Electrolytes
Evidence: weak
Electrolyte supplements are sold far beyond what they do. For most runners they are unnecessary: they do not prevent cramps or hyponatraemia, isolated sodium does not improve performance, and a normal diet already supplies plenty. The genuine case is narrow, replacing large measured losses over long, hot, salty efforts, and even then the rationale is fluid balance, not a performance boost.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Electrolytes are the salts the body uses for nerve and muscle function, chiefly sodium, with smaller roles for potassium, magnesium, calcium and chloride. They are lost in sweat, and an industry has grown up selling tablets, sachets and drinks to replace them, usually on the promise of preventing cramps, staving off dehydration and unlocking performance. The honest position is that for most runners, most of the time, electrolyte supplements solve a problem they do not have. This is a textbook case of the marketing playbook, and the evidence below is graded accordingly.
This is a separate question from hydration, which is well evidenced: drink to thirst. What follows is about whether to add salt to that fluid.
Salt does not reliably prevent cramps
The most aggressively marketed claim is the weakest. The best-supported cause of exercise-associated cramp is altered neuromuscular control driven by fatigue, not dehydration or electrolyte depletion (Schwellnus, in Maughan & Shirreffs). In a 161 km ultramarathon, sodium intake was no different between runners who cramped and those who did not (Hoffman & Stuempfle 2015). There is a sliver of contrary lab evidence: a carbohydrate-electrolyte drink more than doubled the time to an experimentally-induced calf cramp, but 69% of participants still cramped while fully hydrated and supplemented (Jung et al. 2005). Taken together, electrolytes are not a reliable cramp fix, and the depletion theory that sells them is largely refuted.
Salt does not prevent hyponatraemia
Electrolyte products are also sold as protection against the dangerous fall in blood sodium that long events can cause. They are not. Exercise-associated hyponatraemia is driven by drinking too much fluid, not by losing too much sodium, and final blood sodium is governed by fluid volume (Hew-Butler et al. 2017). In the ultramarathon data, sodium intake was no different between those who became hyponatraemic and those who did not (Hoffman & Stuempfle 2015). Salt cannot rescue a runner who is overdrinking; drinking to thirst can.
Most runners already get enough sodium
For most runners, most of the time, ordinary food covers the need, and the electrolyte-supplement industry sells against that fact. Typical sodium intake in most countries is already well above requirement: around 9 to 12 g of salt a day, far more than the recommended limit of under 5 g, roughly 2 g of sodium (WHO sodium guidance). A normal diet, especially with the higher food volume that training appetite drives, supplies plenty to cover an ordinary day’s running. For an easy or moderate run under about an hour, no added electrolytes are needed; the small deficit is replaced at the next meal.
When extra sodium is actually justified
The case for added sodium is specific and conditional, not general. Sweat sodium loss varies enormously between people, from roughly 350 to 1800 mg per litre (sweat sodium variability), so blanket advice is unreliable. Added sodium has a defensible rationale when several factors stack up: efforts beyond about one to two hours, hot or humid conditions, high sweat rates, and being a heavy, salty sweater, the person who finishes crusted in white residue. Even then the firm rationale is replacing a large measured loss and supporting fluid balance over very long events, not a performance gain: isolated sodium supplementation did not improve a cycling time-trial in cool conditions (cool-conditions trial, in Hew-Butler et al. 2017). The longer, hotter and saltier the effort, the stronger the case; the shorter and cooler, the weaker.
The other electrolytes
“Electrolytes” on a label means more than sodium: potassium, magnesium, calcium and chloride. For runners, sodium is the one that carries even the conditional case, and the others are mostly there to complete the picture. Sweat is largely water and sodium chloride; potassium, magnesium and calcium appear in it at far lower concentrations, and the amounts lost in a session are small against the body’s stores and readily replaced by normal meals (Thomas, Erdman & Burke 2016). Chloride simply tracks sodium. There is no good evidence that topping up potassium, magnesium or calcium during exercise improves performance or prevents cramps in a well-fed runner; magnesium in particular is sold hard for cramps and recovery and delivers on neither on top of an adequate diet. Where these minerals matter is the everyday diet, not the drinks bottle, and a chronic shortfall (as in low energy availability) is a nutrition question, not an in-race one.
The practical position
A normal diet covers the average runner; salt food to taste and reserve electrolyte products for long, hot, high-sweat sessions and known salty sweaters, rather than taking them by default. They are not a cramp cure, not protection against hyponatraemia, and not a performance aid for ordinary running. When a product promises otherwise, it is selling certainty the evidence does not support.