Dehydration vs Hyponatremia: The Two Hydration Risks Every Endurance Athlete Must Know

For years the advice shouted along race courses was simple: drink as much as you can. Then researchers in Singapore weighed 1,887 runners before and after the Standard Chartered Marathon, the Sundown Marathon and The North Face 100.

Most finished within a healthy range. But 21% finished dehydrated, and 6% crossed the line heavier than they started, which means they drank more than they lost. Drinking too little and drinking too much can both end your race. Here is how to tell them apart, and how to stay between the two.

Dehydration vs Hyponatremia at a Glance

Dehydration: When You Fall Behind

Dehydration happens when you sweat out more than you drink, and in Singapore's heat the litres add up quickly. Most athletes sweat between half a litre and two litres an hour, and endurance athletes average 1.28 litres.

• The Line to Watch: The American College of Sports Medicine says drinking during exercise should stop you losing more than 2% of your body weight, to protect your performance.
• Some Loss Is Normal: Modest dehydration poses little risk to healthy people, and fluid losses of up to about 3% of body mass are generally well tolerated.

Hyponatremia: The Risk of Drinking Too Much

Exercise-associated hyponatremia (EAH) is diagnosed when blood sodium falls below 135 mmol/L during or soon after exercise. The main causes are drinking beyond thirst and the hormones that make your body hold on to water while you exercise.

Why Electrolytes Alone Will Not Save You

Sports drinks count too. Researchers list them alongside water as fluids that cause EAH when you drink more than you lose. In a study of a 161km ultramarathon, hyponatremia was unrelated to how much sodium runners took in, and sodium supplements do not necessarily prevent it when someone overdrinks. Electrolytes help replace what you sweat out. They are not a licence to drink more.

Electrolytes help replace what you sweat out. They are not a licence to drink more.

Who Is Most at Risk

In the Singapore study, every runner treated for EAH in the medical tents had been racing 42km or further. Longer races mean more time on course and more chances to drink.

If Someone Looks Unwell

The symptoms of hyponatremia are nonspecific, so do not guess. If a runner becomes confused or disoriented late in a long race, do not hand them more water. Get them to the medical tent. Severe EAH is treated by medical staff with concentrated saline, not by drinking.

Three Rules for the Safe Middle

1. Drink to Thirst

Hyponatremia researchers advise drinking according to thirst, and say fixed fluid targets do not work because sweat and urine losses vary so much between people. Treat drink stations as an option, not an instruction.

2. Know Your Sweat Rate

Weigh yourself before and after a one-hour run, add back what you drank, and you have your hourly sweat rate. Our sweat rate guide takes you through it. Then aim to finish no more than 2% lighter, and never heavier.

3. Replace Sodium on Long, Hot Runs

Sodium is the electrolyte you lose most in sweat. On long sessions in the heat, adding it to your bottle helps replace what you lose, as long as it is not your reason to drink more than you need.

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Ready to understand what you lose besides water? Read Electrolytes 101 next.