Electrolytes for Runners: A UK Guide to Salts & Sweat

British runners lose the same sweat as everyone else — the weather just disguises it. A cool, drizzly 90 minutes still costs you a litre or more of fluid and a gram or two of sodium, and “it wasn’t even hot” is how a lot of long-run blow-ups begin. This UK guide to electrolytes for runners covers what you lose, when water stops being enough, and what the authorised UK/EU health claims actually guarantee when a product makes them — because here, the wording on the tub is regulated, and it is worth knowing what it means.

Note on language: UK pages on this site use authorised claim wording only. Where the US version of this guide says a nutrient “supports” something, this page sticks to what the GB/EU register permits — or describes the physiology plainly without a claim.

What do you actually lose in sweat?

Per litre of sweat	Typical range	Notes
Sodium	~460–1,840 mg (20–80 mmol/L)	The electrolyte that drives the plan
Potassium	~100–300 mg	Small losses; food replaces them
Magnesium	A few mg	Diet covers it; magnesium contributes to electrolyte balance (authorised claim)
Fluid overall	Sweat rates ~0.5–2.0 L/hour	Weigh yourself to find your own rate

The fourfold spread in sweat sodium is the single most useful fact on this page. Two runners in the same club, same pace, same weather can finish a long run having lost 1 g and 4 g of sodium respectively. The second runner — the one with white salt lines on their kit, stinging eyes, and a post-run headache — needs a different plan, and no generic label dose knows which runner you are. Measuring is simple: weigh yourself before and after an hour’s run, add what you drank; each kilogram lost is roughly a litre of sweat.

When does water stop being enough?

For efforts under about an hour, water alone is fine — a position UK and international sports-medicine guidance shares. Beyond 60–90 minutes, sodium joins the plan, and the evidence-based shape of a during-run drink is well defined on both sides of the Atlantic. Under the authorised UK/EU rules, a carbohydrate-electrolyte solution must provide 80–350 kcal per litre from carbohydrate (at least 75% from high-glycaemic sources such as glucose or sucrose) and 20–50 mmol of sodium per litre — 460 to 1,150 mg — to carry the claims that it contributes to the maintenance of endurance performance during prolonged endurance exercise and enhances the absorption of water during physical exercise. Those composition rules are a useful shopping filter even if you ignore claims entirely: a drink inside that window is built like the solutions used in the research.

Practical targets, in plain language:

• During runs over an hour: a drink in the authorised composition window above, sipped to thirst, aiming to finish no more than about 2% of body mass down.
• Afterwards: roughly 1.25–1.5 litres of fluid per kilogram lost, with sodium from the drink or food, so the fluid is retained rather than urinated out.
• Fuelling: for runs beyond about 90 minutes, carbohydrate at roughly 30–60 g per hour alongside the fluid — this is where gels, chews, or an energy drink earn their place beside the electrolytes. The marathon-specific protocol is on our electrolytes for long runs page, and the underlying sodium maths on sodium for runners.

If you are deciding whether today’s session even qualifies, when do you need electrolytes works through the thresholds by duration, temperature, and sweater type. And if imported US products with higher doses appeal, remember their labels were written for a different rulebook — the US hub version of this guide is electrolytes for runners.

What about homemade and sugar-free options?

Both are legitimate, with eyes open. A homemade electrolyte drink mixed to real ratios (about ¼ level teaspoon of fine salt per litre for a training drink, with sugar for carbohydrate) lands inside the ranges above for pennies. Sugar-free tablets are tidy and portable, and their sodium is real — but they bring no carbohydrate, so on runs over two hours the fuelling gap has to be filled by gels or food. Our sugar-free electrolytes page compares the two approaches scenario by scenario rather than declaring a winner.

What should you check on a UK label?

UK labels follow different conventions from US ones, and two conversions matter for runners:

Salt versus sodium. UK food labels usually declare “salt,” not sodium. Salt is sodium chloride; sodium is roughly 40% of it. To compare a UK drink with the figures on this page, divide the salt figure by 2.5 to get sodium: a drink listing 1.5 g of salt per litre contains about 600 mg of sodium per litre — inside the authorised-claim window. US products list sodium directly, which is why imported labels can look “stronger” at a glance; they are often just counting differently.

Where the minerals sit. Nutrition information per 100 ml is the legal default; per-serving figures may also appear. A tablet that makes 500 ml at “200 mg sodium per tablet” gives 400 mg per litre — below the 460 mg floor for the authorised claims, and below what a salty sweater needs. Do the per-litre division before judging any product, British or imported.

Testing marks. If you compete under anti-doping rules, look for Informed Sport batch testing — the programme most widely used by UK brands — and check the batch in the programme’s database. A logo on the tub is a claim; the database entry is the evidence.

A British footnote on vitamin D: it is not an electrolyte topic, but UK runners training through winter are the textbook audience for the NHS advice that adults consider a daily 10 microgram vitamin D supplement in autumn and winter. Fuelling the run and covering the season are separate jobs; both are cheap to get right.

Does British weather really justify all this?

It is the most common objection, and it survives about one long run in a June heatwave. The UK’s climate produces the full range of hydration problems in miniature: cool days that suppress thirst while a 2-hour run still costs well over a litre of sweat under layers; sudden warm spells arriving before anyone is acclimatised (the highest-risk window of the year, as the sodium page explains); and humid, still days where sweat stops evaporating and losses climb while the temperature looks modest on paper. London Marathon runners meet all three in a single April. The other British factor is fuelling culture: many club long runs still run on water and willpower into the second hour. Nothing in this page asks you to buy anything for a 45-minute run in March. It asks you to notice that “mild weather” describes the air, not your sweat rate — and to weigh yourself once on a warm day before deciding the topic is an American problem. Runners in genuinely hot climates need bigger plans, set out on our long-runs page; runners here need the same arithmetic at a slightly smaller scale, which is why this UK version exists rather than a copy of the US one.

What about cramp — the question every British club runner asks?

Cramp deserves an honest UK paragraph because it drives more electrolyte purchases than any other symptom. The evidence is genuinely two-sided: cramps cluster in fatigued muscles worked harder than training prepared them for, and they also cluster on the hottest, saltiest days in the runners with the highest measured losses. Sodium planning addresses the second half of that picture — it is the contributor you can fully control in advance — and pacing plus specific conditioning address the first. What the evidence does not support is the magnesium-tablet theory of cramp prevention that fills UK pharmacy shelves each spring: magnesium losses in sweat are a few milligrams per litre, and no good trial has made supplemental magnesium a cramp cure for fed athletes. If cramp ends your races despite honest pacing and a tested sodium plan, that is a conversation for a physio or your GP, not for a stronger tablet.

Who should not follow standard electrolyte advice?

• People with high blood pressure, heart failure, or kidney disease. Long-run fuelling deliberately adds sodium — sometimes several grams in a day. If you have been advised to restrict sodium, agree your race plan with your clinician first.
• Anyone pregnant. Sodium and fluid needs change in pregnancy, and heat tolerance falls. Long efforts in warm weather need your midwife or doctor’s input.
• Under-18 runners. Young athletes should follow water-and-food plans with an adult watching for heat-illness signs, not adult dosing schedules.
• People taking diuretics, lithium, or blood-pressure medicines. These interact directly with fluid and sodium balance. Ask your GP or pharmacist how event-day sodium fits your prescription well before race day.
• Anyone feeling confused, bloated, or worse while drinking more. Those are warning signs of hyponatraemia (low blood sodium from over-drinking) — stop, and seek medical help rather than more fluid.