Electrolytes and sodium for runners: what's being used and what the research says
We report what runners are using and what the research says. We don't recommend taking any supplement. Talk to your GP or a registered sports dietitian before trying one.
Quick answer
Research suggests extra sodium beyond food and drink is generally not needed, even in ultramarathons[1]. Sweat sodium losses vary widely between athletes[2], and drinks with electrolytes and carbohydrate can help more than water alone in some situations[3]. Salt hasn't been shown to prevent cramps[4], and low blood sodium in races mostly comes from drinking beyond thirst[5].
- Research
- Emerging (proposed)Can help replace sweat losses in long, hot events. No good evidence that extra sodium prevents cramps or low blood sodium.
- Proposed by our editor; not yet confirmed by an expert reviewer.
- WADA status
- Permitted
- Distances in the research
- 800 m–5K: not studied10K–half: not studiedMarathon: limited researchUltra: studied
What are electrolytes and sodium?
Electrolytes are minerals dissolved in body fluids. The NHS says sodium and chloride, the two parts of salt, help keep the body's fluid levels balanced, and potassium helps control fluid balance and helps the heart muscle work properly. Magnesium helps turn food into energy[6].
Sweat carries water and electrolytes out of the body, chiefly sodium[2]. Sweat starts out with about the same sodium concentration as blood plasma, around 135–145 mmol/L, but by the time it reaches the skin it typically holds about 10 to 90 mmol/L. Sweat potassium, about 2–8 mmol/L, is similar to blood levels[2].
Runners get electrolytes from sports drinks[3], salt capsules or tablets[1][7], and electrolyte powders, tablets or ready-made drinks, which the Australian Institute of Sport classes as sports foods[8]. Commercial sports drinks hold about 460–575 mg of sodium per litre, less than is typically lost in sweat[9].
The NHS says around three quarters of the salt we eat comes from packaged and everyday foods such as bread, breakfast cereals and ready meals[10], and that potassium is found in most foods[6]. In a 161 km race, two-thirds of runners' sodium, on average, came from food and drink rather than capsules[4].
Also called: salt tablets, salt capsules, electrolyte drinks.
Why are electrolytes getting attention?
Salt tablets are widely used: in a 2024 survey of 5,668 Chinese marathon runners, 54% of those who used supplements said they used salt tablets, men slightly more often (55.3%) than women (51.3%)[7]. At the Western States 161 km race, 93.9% of finishers studied used sodium supplements[11].
Many ultrarunners believe sodium prevents cramps and low blood sodium: in a survey of 1,152, two-thirds (66.4%) said sodium supplements should be available at races, mainly to prevent low blood sodium (65.5%) and cramping (59.1%). Only 12.1% of those against said sodium could raise thirst and cause overdrinking[1].
Low blood sodium also gets attention because it can kill: the deaths of two women charity marathon runners in 2003 brought it to wide notice[5], and a young man died from it after finishing the London Marathon[12]. Yet only 32% of 210 marathoners surveyed could name a symptom[13].
The claims, checked against the research
Fewer crampsNot supported
Cramping was unrelated to sodium intake in a 161 km race[4], and to blood electrolytes and weight change in 210 Ironman triathletes[14]. In marathon and ultra-trail studies, runners who cramped were no more dehydrated or low in sodium, but had higher muscle-damage markers[15][16].
Protection from low blood sodiumMixed
In a trial in 13 women drinking 1 L an hour for 4 hours, mild low blood sodium affected 46% on a high-sodium drink and 92% on water[17]. But at Western States, supplement sodium was only weakly linked with post-race blood sodium and didn't differ between finishers with low and normal levels[11]. The 2015 consensus, restated in the 2017 review, says sodium can soften the fall in blood sodium but can't prevent low blood sodium when people keep drinking too much[18][5].
Faster race timesMixed
In one randomised trial of 26 triathletes, those given salt capsules finished a half-ironman faster than a placebo group[19]. But in 266 ultrarunners, sodium intake had no significant link with pace or finish time[20], and 13 women running for 4 hours performed no differently on water or sodium drinks[17].
Replacing sodium lost in sweatSupported
Sweat removes sodium[2], and sodium drinks limited the fall in blood sodium compared with water in two controlled trials[21][17]. Salt supplements were linked to slightly less weight loss in one triathlon study[22], with a non-significant trend in another[19].
Coping better in the heatMixed
A high-sodium drink before exercise extended time to exhaustion in 13 trained women cycling at 32 °C[23]. But in a 161 km race with a measured on-course peak of 39 °C, sodium intake was unrelated to low blood sodium, and runners taking the least sodium lost no more than 4–5% of body weight[4].
Magnesium stops crampsNot studied
A Cochrane review found no randomised trials of magnesium for exercise-associated cramps. In older adults with ordinary cramps, magnesium was unlikely to give meaningful protection[24].
How do electrolytes work?
Sweat glands reabsorb sodium as sweat travels up the duct, so sweat reaching the skin is more dilute than blood. At higher sweat rates, secretion outpaces reabsorption, so sweat sodium rises as sweating speeds up. Heat acclimation and dietary sodium can change sweat sodium through the hormone aldosterone[2].
Because sweat is more dilute than blood, sweating on its own tends to raise blood sodium[5]. Blood sodium falls when the body holds too much water for its sodium; below 135 mmol/L during or after exercise, this is called exercise-associated hyponatraemia[5][9]. The usual cause is drinking beyond thirst while the hormone vasopressin, released by exercise stress, nausea, heat and some medicines, makes the kidneys hold on to water[5].
An analysis of 2,135 endurance athletes found blood sodium fell as body weight rose, and pointed to three mechanisms: drinking to excess, failing to switch off vasopressin, and failing to release sodium from internal stores[25]. Sodium loss plays a part in fewer cases[5], though in 161 km races in northern California, runners with low blood sodium were more often dehydrated than overhydrated[26].
Two theories compete to explain exercise-associated muscle cramps: dehydration and electrolyte loss, or altered nerve control of the muscle. A 2009 review found the first rests mainly on anecdote, case series totalling 18 cases and one small case-control study, and four prospective cohort studies don't support it[27]. In a lab study, pickle juice shortened induced cramps with little change in blood electrolytes, suggesting a nerve reflex[28].
Do runners need electrolyte supplements?
Research suggests extra sodium beyond food and drink is generally not needed, and sodium supplements haven't been shown to prevent cramps or low blood sodium[1][4][11]. One triathlon trial found salt capsules helped[19], but a study of 266 ultrarunners found no link with performance[20]. Drinks with electrolytes and carbohydrate can help more than water alone in some situations[3]; the clearest danger in long races is drinking too much[5].
800 m to 5K (Not studied)
We found no studies of electrolyte supplements in races of these lengths. The research is on long events, where sweat losses are more likely to upset water and electrolyte balance[2]. A review and a position stand describe drinking to thirst as possibly enough for exercise under about 60 to 90 minutes, in cool conditions or at low intensity[29][9].
10K to half marathon (Not studied)
We found no trials of electrolyte supplements at these distances. Sweat rates and sweat sodium vary so much between athletes that losses over an hour or two can be small for one person and large for another[2]. In sweat tests on 1,303 athletes, the 255 endurance athletes (runners, cyclists and triathletes) averaged 1.28 L of sweat and 51.7 mmol (about 1.2 g) of sodium an hour, with a wide spread[30].
Low blood sodium has also been measured in mixed groups of marathon and half-marathon runners: in one study, 5% of 161 tested before the race and 8% of 192 tested after it had low blood sodium without symptoms[5].
Marathon (Limited research)
Studies at big-city marathons have found low blood sodium in about 1 in 8 finishers tested: 13% of 488 in the 2002 Boston Marathon, 0.6% at critical levels[31], and 12.5% of 88 London Marathon volunteers, none with symptoms[12]. In Boston, weight gain, a finish over 4 hours and body-mass extremes were linked to it; the type of drink wasn't[31].
In a survey of 210 marathon runners, fewer than 5% named drinking to thirst, the guidance of the 2015 consensus on low blood sodium, and slower runners tended to plan to drink more often, though the difference was not statistically significant[13]. Drinking beyond thirst is the most common cause of low blood sodium[5].
In one study, the 20 of 88 finishers who cramped were no more dehydrated and had similar blood sodium and potassium, but higher muscle-damage markers[15].
Ultra (Studied)
In a 161 km race where the researchers measured an on-course peak of 39 °C (nearby weather stations peaked at 31.7 °C), cramping, dehydration, low blood sodium and nausea were unrelated to how much sodium runners took in. Runners who took in more sodium lost less weight, but the authors judged the losses in low-sodium runners, up to 4 to 5%, acceptable[4].
Across 5 years of 161 km races in northern California, 15.1% of 669 finishers tested had low blood sodium, from 4.6% to 51.0% by year, and more in hotter years[26]. At Western States, supplement sodium didn't differ between finishers with low and normal blood sodium, and overhydration was the main feature of those affected[11].
Among 266 runners in 80 km stages of multi-stage ultramarathons, average sodium intake ran from about 2 g in the lowest-intake third to 7.2 g in the highest, and intake was not significantly linked with pace or finish time[20]. In a different long event, a half-ironman triathlon, the group given salt capsules in a randomised trial finished faster and had higher blood sodium than the control group[19].
Study by study
| Study | Who took part | What they took | What was measured | What they found |
|---|---|---|---|---|
| Almond 2005 [31]Observational study | 488 runners in the 2002 Boston Marathon | Observational: pre-race survey, weights, fluid questionnaire and finish-line blood test | Blood sodium and its predictors | 13% had low blood sodium, 0.6% critical. Linked to weight gain, times over 4 h and BMI extremes, not to sex, drink type or NSAIDs |
| Kipps 2011 [12]Observational study | 88 London Marathon volunteers | Observational: weights and blood tests before and after the race | Blood sodium, fluid intake, weight change | 12.5% had low blood sodium, none with symptoms; they drank more and gained more weight, though 4 lost weight |
| Hoffman 2013 [26]Observational study | 669 finishers of 161 km races over 5 years | Observational: weights before and after, blood sodium after | Low blood sodium, hydration, finish time | 15.1% had low blood sodium (4.6–51.0% by year), more in hotter years and more often with dehydration |
| Hoffman 2015 [4]Observational study | 20 Western States 161 km finishers, 2014 | Observational: full records of food, drink and capsules during the race | Cramping, nausea, weight change, blood sodium | Sodium intake 2.5–38 g. Cramping or near-cramping (39%), nausea or vomiting (57%) and low blood sodium (10%) unrelated to intake |
| Hoffman 2015 [11]Observational study | Western States 161 km finishers with survey and blood data | Observational: drinking and supplement questionnaire, weights, blood sodium after the race | Blood sodium v supplement sodium intake | 6.6% had low blood sodium; their supplement sodium intake didn't differ from that of finishers with normal levels |
| Lipman 2021 [20]Observational study | 266 runners enrolled (161–217 analysed) in 80 km stages of multi-stage ultramarathons | Observational: sodium intake questionnaire, weights and finish-line blood sodium | Pace, race time, ranking | No significant link with sodium intake (group averages 2–7.2 g); dehydrated runners were faster than overhydrated ones |
| Del 2016 [19]Randomised controlled trial | 26 experienced triathletes | RCT: salt capsules (113 mmol sodium) or placebo capsules before and during a half-ironman | Race time, strength, jump, weight, blood sodium | Salt group faster (P = 0.04) with higher blood sodium; smaller weight loss not significant; strength and jump similar |
| Twerenbold 2003 [17]Randomised controlled trial | 13 female endurance athletes | Randomised crossover: 4-hour runs drinking 1 L an hour of water or drinks with 410 or 680 mg/L sodium | Blood sodium, performance | Mild low blood sodium in 92% (water), 69% (low sodium) and 46% (high sodium); no performance difference |
| Vrijens 1999 [21]Randomised controlled trial | 10 male endurance athletes | Randomised crossover: up to 3 hours' cycling at 34 °C replacing sweat with water or Gatorade (18 mmol/L sodium) | Blood sodium, exercise time | Blood sodium fell faster on water, and one rider developed low blood sodium; lower sodium went with shorter exercise time |
| Sims 2007 [23]Non-randomised trial | 13 trained female cyclists | Double-blind crossover (order alternated after a random first allocation): about 628 mL of a high- (164 mmol/L) or low-sodium (10 mmol/L) drink before cycling at 32 °C | Plasma volume, core temperature, time to exhaustion | High sodium raised plasma volume and extended time to exhaustion (98.8 v 78.7 min) |
| Schwellnus 2011 [14]Observational study | 210 Ironman triathletes | Prospective cohort: blood electrolytes and weight before and after the race | Cramping | 43 cramped. Electrolytes and weight change similar; faster racing and past cramps predicted cramping |
| Martínez-Navarro 2022 [15]Observational study | 88 marathon finishers | Observational: blood, urine and weight before and after the race | Cramping v hydration, electrolytes, muscle damage | 20 cramped. Hydration and blood sodium and potassium similar; muscle-damage markers higher |
| Garrison 2020 [24]Systematic review | 11 trials, 735 people; none in athletes | Cochrane systematic review of randomised magnesium trials | Cramp frequency, intensity, side effects | No meaningful benefit in older adults (moderate certainty); pregnancy trials conflict; no exercise-cramp trials |
The key studies, in order
- 1999Trial10 male athletes cycling at 34 °C: blood sodium fell faster when sweat was replaced with water than with a sodium sports drink. [21]
- 2002Non-randomised trial38 Ironman triathletes given salt tablets v matched controls: slightly less weight loss, no significant difference in blood sodium. [22]
- 2003Trial13 women running 4 hours: a high-sodium drink limited the fall in blood sodium compared with water; performance didn't differ. [17]
- 2005Observational study488 Boston Marathon runners: 13% had low blood sodium and 0.6% critical levels; linked to weight gain, times over 4 hours and BMI extremes. [31]
- 2005Observational study2,135 endurance athletes: blood sodium fell as body weight rose; drinking to excess was the main cause of low blood sodium. [25]
- 2007ConsensusSweat rates and electrolytes vary between people; drinks with electrolytes and carbohydrate can help more than water alone in some circumstances. [3]
- 2007Non-randomised trial13 female cyclists at 32 °C: a high-sodium drink before exercise raised plasma volume and extended time to exhaustion. [23]
- 2009Expert reviewThe electrolyte theory of cramps rests mainly on anecdote and case series, and cohort studies don't support it; evidence points to altered neuromuscular control. [27]
- 2010Non-randomised trialElectrically induced cramps in dehydrated men ended sooner after pickle juice than after water, with little change in blood electrolytes. [28]
- 2011Observational study88 London Marathon volunteers: 12.5% finished with low blood sodium, without symptoms; they had drunk more and gained more weight. [12]
- 2011Observational study210 Ironman triathletes: cramping wasn't linked to blood electrolytes or weight change; faster racing and past cramps predicted it. [14]
- 2013Observational study669 finishers of 161 km races: 15.1% had low blood sodium, more in hotter years and more often with dehydration than overhydration. [26]
- 2015Consensus2015 international consensus: drinking more than sweat and urine losses is the main cause of exercise-associated low blood sodium; it advises using thirst to guide drinking, and says sodium can't prevent it when overdrinking persists. [18]
- 2015Observational study20 finishers of a 161 km race (on-course peak 39 °C; nearby stations 31.7 °C): cramping, low blood sodium and nausea were unrelated to sodium intake; low-sodium runners lost at most 4–5% of body weight. [4]
- 2015Observational studyWestern States finishers: supplement sodium didn't differ between the 6.6% with low blood sodium and those with normal levels; overhydration was the main feature. [11]
- 2016Trial26 triathletes: the salt-capsule group finished a half-ironman faster, with higher blood sodium afterwards, than the control group. [19]
- 2017Expert reviewSweat rate (about 0.5–2.0 L an hour) and sweat sodium (about 10–90 mmol/L) vary widely; sweat tests need careful methods. The author works for a sports-drink maker. [2]
- 2017Expert reviewDrinking beyond thirst, with the hormone vasopressin, is the main cause of low blood sodium in exercise; sports drinks won't prevent it in people who overdrink. [5]
- 2018Expert reviewPlanned drinking suits long, hot or intense exercise; drinking to thirst may suffice for shorter, cooler or easier sessions. In a supplement backed by a sports-drink maker's institute, which paid the author an honorarium. [29]
- 2018ConsensusOnly a few supplements, including caffeine, creatine, nitrate and sodium bicarbonate, have good evidence. Contamination is a known anti-doping risk. [41]
- 2019Observational studySweat tests on 1,303 athletes: endurance athletes averaged 1.28 L of sweat and 51.7 mmol of sodium an hour, with wide variation. Funded by a sports-drink maker's institute, where the authors work. [30]
- 2019ConsensusISSN ultramarathon position stand: advises 500–700 mg of sodium per litre of fluid; sports drinks hold less sodium than typical sweat. [9]
- 2020Systematic review11 trials, 735 people: magnesium is unlikely to give older adults meaningful protection from ordinary cramps; no trials in exercise-associated cramps. [24]
- 2020Survey1,152 ultrarunners: two-thirds said sodium supplements should be available at races, believing they prevent low blood sodium and cramps. [1]
- 2020ConsensusHeat strains fluid balance and performance; sodium or glycerol loading can hyperhydrate athletes before exercise. [33]
- 2021Observational study266 ultrarunners: sodium supplement intake wasn't significantly linked with performance; dehydrated runners were faster than overhydrated ones. [20]
- 2022Observational study88 marathon finishers: those who cramped weren't more dehydrated or low in electrolytes, but had higher muscle-damage markers. [15]
- 2022Survey210 marathon runners: fewer than 5% named drinking to thirst; slower and first-time runners knew least about low blood sodium. [13]
- 2025Survey5,668 marathon runners surveyed in 2024: among supplement users, more than 90% used energy gels and 54.4% used salt tablets. [7]
- 2026Observational study58 ultra-trail finishers: those who cramped weren't more dehydrated or low in sodium; potassium and muscle-damage markers were higher. [16]
Who has the research been done in?
Most of this research was in endurance athletes at mass-participation events: marathoners in Boston and London, 161 km ultrarunners in California and Ironman triathletes[31][12][26][14]. In one Western States study, runners averaged 43 years old and a quarter were women[4].
The controlled trials had 10 to 26 people[21][17][23][19], and several trials were in cyclists or triathletes rather than runners[21][23][19]. The large sweat datasets came from athletes tested by a sports-drink maker's institute, mostly men: 404 of 506[2] and 1,103 of 1,303[30].
These were healthy adult athletes; the findings may not apply to teenagers, older runners or people with heart, kidney or blood pressure conditions. The NHS notes older people's kidneys may be less able to remove potassium[6].
What we know about women
Most recorded cases of exercise-associated low blood sodium have been in women, but in the Boston Marathon study the sex difference disappeared once body-mass index and race time were accounted for[5][31]. Researchers have suggested oestrogen may impair the brain's adaptation to rapid swelling in women who menstruate; that remains a hypothesis, and men are not immune[5]. In adults, sweat sodium doesn't seem to differ by sex[2].
Two trials were only in women. In 13 endurance athletes running for 4 hours while drinking 1 L an hour, mild low blood sodium developed in 92% on water, 69% on a low-sodium drink and 46% on a high-sodium drink, with no difference in performance[17]. In 13 trained cyclists in the high-hormone phase of their natural or pill cycle, a high-sodium drink before exercise in heat extended time to exhaustion[23]. We found no studies in pregnant runners; the NHS lists pregnancy among causes of leg cramps[32], and magnesium trials for pregnancy cramps conflict[24].
What the research can't tell us yet
Most race evidence is observational: runners chose their own sodium and fluid, so the studies show links, not cause and effect[4][11][20]. The ultramarathon sodium-intake findings come from 20 finishers[4], the salt capsule trial from 26 triathletes in one race[19], and the Ironman salt study wasn't randomised[22].
Sweat rate is most simply and accurately measured from the change in body weight during exercise, and sweat sodium in the field with absorbent skin patches[2][30]. Results vary with the collection method, timing, skin cleaning and lab analysis; sweat sodium can differ by 80–120% between body sites, and repeat tests vary by 5–16% from day to day, so tests can mislead unless done carefully[2].
Guidance differs: the hyponatraemia consensus advises drinking to thirst[18], while the ISSN ultramarathon position stand, though it calls drinking to thirst the most appropriate way to limit over- and under-hydration, warns that it may leave sodium intake short[9]. Two reviews cited here appeared in journal supplements supported by a sports-drink maker's institute: one author works there, and the other received an honorarium from it for attending its meeting and writing the review[2][29]. The largest sweat dataset was funded by the same institute, whose staff wrote it up[30].
We found no trials of electrolyte supplements in runners over 5K to half-marathon distances, and a Cochrane review found none of magnesium for exercise cramps[24].
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How and when are electrolytes used in studies?
Trials gave sodium as capsules, tablets or drinks. Triathletes in the salt capsule trial took 113 mmol of sodium (about 2.6 g) before and during a half-ironman[19], and Ironman triathletes in another study took salt tablets giving about 700 mg of sodium an hour[22].
In the 161 km ultramarathon, runners' total sodium intake ranged from about 2.5 g to 38 g, and on average two-thirds of it came from food and drink rather than capsules; across 30 runners the average rate was 438 mg an hour[4]. In 80 km ultramarathon stages, average intakes ran from about 2 g in the lowest-intake third of runners to 7.2 g in the highest[20].
Drinks tested held 410 to 680 mg of sodium per litre in a 4-hour run in women[17] and 18 mmol/L (about 410 mg per litre) in a sports drink trial[21]. The ISSN advises ultrarunners take 500–700 mg of sodium per litre of fluid[9].
On fluids, the 2015 consensus advises drinking to thirst[18], while the ACSM advises individual plans, with sweat rate estimated by weighing before and after exercise, to prevent body-weight loss of more than 2%[3].
When studies gave it
Before: in a pre-loading trial, women drank about 628 mL of a concentrated sodium drink over 60 minutes, starting 105 minutes before cycling in heat[23]. Sports Dietitians Australia says pre-exercise hyperhydration may be desirable in some scenarios and can be achieved by sodium or glycerol loading[33].
During: most studies gave sodium during the event, as capsules or drinks[19][22][17]. Kenefick's review argues planned drinking suits exercise over 90 minutes, especially in heat, while drinking to thirst may be enough for shorter, cooler or easier sessions[29].
After: the ACSM says the aim is to replace any fluid and electrolyte deficit[3]. The ISSN notes plain water alone is unlikely to restore fluid balance, because it lowers blood sodium and increases urine output[9].
| What studies used | When | Ref. |
|---|---|---|
| Salt capsules totalling 113 mmol of sodium (about 2.6 g) | Before and during a half-ironman triathlon | [19] |
| Salt tablets giving about 700 mg of sodium an hour | During an Ironman triathlon | [22] |
| Self-chosen: 2.5–38 g of sodium in total from food, drink and capsules | During a 161 km ultramarathon | [4] |
| Drinks with 410 or 680 mg of sodium per litre, at 1 L an hour | During a 4-hour run | [17] |
| About 628 mL (10 mL per kg) of a drink with 164 mmol/L of sodium | Over 60 minutes, starting 105 minutes before exercise in heat | [23] |
| 500–700 mg of sodium per litre of fluid (position stand guidance, not a trial) | During ultramarathons | [9] |
What are the side effects, and who should be careful?
The main risk isn't the salt, it's drinking too much: low blood sodium during exercise is most often caused by drinking beyond thirst[5], and ultrarunners rarely recognise that sodium supplements could lead to overhydration[1]. The 2017 review describes a runner who took large amounts of sodium on advice and still developed symptomatic low blood sodium on a long run[5].
Low blood sodium ranges from cases found only by research blood tests to a potentially fatal condition in which cells swell[5][9].
Gut symptoms are common in long races: 57% of runners in the 161 km race reported nausea or vomiting, unrelated to sodium intake[4]. The NHS says more than 400 mg of magnesium for a short time can cause diarrhoea, and too much potassium can cause stomach pain, sickness and diarrhoea[6]. In magnesium trials, side effects, mostly gut symptoms such as diarrhoea, affected 11–37% of people taking magnesium and 10–14% of controls[24].
Salt tablets add to the salt already in the diet. The NHS says adults should have no more than 6 g of salt a day (2.4 g of sodium), and that too much can cause high blood pressure, which increases the risk of heart attacks and strokes[10][6]. People in the UK eat about 8 g a day on average[6]. The NHS also notes that the Department of Health and Social Care says sodium chloride should not be used in supplements[6].
Medicines and health conditions
Lithium: to avoid high lithium levels, the NHS says people taking lithium shouldn't reduce their salt intake suddenly, and should drink plenty of fluids, especially during intense exercise or in hot weather when they sweat more[34].
Diuretics and blood pressure medicines: the NHS says to tell a doctor before starting the diuretic bendroflumethiazide if you have low sodium, potassium or magnesium levels[35]. It lists potassium-raising medicines or supplements among those that may not mix well with furosemide[36], and potassium among medicines to discuss when taking ramipril[37]. Diuretics are also among the NHS's listed causes of leg cramps and dehydration[32][38].
Painkillers and antidepressants: the 2017 review lists NSAIDs and SSRI antidepressants among possible triggers of vasopressin during exercise[5], and the ISSN notes concern about a role for NSAIDs in low blood sodium[9]. In Boston, though, NSAID use wasn't linked to it[31].
Be careful if
- You have high blood pressure or have been told to eat less salt: too much salt can cause high blood pressure[10].
- You plan to drink to a fixed schedule in a long race: drinking beyond thirst is the most common cause of low blood sodium[5], and weight gain during the race was linked to it in Boston Marathon runners[31].
- You're a slower or first-time marathoner: these runners knew least about low blood sodium in one survey[13], and finishing in over 4 hours was linked to it in Boston[31].
- You take lithium, a diuretic or a blood pressure medicine such as ramipril: the NHS gives specific guidance on salt, fluids or potassium for these medicines[34][35][37].
- You're older and thinking about potassium supplements: the NHS says older people should not take them unless advised to by a doctor[6].
- You're racing in heat: low blood sodium was more common in hotter years at 161 km races[26], and the NHS says the risk of heat exhaustion is high during hot weather or exercise[39].
Are electrolytes allowed in competition?
Sodium and other electrolytes are not on the World Anti-Doping Agency's Prohibited List, so taking them by mouth is permitted. Intravenous infusions or injections of more than 100 mL in 12 hours are a prohibited method at all times, whatever they contain, unless received legitimately in hospital treatment, surgery or clinical diagnostic investigations[40]. WADA says the rule is there to stop blood dilution and overhydration by drip, and that outside those settings, in situations such as severe dehydration or intractable vomiting, an athlete should receive appropriate treatment and apply for a retroactive Therapeutic Use Exemption as soon as reasonable[40].
The anti-doping risk with any supplement product is contamination. Products can contain prohibited substances that aren't on the label, and under the World Anti-Doping Code an athlete is responsible for anything found in their sample, even if it got there by accident[41].
Independent quality-assurance schemes, often called batch testing, check supplement products for prohibited substances[41]. WADA says using a product from one of these schemes can reduce, but not eliminate, the risk of an anti-doping violation[40], and the IOC says no product can be guaranteed entirely safe[41].
Myth or evidence?
5 things runners hear about electrolytes and sodium. Pick an answer to see what the research says.
1.Salt tablets prevent muscle cramps.
2.Drinking too much is the most common cause of low blood sodium in runners.
Show the answer
Evidence. Drinking beyond thirst is the most common cause[5].
3.Every runner loses about the same amount of sodium in sweat.
Show the answer
Myth. Sweat rate and sweat sodium vary widely between athletes[2].
4.Salt capsules improved race times in a triathlon trial.
Show the answer
Evidence. In a half-ironman trial, the salt-capsule group finished faster than the control group[19].
5.Sports drinks can't cause low blood sodium.
Show the answer
Myth. Low blood sodium develops when athletes drink water or sports drinks, both more dilute than blood, in excess of their losses; sports drinks won't prevent it in people who overdrink[5].
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Frequently asked questions
- Do runners need electrolyte supplements?
- Research suggests extra sodium beyond food and drink is generally not needed, even in ultramarathons[1]. Drinks with electrolytes and carbohydrate can help more than water alone in some situations[3].
- Do salt tablets prevent cramps?
- The evidence doesn't support that. Cramping was unrelated to sodium intake in a 161 km race[4] and to blood electrolytes in Ironman triathletes[14]; research points to altered nerve and muscle control instead[27].
- What causes low blood sodium in runners?
- Most often, drinking beyond thirst[5]. Fluid retention by the hormone vasopressin plays a part[25], and in long ultramarathons some cases occur in runners who have lost weight[26].
- How common is hyponatraemia in marathons?
- Studies have found low blood sodium in about 1 in 8 finishers tested: 13% in the 2002 Boston Marathon[31] and 12.5% in a London Marathon study[12]. Critically low levels were rarer, at 0.6% in Boston[31].
- How much sodium do runners lose in sweat?
- It varies widely between athletes and conditions[2]. Endurance athletes in one large dataset lost an average of 51.7 mmol (about 1.2 g) of sodium an hour[30].
- Is drinking to thirst enough?
- Guidance differs. The 2015 hyponatraemia consensus advises drinking to thirst[18], while the ACSM advises individual drinking plans to prevent body-weight loss of more than 2%[3]. Drinking beyond thirst is the most common cause of low blood sodium[5].
- Are sports drinks better than water on long runs?
- Sometimes. The ACSM says they can help more than water alone in certain circumstances[3], but they are more dilute than blood, so they won't prevent low blood sodium in someone who drinks too much[5].
- Are women more at risk of hyponatraemia?
- More recorded cases have been in women, but in the Boston Marathon study the difference disappeared after accounting for body size and finish time[5][31]. A hormonal explanation has been proposed, not confirmed[5].
- Do salt tablets help in hot races?
- The research doesn't settle it. Sodium intake was unrelated to low blood sodium in a 161 km race with a measured on-course peak of 39 °C[4], but a high-sodium drink before exercise in heat extended endurance in 13 women cyclists[23].
- Does magnesium help with cramps?
- The evidence doesn't show that. A Cochrane review found no trials in exercise-associated cramps, and magnesium was unlikely to give older adults with ordinary cramps a meaningful benefit[24].
- Do runners need potassium supplements?
- The NHS says you should be able to get all the potassium you need from a varied and balanced diet[6]. In an ultra-trail study, runners who cramped had higher, not lower, blood potassium[16].
- Does pickle juice work for cramps?
- In one lab study it shortened electrically induced cramps in dehydrated men, to 85 seconds from 134 on water, with little change in blood electrolytes[28].
- Are IV drips allowed for runners?
- Not above 100 mL in 12 hours. WADA prohibits intravenous infusions or injections of more than 100 mL per 12 hours, unless received legitimately in hospital treatment, surgery or clinical diagnostic investigations; otherwise, WADA says, they need a Therapeutic Use Exemption[40].
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Expert reviewSweat rate (about 0.5–2.0 L an hour) and sweat sodium (about 10–90 mmol/L) vary widely; sweat tests need careful methods. The author works for a sports-drink maker.
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RegulatorAdults need 3,500 mg of potassium and 270–300 mg of magnesium a day. 6 g of salt is 2.4 g of sodium. The DHSC says sodium chloride should not be used in supplements.
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NHS Salt in your diet. NHS, 2026.
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RegulatorLeg cramps are common and usually harmless; causes include exercise strain (worse in heat), dehydration, pregnancy and diuretics.
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NHS Side effects of lithium. NHS, 2026.
RegulatorSays not to cut salt intake suddenly, and to drink plenty during intense exercise or hot weather, to avoid high lithium levels.
www.nhs.uk/medicines/lithium/side-effects-of-lithium/ - [35]
NHS Who can and cannot take bendroflumethiazide. NHS, 2026.
RegulatorTell a doctor before starting this diuretic if you have low levels of sodium, potassium or magnesium in your blood.
www.nhs.uk/medicines/bendroflumethiazide/who-can-and-cannot-take-bendroflumethiazide/ - [36]
NHS Furosemide: a medicine to treat oedema. NHS, 2026.
RegulatorThis diuretic can cause dehydration; potassium-raising medicines or supplements, and some NSAIDs, may not mix well with it.
www.nhs.uk/medicines/furosemide/ - [37]
NHS Taking ramipril with other medicines and herbal supplements. NHS, 2026.
RegulatorLists potassium, and NSAIDs such as ibuprofen, among medicines to tell a doctor about when taking ramipril.
www.nhs.uk/medicines/ramipril/taking-ramipril-with-other-medicines-and-herbal-supplements/ - [38]
NHS Dehydration. NHS, 2026.
RegulatorDehydration happens more easily with heavy sweating after exercise or with diuretics; symptoms include thirst and headache.
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NHS Heat exhaustion and heatstroke. NHS, 2026.
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World Anti-Doping Agency The Prohibited List. World Anti-Doping Agency, 2026.
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www.wada-ama.org/en/prohibited-list - [41]
Maughan RJ, Burke LM, Dvorak J, et al. IOC consensus statement: dietary supplements and the high-performance athlete. British Journal of Sports Medicine, 2018.
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doi.org/10.1136/bjsports-2018-099027
Who wrote and reviewed this
Written by Grant MacMillan, Editor, RunV. Not yet reviewed by an expert.
Last checked against its references . Next review due . How pages are reviewed · How we grade research
