Low sodium in runners and heavy water drinkers comes from drinking more water than the kidneys can clear

Low sodium in runners and heavy water drinkers usually comes from drinking more water than the kidneys can clear, which dilutes the sodium in your blood below 135 mmol/L. During long exercise, vasopressin, a hormone that tells the kidneys to hold on to water, can stay switched on, so they clear even less.

Doctors call this exercise-associated hyponatremia (EAH), and overdrinking water or sports drinks is the biggest risk. In the 2002 Boston Marathon, 13% of 488 runners tested at the finish had sodium of 135 mmol/L or less, and those who gained weight during the race had about four times the odds.

Heavy water drinkers hit the same limit, especially when they eat very little, because the kidneys need salt and the urea made from protein to carry water out.

Why know your sodium before anything goes wrong?

Low sodium isn't only an athlete's problem: in a US national survey, about 1.7% of adults had it. Your kidneys set that balance, and kidney disease is often silent. About 1 in 7 US adults have it, and only 12% of them know.

A sodium result reads best next to your kidney markers. Numia Health, a physician-ordered, at-home blood testing service in Greater Vancouver, measures sodium with creatinine and urea on its Electrolytes Panel, giving you a baseline to compare against later.

What sodium level is too low, and how does it feel?

When is low sodium an emergency?

Most low results, Numia's included, mean a talk with your doctor. Race day is different:

• Call 9-1-1 for vomiting, confusion, a seizure, collapse or trouble breathing during or within 24 hours of a long event or heavy water drinking.
• Don't treat it with more fluid. Hospital teams restrict fluids and may give a concentrated salt solution by IV.
• For milder symptoms such as headache or nausea, call HealthLink BC at 8-1-1.

Race-day symptoms need care on the spot, while a home panel like Numia's tracks your usual numbers. BC's sodium critical values are in our eGFR guide.

Where does Numia fit?

The usual route, publicly funded when clinically indicated, runs through an appointment, a lab visit, a wait and a follow-up. In BC, a request for "electrolytes" means sodium and potassium only, unless more is requested. Numia Health keeps it in one place: a licensed Canadian physician approves your requisition (lab order) online in about 3 minutes, then a BC-licensed clinician does a 10-minute morning draw at your home. You get sodium, potassium, chloride, creatinine and urea with no fasting, plain-English results in the Numia app in 7 days, and the Numia Companion, an AI that has read them, to answer questions. Out-of-range results are physician-reviewed with next steps, and an optional wellness consultation is available. Numia is an HSA-eligible service.

Frequently asked questions

What causes exercise-associated hyponatremia?

Mostly overdrinking: taking in more water or sports drink than you lose, while vasopressin keeps the kidneys holding on to water.

Can drinking too much water every day lower your sodium?

Yes, if intake outruns what your kidneys can clear: roughly 10–20 litres a day on a normal diet, less when you eat very little. Compulsive drinking (psychogenic polydipsia) can push sodium very low. Numia's panel pairs sodium with kidney markers for context.

How can runners avoid low sodium?

Drink to thirst, as the Wilderness Medical Society advises, and weigh yourself before and after long runs: a gain means you drank more than you lost. Salt tablets and sports drinks won't prevent it if you overdrink.