Why do you cramp when you're drinking plenty?
If you cramp during a run despite drinking to plan, the honest answer points away from the story you were sold: for most runners, the evidence linking cramps to a sodium deficit is weaker than the marketing around electrolyte tablets suggests. Studies comparing runners who cramp with those who don't have generally failed to find the salt-and-water difference the deficit theory predicts. That does not mean sodium is irrelevant to running – it matters a great deal for fluid balance over long efforts – but it means the specific claim my cramps are a sodium problem is not well supported.
Sodium does real work in your body during a run. Whether topping it up prevents your cramps is a separate question, and the data are not on the marketing's side.
What is sodium actually doing during a run?
Two mechanisms are worth separating, because they get merged in the marketing and they are not the same tier of evidence.
Fluid balance is established physiology. Sodium is the main dissolved particle in the fluid outside your cells, and the concentration of that fluid is what your body defends. You lose sodium in sweat, and how much varies widely between people. Over a long enough effort, if you drink a lot of plain water and replace little sodium, the concentration of sodium in your blood can fall. That is not a theory; it is measurable. This is the mechanism behind exertional hyponatraemia – dangerously low blood sodium – which is a genuine and occasionally fatal problem in very long events. Sodium's role here is real and is not the subject of any serious dispute.
Cramping is a different claim. The idea is that losing sodium (and other electrolytes) in sweat changes the fluid around your muscle nerve endings enough to make the muscle fire uncontrollably. That is a plausible mechanism. The trouble is the step from "plausible mechanism" to "this is why you, specifically, cramped" – and that step is where the measured evidence thins out.
Not to be confused with: two different problems that both involve sodium
Exercise-associated muscle cramps and exertional hyponatraemia are frequently discussed in the same breath and treated as the same electrolyte story. They are not.
- Exercise-associated muscle cramps (EAMC) – the localised, painful seizing of a muscle you're using hard, typically late in a race. Uncomfortable, usually harmless.
- Exertional hyponatraemia – blood sodium falling too low, most often from drinking too much fluid over a long event rather than too little sodium. This one can be serious.
The important asymmetry: the standard fix for cramps that marketing pushes – drink more, add salt – is closer to the cause of hyponatraemia than to its cure. A retrospective field study of ultra-endurance cyclists found that low blood sodium during prolonged exercise could occur with loss, gain, or no change in body mass, and that its main modifiable driver is excessive drinking (Int J Sport Nutr Exerc Metab, 2017). Sodium here is a level to defend, not a supply to keep maximising.
Where does the cramp chain actually break?
Set the deficit theory out as a chain, step by step, because written as continuous prose it hides its own weak join:
- You sweat and lose sodium – established, and highly variable between people.
- Over a long effort this can lower fluid volume and, sometimes, blood sodium concentration – measurable in some runners.
- That electrolyte change is what triggers the cramp – this is the step that has not held up.
Step three is where the measurement stops matching the story. When researchers compared marathon runners who developed cramps with those who didn't, the cramping runners did not show the dehydration and serum electrolyte differences the deficit theory predicts; markers of muscle damage were the more relevant distinction (J Strength Cond Res, 2022). Twenty of the 88 finishers in that study cramped, and body-mass change, post-race urine concentration and serum sodium and potassium were all indistinguishable between them and the runners who stayed cramp-free. What did separate the two groups was muscle damage: creatine kinase and lactate dehydrogenase were significantly higher in the crampers immediately after the race and still higher 24 hours later. A later comparison in ultra-trail runners reached a similar conclusion, again pointing towards muscle damage rather than dehydration and electrolyte depletion as the distinguishing factor (J Strength Cond Res, 2026). The ultra-trail numbers push against the deficit story from one more angle: serum sodium again did not differ, and post-race potassium was higher in the runners who cramped rather than lower, while their creatine kinase ran roughly two to three times the non-cramper value at 24 and 48 hours.
The competing explanation for EAMC is that the cramp comes from the nervous system, not the fluid: when a muscle is fatigued and working past what it's trained for, the reflex control of its contraction and relaxation goes haywire. That fits the everyday observation that cramps hit the muscles you're overusing, late, when you're tired – not evenly across a body that is uniformly low on salt.
Does pre-loading sodium help – and with what?
Some runners take concentrated salt before a race, and pickle juice has a following as a cramp remedy. Here the mechanism and the outcome need to be kept apart in the same sentence, because they are different tiers of evidence.
A study on ingesting pickle juice, hypertonic (very salty) saline, or plain water before exercise looked at aerobic performance and thermoregulation – how the body handles heat (J Athl Train, 2014). The relevant surrogate there is thermoregulation and performance, not "did it stop a cramp". A high-sodium drink can plausibly help the body hold onto fluid and manage heat over a long, hot effort. That is a fluid-balance and heat argument, and it is a reasonable one. It is not the same as demonstrating that the salt prevented the cramp – and if pickle juice does anything for an acute cramp, a rapid nervous-system reflex from the strong taste is at least as plausible as anything the sodium does, given how fast relief is sometimes reported relative to how slowly swallowed sodium reaches the blood.
Does distance change the answer?
Yes, and this is where the sodium story earns back some ground – just not for cramps.
For a sub-one-hour run, sodium replacement is close to irrelevant to fluid balance; the effect on the relationship between body-mass change and hydration over short exercise is trivial. The picture changes over very long events. A review of hydration for ultra-endurance activities (over roughly four hours) describes how athletes in these events can drift into either too little or too much body water, and how both states carry performance costs and, at the extremes, danger (Res Sports Med, 2019). Over that duration, managing sodium and fluid together is a genuine part of not getting into trouble.
So the mechanism belongs to a regime. Over ultra distances, sodium and fluid strategy matters for hydration and heat, and getting it badly wrong in either direction has consequences. That is a fluid-and-heat conclusion. It does not license the marathon runner buying electrolyte tablets to stop their calf seizing at mile 20 – that is the claim the comparison studies did not support.
Individual variation is the part the label ignores
Sweat sodium loss varies several-fold between people. A high-sweat, salty-sweating runner in the heat over six hours is in a different physiological situation from a lighter sweater doing a cool half-marathon. When you see a single dosing figure on a tab of electrolytes, that figure cannot be right for both, because the underlying loss it claims to replace is not the same quantity in both bodies. This is why the question of how much sodium to take has no clean universal answer, and why a strategy that works for one runner tells you little about yours.
Verdict: what the mechanism does and does not license
Sodium does real, established work during a run: it is the main solute your body uses to defend the concentration of your body fluids, and over long, hot efforts, managing it alongside fluid intake is part of staying safe and running well. That much is licensed.
What is not licensed is the leap most cramp marketing makes. In head-to-head comparisons, runners who cramped did not reliably differ from those who didn't on the dehydration and electrolyte measures the deficit theory rests on – muscle damage and, plausibly, neuromuscular fatigue looked more relevant. If you cramp despite drinking enough, the most probable culprits are doing more than you're trained for and fatiguing the muscle, not a salt shortfall.
A practical way to weigh the criteria, by strength of evidence:
- Sodium for fluid balance and heat over ultra-distance efforts – biomarker-supported and mechanistically sound.
- Avoiding over-drinking to prevent low blood sodium – biomarker-supported; the more common failure than under-drinking in long events.
- Sodium supplementation to prevent muscle cramps – mechanistically plausible but not confirmed; the direct comparisons point elsewhere.
What would settle the cramp question is a trial that manipulates sodium intake and measures cramp incidence directly in runners of your type and distance, rather than comparing electrolyte levels after the fact. Until that exists, treat the anti-cramp claim on an electrolyte label as the weakest link in the chain, and treat your training load – not your salt intake – as the first thing to examine when your legs seize. One concrete difference did separate the groups in the ultra-trail comparison: regular lower-limb strength training was reported by 56 % of the runners who cramped against 88 % of those who did not. The marathon study pointed the same way (25 % against 48 %) without reaching statistical significance. Two observational comparisons are not a trial of strength work as a cramp treatment, but they point where the biomarkers point – at how ready the muscle is for the load, not at how much salt it has left.
Sources
- Muscle Cramping in the Marathon: Dehydration and Electrolyte Depletion vs. Muscle Damage (J Strength Cond Res, 2022) – marathon runners who cramped did not differ from non-crampers in the expected dehydration/electrolyte measures, with muscle damage more relevant.
- Muscle Cramping in Ultra-Trail: Dehydration and Electrolyte Depletion versus Muscle Damage (J Strength Cond Res, 2026) – comparison in ultra-trail runners pointing to muscle damage over dehydration and electrolyte depletion.
- Exertional Hyponatremia and Serum Sodium Change During Ultraendurance Cycling (Int J Sport Nutr Exerc Metab, 2017) – low blood sodium during prolonged exercise occurred with loss, gain, or no change in body mass, with excessive drinking the main modifiable factor.
- Pre-exercise ingestion of pickle juice, hypertonic saline, or water and aerobic performance and thermoregulation (J Athl Train, 2014) – examined pre-exercise high-sodium drinks for their effect on aerobic performance and thermoregulation.
- Considerations for ultra-endurance activities: part 2 - hydration (Res Sports Med, 2019) – over ultra-endurance durations athletes can drift into under- or over-hydration, both carrying consequences; short-exercise effects on hydration are trivial.
