. PubMed 17277604.
4. Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 2017. PubMed 28332116.
5. NHS. Salt in your diet. 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. PubMed 33314019.
7. Sarnowski A, et al. Hyperkalemia in chronic kidney disease: links, risks and management. International Journal of Nephrology and Renovascular Disease, 2022. 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. 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. 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. 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. PubMed 18981039.
12. Great Britain nutrition and health claims (NHC) register. 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. PubMed 8897383.
14. NHS. Heat exhaustion and heatstroke. nhs.uk/conditions/heat-exhaustion-heatstroke.

This article is general information, not medical advice. If you have high blood pressure, heart failure or kidney disease, or take a diuretic or a blood-pressure medicine, ask a pharmacist before using a sodium-and-potassium sachet or salt tablets, and treat heat-exhaustion or low-sodium symptoms that do not settle as a 111 matter.