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Electrolytes for athletes: why sodium, potassium and their team matter

We break down what electrolytes are, how the kidneys keep them balanced, and why on a long course it matters not only to drink water but also to replace salts.

AL
Andrey Leskov

The thirtieth kilometer of a marathon: the pace is still holding, but your calves cramp so hard you want to sit right down on the asphalt. Or a different picture: a hot bike race, you dutifully drank two bottles of plain water — and by the finish you feel queasy, your head is spinning, and somehow things have only gotten worse. In both cases electrolytes are working (or rebelling) behind the scenes. This isn't a marketing word on a sports-drink label, but very concrete physiology. Let's dig in.

What electrolytes are and what they do

By the Merck Manual's definition, electrolytes are minerals that carry an electric charge when dissolved in a fluid, such as blood. It's exactly that charge that turns ordinary salt into an "electrical" signal on which the body's work depends.

The classic set of blood electrolytes checked in lab tests is sodium, potassium, chloride and bicarbonate. Their work, according to the reference, comes down to three big tasks:

  • regulating the function of nerves and muscles;
  • maintaining acid-base balance;
  • keeping the body's water balance.

Sodium plays the key role here: it helps the body maintain a normal level of fluid in the body's water compartments. For an athlete this translates directly: the nerve impulse that makes a muscle contract, and the stability of the internal environment during hours of work, depend on these minerals being in the normal range.

To this quartet an athlete should add calcium and magnesium — also electrolytes, closely tied to muscle contraction and the transmission of nerve signals. They aren't part of the standard "electrolyte" blood panel, but to understand why muscles work smoothly instead of twitching, they're worth keeping in mind.

Who keeps the balance: the kidneys

Electrolytes can't simply be "stockpiled for later." Their concentration must stay within a narrow corridor, and the chief conductor here is the kidneys. The Merck Manual describes the mechanism like this: the kidneys filter electrolytes and water out of the blood, return some of it, and excrete the excess in urine. In essence, every day they balance what we take in from food and drink against what leaves in urine.

An important detail that explains half the problems on the course is osmosis. If the concentration of electrolytes in a compartment is high, fluid moves toward it; if it's low, fluid moves out. In other words, water in the body follows the salts, not the other way around. Hence a simple conclusion: pouring in water without electrolytes means diluting sodium and driving fluid to the wrong place.

What you lose in sweat

The reference lists the causes that throw the balance off: dehydration or, conversely, an excess of water, certain medications, heart, kidney and liver diseases, incorrect volumes of intravenous fluids. For a healthy athlete the main one of these is water: both too little and too much.

With sweat we lose sodium first and foremost (the salty taste of sweat and the white streaks on your cap — that's it). The longer and hotter the effort, the greater the total losses. And here amateurs make a classic mistake: they drink lots of plain water, thinking "the main thing is not to get dehydrated." But if you replace only water and don't return sodium, the sodium concentration in the blood falls — this condition is called hyponatremia, and in ultramarathons it's more dangerous than mild dehydration.

How to put it into practice

  • Don't drink "empty" on long distances. Anything longer than roughly an hour to an hour and a half of intense work in the heat is a reason to add electrolytes to your drink, not just water.
  • Know your sweat. Salty streaks on your clothes and face are a sign that you're losing a lot of sodium and need a "saltier" drinking plan.
  • Plan, don't scramble to rescue yourself. Estimate your fluid losses in advance (a water-loss calculator helps here) and drink to a plan, not by thirst, which lies during a race.
  • Don't treat every cramp with magnesium. Cramps have many causes — fatigue, pace, salts — and one magic pill doesn't close the problem. Work with your overall drinking and nutrition strategy.
  • The "more water is always better" myth is dangerous. Overdrinking is just as real as underdrinking, and the consequences can be more serious.

The main points

  • Electrolytes are minerals with an electric charge; the key ones in blood are sodium, potassium, chloride, bicarbonate, and for an athlete's muscles calcium and magnesium matter too.
  • They regulate the work of nerves and muscles, and acid-base and water balance.
  • The kidneys keep the balance: they filter, return some, excrete the excess in urine, reconciling intake and output.
  • Water in the body follows the salts (osmosis) — which is why not just water but the electrolytes in it matter.
  • Sweat carries away sodium above all; on a long, hot start replace salts, not just water, to avoid catching hyponatremia.

Source: Merck Manual — Overview of Electrolytes. https://www.merckmanuals.com/home/kidney-disorders/electrolyte-balance/overview-of-electrolytes

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