Sodium: the electrolyte that runs your body over the distance

The last hour of a marathon or a trail race is rarely lost by the legs. More often it's the chemistry that loses: thoughts get muddled, muscles cramp, water sloshes in your stomach but relief just won't come. Behind the scenes of this drama stands a single mineral — sodium. For the average person it's "the salt there's so much of in chips." For an endurance athlete it's the key electrolyte that the body loses by the liter in sweat — and that decides whether you finish smoothly or hit the wall.

What sodium does in the body

Sodium is an electrolyte, and it lives mainly in the blood and in the fluid around the cells. It has two key jobs.

The first is to keep fluid in balance. The amount of sodium in the body directly sets the blood volume: where the sodium goes, water gets "pulled" along. The second job is to ensure the normal functioning of nerves and muscles. It's precisely the sodium gradient across the two sides of the cell membrane that creates the electrical impulse that makes a nerve pass a signal and a muscle contract. When that gradient falls short, the signal gets distorted: hence the cramps and the "fog" in your head.

Sodium enters the body with food and drink, and it leaves mainly with sweat and urine. And here the athlete is a special case: for the average person the main channel of loss is urine, but for us in prolonged heat it's sweat, and the losses can be huge.

How the body keeps balance on its own

The body doesn't leave sodium to chance — it's a finely tuned system.

When sodium (and blood volume) becomes too much, sensors in the heart, blood vessels, and kidneys catch it and command the kidneys to excrete sodium more actively in the urine. When the opposite happens — the level drops — two hormones kick in:

Aldosterone (from the adrenal glands) — makes the kidneys retain sodium and shed potassium, lowers urine volume, and raises blood volume.
Vasopressin (ADH) — makes the kidneys conserve water.

On top of all this sits thirst — a crude but important signal. An important nuance: thirst isn't a perfect sensor. With age its sensitivity declines, as does the kidneys' ability to concentrate urine. Don't rely blindly on "I'll drink when I feel like it": over the distance, thirst often lags behind.

Too much, too little, and "salty sweaters"

The extremes have simple names. Hypernatremia — too much sodium in the blood; in its severe form it's confusion, coma, and death. Hyponatremia — too little sodium.

For the athlete, the second is more dangerous. The classic scenario: on a long, hot race a person fears dehydration and pours plain water into themselves by the liter. Sweat carries away both water and salt, but only water goes in — it dilutes the blood, and the sodium concentration drops to < normal. The head goes hazy, nausea sets in, and in severe cases it's genuinely dangerous. The paradox: the person "drank enough" and collapsed precisely because of drinking.

A separate topic is "salty sweaters." If after a workout there are white streaks on your temples and cap, and your skin feels like it's been rubbed with salt — you're most likely losing more sodium in your sweat than the person next to you at the start. This is neither a myth nor a weakness: people really do have different sweat "saltiness." For such athletes, plain water on the distance is categorically not enough.

How to put it into practice

Drink to a plan, not to panic. For efforts longer than ~1–1.5 hours, especially in the heat, water should come together with sodium — an isotonic drink, salt capsules, or salted fluid, not just plain water.
Assess your losses. Weigh yourself before and after a long workout: the weight difference is mostly lost fluid. The water-loss calculator below will help you estimate the volume.
Look for the "salty" signs. White streaks on your kit, a salty taste on your lips, cramps in the second half of the distance — a signal that you need more sodium.
Don't overdo the water. More isn't always better. If your weight went up after the race instead of down, you overdrank.
Rehearse it in training. You can't test your salt strategy for the first time at the start.

The bottom line

Sodium keeps water in the body and powers the work of nerves and muscles — it's not "harmful salt" but a working tool.
It leaves mainly with sweat and urine, and for an athlete in the heat, sweat is the main channel of loss.
Balance is regulated by the kidneys, aldosterone, vasopressin (ADH), and thirst, but thirst is an inexact sensor.
Excess and deficiency are both dangerous; for an athlete, a common trap is hyponatremia from too much plain water.
"Salty sweaters" need to top up sodium deliberately — plain water isn't enough for them.

Source: Merck Manual — Overview of Sodium's Role in the Body. https://www.merckmanuals.com/home/kidney-disorders/electrolyte-balance/overview-of-sodium-s-role-in-the-body