Hydration and electrolytes
Evidence: strong
Drink to thirst. Overdrinking, not dehydration, is the real danger. Salt supplements do not prevent cramps, and most people already eat plenty of sodium.
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.
The evidence-based position on hydration overturns much of what is sold to runners. Drink to thirst rather than to a fixed schedule. Overdrinking does not prevent fatigue, cramps or heat illness, and it raises the risk of a dangerous fall in blood sodium (Hew-Butler et al. 2017). The guidance shifted from the “drink as much as possible” advice of the 1990s precisely because that advice was harming people.
Hyponatraemia is the real danger
Exercise-associated hyponatraemia, a blood sodium below 135 mmol/L, is caused primarily by drinking too much hypotonic fluid combined with inappropriate fluid retention, not by losing too much sodium (Hew-Butler et al. 2017). Final blood sodium is governed by fluid volume, not sodium intake, which is why salt supplements cannot prevent it if fluid intake is excessive. It can be fatal, and symptomatic cases are treated with hypertonic saline, never more water. Risk rises with overdrinking and weight gain during an event, low body mass, female sex, slow pace, events over four hours, and freely available fluid on course.
Electrolytes and cramps
Electrolyte and salt supplements do not reliably prevent muscle cramps. The best-supported cause of exercise-associated cramp is altered neuromuscular control driven by fatigue, not dehydration or electrolyte depletion (Schwellnus, in Maughan & Shirreffs). The dehydration-and-salt theory of cramping is now largely refuted. In-race sodium has minimal evidence for improving performance, with guidance figures around 300 to 600 mg per hour resting on weak support.
Do you already get enough salt?
For most runners, most of the time, the answer is yes, and the electrolyte-supplement industry is selling against that fact. Typical sodium intake in most countries is already well above need: around 9 to 12 g of salt a day, far more than the recommended limit of under 5 g of salt, roughly 2 g of sodium (WHO sodium guidance). A normal diet, especially one with the higher food volume that training appetite drives, supplies plenty of sodium to cover an ordinary day’s running. For an easy or moderate run under about an hour, no added electrolytes are needed; the deficit is small and replaced at the next meal.
The case for extra sodium is specific and conditional, not general. Sweat sodium loss varies enormously between people, from roughly 350 to 1800 mg per litre of sweat (sweat sodium variability), so blanket advice is unreliable. Added sodium during exercise 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. The longer, hotter and saltier the effort, the stronger the case; the shorter and cooler, the weaker.
Even where extra sodium is reasonable, the evidence that it improves performance is weak; the firmer rationale is replacing a large measured loss and supporting fluid balance over very long events. And sodium does not protect against hyponatraemia if fluid intake is excessive, because that condition is driven by overdrinking, not sodium loss. The practical position: a normal diet covers the average runner, salt your food to taste, and reserve electrolyte products for long, hot, high-sweat sessions and for known salty sweaters, rather than taking them by default.
The 2% rule is contested
The familiar claim that losing 2% of body mass in fluid impairs performance comes from fixed-intensity laboratory tests. In self-paced time-trials that mimic real racing, exercise-induced dehydration changed performance negligibly, because thirst-guided drinking suffices (Goulet 2011). The 2% figure may still matter in fixed-pace efforts in the heat, but for self-paced racing it is not the hard limit it is often presented as.