Electrolytes for Athletes: Sweat, Sodium per Litre and the Trials

For most training sessions, water to thirst and your next meal replace what you sweat out. A measured sodium drink earns its place in the hours that cost salt: beyond about 90 minutes, in heat, or when you finish with salt on your kit. In 506 athletes, sweat carried about 0.8 g of sodium a litre at about 1.2 litres an hour, with a fourfold range between people, so the useful number on a label is sodium per litre as mixed, read against the NHS 6 g of salt a day.^{1, 2} This is the endurance sub-hub of the Wellgard electrolyte guide.

Evidence at a glance

• Studies: sweat datasets of 506 athletes and 1,944 tests; race cohorts of 488 and 669 runners; one 26-triathlete trial; a Cochrane review.
• Largest single study: 1,944 sweat tests from 1,304 people.³
• Strongest measured outcome: 13% of 488 Boston Marathon runners tested finished with blood sodium at or below 135 mmol/L; weight gain multiplied the odds by 4.2.⁴
• Not tested: a sugar-free electrolyte sachet in athletes with a performance outcome.

Sources checked on 17 October 2026.

How much sodium do athletes lose in sweat?

About 0.8 g a litre on average, and the average hides the useful part. In 506 team-sport and endurance athletes, whole-body sweat sodium averaged 36 mmol/L (0.8 g a litre) and ranged from 18 to 71 mmol/L (0.4 to 1.6 g); sweat rate averaged 1.2 litres an hour and ranged from 0.26 to 5.73.¹ An hour of hard training costs an average athlete about a gram of sodium, 2.5 g of salt; a salty, heavy sweater in heat loses several times that.

What makes sweat saltier? In 1,944 sweat tests from 1,304 people, warmer months (heat acclimatisation) went with lower sweat sodium, while cycling, being male and a higher sweat rate went with more; how much salt the person ate made no difference, and everything measured explained only 17 to 23% of the variation.³ The rest is you: salty-tasting sweat and white crusts on kit are the signs, and your own sweat rate comes from the weigh-yourself method on the runners' page.

What does "sodium per litre" mean on an electrolyte label?

The strength of the drink you swallow: sodium per tablet or sachet divided by the water the instructions call for, in milligrams per litre, which compares with sweat (400 to 1,600 mg/L) and with the 460 to 1,150 mg/L band the GB claims register sets for carbohydrate-electrolyte sports drinks.^{1, 5} The mixing volume is half the number.

Above the band is not a fault: the band defines a sugary sports drink, not a safety limit, and a sugar-free sachet sits outside it at any strength. A high-sodium sachet of about 2 g is concentrated on purpose, putting back about two litres of average sweat in one drink, and it counts for about 80% of the NHS 6 g of salt for the day. Are high-sodium electrolytes better? covers who that suits.

Why hydration and fuelling are two different jobs

Sodium does the salt job; carbohydrate does the energy job. The GB register authorises two wordings for carbohydrate-electrolyte solutions, that they "enhance the absorption of water during physical exercise" and "contribute to the maintenance of endurance performance during prolonged endurance exercise", only for drinks with 80 to 350 kcal a litre from carbohydrate, at least 75% of it from fast sugars, 20 to 50 mmol/L of sodium and an osmolality of 200 to 330 mOsm/kg.⁵ A sugar-free sachet supplies the sodium and minerals with no energy, so the carbohydrate comes from gels, food or a sugared drink: a 2014 review puts the need at up to about 60 g an hour for two to three hours and about 90 g an hour in ultra-endurance events.⁶ Sugar-free electrolytes vs sports drinks sets the two side by side.

Do you need electrolytes for a gym session, HYROX or a 5k?

Usually the meal afterwards does it. A 5k at average rates loses about half a gram of sodium, an hour in the gym about a gram, and a UK diet already runs at about 8.4 g of salt a day against the 6 g maximum.^{1, 2} A 2023 model found sodium needs are driven by how salty the sweat is and how much of the lost fluid is drunk back: for a football match or an elite marathon it found little need in any realistic scenario, while a 160 km ultramarathon with sweat at 40 mmol/L or more and most losses replaced needed about half the sodium put back.⁷ A HYROX or a long, hot gym session in a salty sweater sits between those cases; the scales say which side.

Cycling, triathlon and hot weather

Cycling went with higher sweat sodium in the 1,944-test analysis, so the per-litre arithmetic matters most on the bike.³ In the one randomised trial in a real race, 26 experienced triathletes took salt capsules carrying 113 mmol of sodium (about 2.6 g) or a cellulose placebo before and during a half-ironman: the salt group finished faster (P = 0.04), lost slightly less body mass and ended with higher blood sodium; funding is not stated in the abstract.⁸ One trial of 26 is a result, not a prescription. Heat multiplies everything: ultramarathon hyponatraemia rose in hot years, and the NHS lists cramps in the arms, legs and stomach among the signs of heat exhaustion, with 999 if someone is still unwell after 30 minutes of cooling and drinking.^{9, 10}

Do you need magnesium and potassium in a sports electrolyte?

They do their jobs at the label amounts, and sweat is not where you lose much of either. Where a serving carries 15% of the reference intake or more (56.25 mg of magnesium, 300 mg of potassium), magnesium contributes to electrolyte balance and potassium contributes to normal muscle function; sweat carries little of either beside its sodium.^{1, 5} On cramp, the cohorts are clear and the trials are missing: in 210 Ironman triathletes the 43 who cramped had the same blood electrolytes and weight change as the rest, and the 2020 Cochrane review of magnesium for cramps, 11 trials and 735 people, found no trial in exercise cramps.^{11, 12} Sodium for a heavy sweater stays sensible, with no promise that any mineral stops cramp.

Can athletes have too much? Over-drinking and hyponatraemia

Yes, and the documented route is water, not salt. At Boston in 2002, 13% of 488 runners tested finished with blood sodium at or below 135 mmol/L, weight gain multiplied the odds by 4.2 and the type of drink made no difference; in 669 ultramarathon finishers tested, 15.1% were hyponatraemic.^{4, 9} Drink to thirst; confusion, vomiting or a seizure after hours of heavy drinking is a 999 call. Dehydration vs hyponatraemia in long events separates the two with a comparison table.

Who should be careful with electrolytes?

Anyone whose sodium or potassium is already a medical matter. A high-sodium sachet of about 2 g matches about two litres of sweat and about 80% of the NHS 6 g of salt in one drink, so it is sized for the long, hot session, not the office day.^{1, 2} With high blood pressure, heart failure or kidney disease, or on an ACE inhibitor, an angiotensin receptor blocker, spironolactone or amiloride, where high blood potassium is common, ask a pharmacist or GP first; the blood-pressure trials are on the high-sodium page.¹³

What we don't know

• No trial has given a sugar-free electrolyte sachet to athletes and measured performance; the race trial used salt capsules.
• Four in five athletes in the 506-person sweat dataset were male; no trial has set sodium amounts for women.¹
• Everything measured in 1,944 sweat tests explained under a quarter of the variation in sweat sodium.³

Every guide in this series

Are high-sodium electrolytes better? (per litre and the performance trials); sugar-free electrolytes vs sports drinks (the two jobs); dehydration vs hyponatraemia in long events (safety). Also: electrolytes for runners, when to take electrolytes and how to read an electrolyte label.

Frequently asked questions

Do athletes need electrolytes every day?

On the days that cost salt; on rest days a UK diet already supplies about 8.4 g of salt against the NHS 6 g.² See do you need electrolytes every day?

How much sodium should an athlete have per hour?

What you lose: about 1 g an hour on average and 0.4 to 1.6 g a litre between people, so the figure that counts is your own from the scales.¹

Is salt the same as sodium on a label?

No. Salt is about 39% sodium, so salt divided by 2.5 is sodium; 6 g of salt is 2.4 g of sodium.²

Sources

1. Baker LB, et al. Observational, 506 athletes (404 male); sports-drink-maker research institute. J Sports Sci 2016. PubMed 26070030.
2. NHS. Salt in your diet. Reviewed 17 April 2023 (overdue); checked 17 October 2026. nhs.uk. UK 8.4 g: Public Health England, NDNS urinary sodium 2018 to 2019. gov.uk.
3. Baker LB, et al. Retrospective analysis, 1,944 sweat tests, 1,304 people; sports-drink-maker employees. J Appl Physiol 2022. PubMed 36227164.
4. Almond CS, et al. Cohort, 488 marathon runners tested; publicly funded. N Engl J Med 2005. PubMed 15829535.
5. Department of Health and Social Care. GB nutrition and health claims register; retained Regulation (EU) 432/2012 annex. gov.uk.
6. Jeukendrup A. Narrative review; author at a sports-drink-maker research institute. Sports Med 2014. PubMed 24791914.
7. McCubbin AJ. Modelling study; single academic author. Eur J Sport Sci 2023. PubMed 35616504.
8. Del Coso J, et al. Randomised controlled trial, 26 triathletes; funding not stated in the abstract. Scand J Med Sci Sports 2016. PubMed 25683094.
9. Hoffman MD, et al. Cohort, 887 ultramarathon finishers, 669 with blood sodium, 5 years; publicly funded. Med Sci Sports Exerc 2013. PubMed 23135369.
10. NHS. Heat exhaustion and heatstroke. Reviewed 28 May 2026; checked 17 October 2026. nhs.uk.
11. Schwellnus MP, et al. Prospective cohort, 210 triathletes, 43 with cramp; publicly funded. Br J Sports Med 2011. PubMed 21148567.
12. Garrison SR, et al. Cochrane review, 11 randomised trials, 735 people; one author ran an included trial. Cochrane Database Syst Rev 2020. PubMed 32956536.
13. Sarnowski A, et al. Narrative review; one author reports pharmaceutical grants and honoraria. Int J Nephrol Renovasc Dis 2022. PubMed 35942480.

General information, not medical advice. With high blood pressure, heart failure or kidney disease, or a medicine that affects sodium or potassium, ask a pharmacist or your GP before using any electrolyte product.

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