Electrolytes for Running: Sodium, Potassium and Magnesium

Colorful effervescent tablets, salt capsules, isotonic powders — the sports industry makes good money from electrolytes. Advertising suggests that without a steady supply of sodium, potassium and magnesium, runners will cramp, fade and dehydrate. Sports science takes a more sober view. In fact, exactly one electrolyte is practically relevant when running — and that precisely during the least glamorous sessions: the long, hot ones. Here you will learn which minerals you actually lose in sweat, when intake is necessary, when it is a waste of money — and why reaching for the magnesium bottle will not prevent your cramps.

What electrolytes do in the body

Electrolytes are electrically charged minerals dissolved in blood and body fluids. They control three processes that matter for runners: fluid balance — how much water the body retains and where it sits — the transmission of nerve impulses, and muscle contraction. The four relevant ones are sodium, potassium, magnesium and calcium.

What matters for training: you lose electrolytes through sweat — but in dramatically different amounts. One litre of sweat contains between 300 and over 1,500 milligrams of sodium, but only around 150 to 300 milligrams of potassium and a few milligrams of magnesium. On top of that comes an individual sweat rate of 0.4 to over 2 litres per hour — how to measure yours is covered in the article hydration for running. From these two numbers — sweat rate times concentration — your real loss emerges. And it varies from runner to runner by a factor of ten.

Sodium: the electrolyte that counts when running

Sodium sits mainly outside the cells and is the most important regulator of extracellular fluid volume — and therefore of blood volume. If you sweat for hours, you lose with sodium the ability to retain fluid in the body: water you drink practically runs straight through without it. A salty sweater losing 1,200 milligrams of sodium per litre at 1.5 litres of sweat per hour loses more than five grams of sodium on a three-hour summer long run — over twelve grams of salt.

Three functions make sodium indispensable on long sessions:

• Fluid retention: Sodium keeps water in the bloodstream. Without it, blood volume drops despite drinking — heart rate rises, performance falls.
• Thirst regulation: Sodium keeps the thirst drive alive. Plain water lowers the sodium concentration and paradoxically switches off thirst even though the need remains.
• Hyponatremia protection: During efforts of three, four hours or more, plain water without sodium can dangerously dilute the sodium concentration in your blood. This is one of the few acutely life-threatening conditions in marathoning.

Potassium, magnesium, calcium: much myth, little loss

Potassium sits 98 percent inside the cells — sweat losses of 150 to 300 milligrams per litre are small and easily replaced by a normal diet: a banana contains around 400, a large potato over 800 milligrams. Extra potassium during exercise is unnecessary for healthy runners and, in overdose, even risky for heart rhythm.

Magnesium is the most stubborn myth in the scene. Sweat losses are in the range of a few milligrams per litre — physiologically irrelevant. And the belief that "cramps come from magnesium deficiency" does not survive scrutiny: controlled studies find neither lower magnesium levels in cramping athletes nor any effect of magnesium supplementation on cramp frequency or severity. If you reach for the magnesium bottle in the evening and stay cramp-free the next day, you probably simply trained shorter or slower.

Calcium, finally, is lost in sweat in tiny amounts and has no bearing on acute performance — it matters long-term for bone health, especially for female runners with low energy availability. That is a topic for everyday nutrition, not for your sports drink.

When you really need electrolytes

The loss numbers lead to clear rules — and they are simpler than advertising suggests:

• Under 60 minutes: Never. Body stores and a normal diet cover everything. Electrolytes on a 45-minute easy run are money down the drain.
• 60 to 90 minutes: Only in heat of about 25 degrees Celsius or more, or if you are a heavy salty sweater. Otherwise water is enough.
• Over 90 minutes: Now sodium counts: 300 to 800 milligrams per hour, matched to sweat rate and temperature. For ultra distances and great heat, up to 1,000 milligrams per hour may be needed.
• After the run: Here sodium helps with rehydration — without it, replacement fluid ends up in the bladder. A salted meal takes care of this incidentally.

Whether you belong at the upper or lower end of the range can be told from three signs: white salt stains on your shirt and cap, burning eyes when sweating, and a distinctly salty taste on your skin. All three together mean: high concentration, replace generously.

Sources compared: from sports drink to salted pretzel

Not everything labelled "electrolytes" delivers relevant amounts. A look at the label pays off:

• Sports drinks (isotonic): 400 to 1,100 milligrams of sodium per litre, plus carbohydrates. The most practical solution for long sessions — fluid, energy and sodium in one.
• Gels: highly variable, from 20 to 200 milligrams of sodium per gel. If you sweat heavily, deliberately choose sodium-rich products.
• Salt tablets: 200 to 500 milligrams of sodium per capsule. Useful in ultras and heat when drink volume is limited — but always take with water, never dry.
• Normal food: salted pretzels, broth, salted rice cakes or simply generously salted meals before and after training. Entirely sufficient for most training weeks.
• Effervescent tablets: caution — many contain only 50 to 150 milligrams of sodium per tablet. For replacing sweat losses that is practically homeopathic.

The cheapest option sits in your kitchen: a homemade sports drink from one litre of water, a quarter teaspoon of salt (1 to 1.5 grams), 40 to 60 grams of sugar and a squeeze of lemon juice. That yields around 400 to 600 milligrams of sodium plus carbohydrates per litre — the same effect as the branded product for a fraction of the price.

Muscle cramps: what the research really shows

Exercise-associated cramps are common in the marathon finish — and electrolyte theory was the standard explanation for decades. The data, however, is thin: field studies of marathon and ultra runners find no meaningful differences in blood electrolytes between cramp sufferers and the cramp-free. The dominant explanation today is neuromuscular: cramps occur when muscles fatigue beyond their trained level — typically because race pace exceeds what training supports.

Effective prevention therefore lies elsewhere: a realistic goal pace that matches your fitness, regular strength training for runners against muscular fatigue, and enough carbohydrates — details in the article marathon fueling. Acute cramp management: stop briefly, gently stretch the muscle, then markedly slow the pace. Salt plays at most a supporting role on the placebo plate.

Practical example: electrolyte strategy on race day

Concrete plan for a marathon with a target time of around 3:30 in mild weather: breakfast three hours before the start, salted as normal — bread rolls with a salted topping or salted porridge, plus 400 to 500 millilitres of fluid. During the race, carbohydrates come primarily from gels (60 to 90 grams per hour), sodium from isotonic sports drink at the aid stations: two to three cups of 150 millilitres each per hour deliver 100 to 250 milligrams of sodium at typical mixing strength — combined with sodium-containing gels you land in the target corridor of 300 to 500 milligrams per hour.

In heat above 20 degrees Celsius or with a known high salt loss, shift the strategy upward: visit stations more often, choose sodium-richer gels, add salt capsules in ultra events. The dress rehearsal matters: you train the exact combination of drink, gel and capsule during the long runs of the final eight weeks — fueling is part of marathon training, not improvisation at the start line. The days before count too: if you combine your carb loading with normally salted food, you store about three grams of water with every gram of glycogen — the best base hydration of all.

Common electrolyte mistakes

• Magnesium as cramp insurance. The study situation is clear: no reliable effect. The money is better spent on shoes, physio or simply better training.
• Salt tablets prophylactically on every run. Without high losses, excess sodium is ballast — the body must dilute and excrete it with additional water.
• Only plain water for hours. The most dangerous variant: drink a lot, add no salt — that is how hyponatremia develops. For efforts over 90 minutes, sodium belongs in the plan.
• Looking at the packaging instead of the label. "With electrolytes" says nothing about quantity. Under 300 milligrams of sodium per litre, a drink is underdosed for long sessions.
• Electrolytes without fluid. Salt capsules without water irritate the stomach and shift concentration in the wrong direction. Every capsule needs its 150 to 250 millilitres.

How Peakora plans your fueling

The fueling calculator in Peakora factors sodium in directly: based on your estimated race time, you get an hourly plan for carbohydrates, sodium and water — matched to race duration and the typical aid-station spacing of your goal event. So on race day, not only your pace is set, but also how much salt goes into the cup — and during the long runs you rehearse your fueling for weeks.

Frequently asked questions about electrolytes for running

Do I need electrolytes on every run?

No. For runs under 60 to 90 minutes, water and a normally salted diet are completely sufficient. Extra sodium only pays off on long runs over 90 minutes, in hot conditions, or if you are a salty sweater — recognizable by white salt stains on your clothing.

Does magnesium help against muscle cramps while running?

No, that is largely a myth. Studies show no reliable link between magnesium levels and cramps. Exercise-associated cramps are mainly caused by neuromuscular fatigue — better prevention comes from realistic pacing, strength training and enough carbohydrates.

How much sodium should I take per hour during a marathon?

A framework of 300 to 800 milligrams of sodium per hour applies for efforts over 90 minutes. Heavy salty sweaters and athletes in hot conditions may need up to 1,000 milligrams per hour. The exact amount depends on your sweat rate and the salt concentration of your sweat.

Can I make my own electrolyte drink?

Yes, easily: per litre of water add 1 to 1.5 grams of salt — about a quarter teaspoon — plus 40 to 60 grams of sugar and a squeeze of lemon juice. That yields roughly 400 to 600 milligrams of sodium per litre plus carbohydrates for fueling, at a fraction of the price of branded products.