Electrolytes 101: Sodium, Potassium, and Why Plain Water Isn't Enough

"Stay hydrated and drink plenty of water" is the oldest advice in sport. Then you finish a Sunday long run along East Coast Park with white salt lines drying on your cap, and it is obvious that sweat is not just water.

Every litre you sweat takes sodium with it, and in Singapore's heat the litres add up fast. Here is what electrolytes actually do, how much sodium runners lose, and where plain water stops being enough.

Photo: SportPlus.sg / Isis Ang

What Runners Lose in Sweat

Why Sodium Matters Most

Electrolytes are minerals that carry an electrical charge in your body fluids. Sweat carries several of them, but one does most of the work.

• Sodium: It is the electrolyte you lose in the greatest amounts, and the one with the biggest effect on your fluid balance, because it helps your body hold on to water and keep your blood volume up.
• Potassium: You lose it too, but at much lower concentrations, close to the levels already in your blood.
• Your Own Mix: How much you sweat, and how salty it is, changes with effort, weather, heat acclimation, fitness and body size. Two runners on the same route can lose very different amounts.

Sweat sodium ranges from about 10 to 90 mmol per litre. The saltiest sweaters lose many times more than the least salty.

When Plain Water Stops Being Enough

On long, hot sessions you can sweat a litre or more an hour, and a meaningful amount of sodium goes with it. The American College of Sports Medicine says drinking during exercise should prevent both excessive dehydration and big swings in electrolyte balance, and that drinks with electrolytes and carbohydrate can beat water alone in some situations. Because everyone sweats differently, it recommends an individual plan rather than one rule for all.

What Electrolytes Will Not Do

Here is the catch: electrolytes do not protect you from drinking too much. Exercise-associated hyponatremia, where blood sodium falls below 135 mmol/L, is driven mainly by drinking beyond thirst. In a study of a 161km ultramarathon, it was unrelated to how much sodium runners took in, and sodium supplements do not necessarily prevent it when someone overdrinks.

The symptoms can include confusion, headache, lethargy and, in severe cases, seizures. If a runner seems confused late in a long race, get them to the medical tent.

How to Find Your Own Numbers

• Your Sweat Rate: Weighing yourself before and after a run is the simplest and most accurate way to measure it. Our sweat rate guide walks you through the test.
• Your Sweat Sodium: A sweat test is the only way to know your number. Until you have one, treat your long, hot runs as the sessions that need sodium most.

Dial In Your Sweat Balance with SUPERnatural+ on ATHLO+

SUPERnatural+ SUPERsalty puts 1,000mg of sodium, 200mg of potassium and 60mg of magnesium in a single sachet, close to the average hourly sodium loss in the table above. Each sachet is sized for one 500ml SUPERflask, a soft flask that shrinks as you drink. The full range is in the SUPERnatural+ collection.

Ready to work out how much you sweat? Read our guide to calculating your sweat rate next.