Why Sodium Matters More Than You Think

Ask most people what hydration means and they will say water. Ask a sports physiologist and they will talk about sodium. The two answers are connected, and understanding how is the quickest way to make better decisions about what you drink when you train, travel, or spend time in the heat.

What sodium actually does

Sodium is the main electrolyte in the fluid outside your cells. Electrolytes are minerals that carry an electrical charge when dissolved, and sodium's charge is part of what lets nerves fire and muscles contract. Sodium also strongly influences how much water your body holds in the blood and surrounding tissues. When sodium is low relative to water, your body tends to make more urine. When it is adequate, the fluid you drink is retained more effectively.

What sweat takes with it

Sweat is mostly water with dissolved salts. Sodium and chloride make up most of those salts, with smaller amounts of potassium, magnesium, and calcium. The amount of sodium in sweat varies enormously. Published reviews report a range of roughly 200 to 2,000 milligrams of sodium per liter of sweat, and sweat rates during exercise commonly fall between about 0.5 and 2 liters per hour, higher in hot conditions.

That means two people finishing the same hour-long session in the same room can have lost very different amounts of sodium. It also explains why some people finish a workout with white salt marks on their clothes or hat, and others never do.

Why plain water can fall short

For short, easy sessions, water is enough and your normal diet restores the rest. The picture changes during long or hot efforts. If you sweat heavily for hours and replace the loss with only plain water, blood sodium can be diluted. In endurance events this can progress to exercise-associated hyponatremia, a condition of low blood sodium that can be serious.

The international consensus statement on the condition is clear that its main cause is drinking more than you lose, and its main advice is to drink according to thirst rather than on a fixed schedule. Replacing sodium through food or a drink can help offset what sweat removes, but it does not make overdrinking safe.

A practical way to think about it

• Under about an hour at moderate effort: water is usually enough.
• Long sessions, heat, or heavy sweating: include sodium from food or a drink, and drink to thirst.
• Salt crusting on your skin or clothing: you may be a saltier sweater than average.
• Weigh yourself before and after a hard session. Each kilogram lost is roughly a liter of fluid, which gives you a rough idea of your sweat rate.

Those are starting points, not rules. Needs depend on the person, the conditions, and what else you eat.