The Science of Hydration for Runners: Fluids and Electrolytes

A breakdown of the importance of fluids and electrolytes along with practical tips and sports nutrition products recommended by a sports dietitian.

As a runner, it's no secret that staying properly hydrated is essential for optimal performance, but with so much conflicting information out there, it can be hard to know how much water you really need. Do you need more fluids than a non-runner? What about on a long run or hard workout day? Should you weigh yourself before and after your run to figure out how much fluid you've lost?

In this blog post, we'll explore research on hydration for runners, including how much water you need, how to monitor your hydration status, and the best strategies for staying hydrated during training and racing, as well as day to day.

Why hydration matters for runners

The risk of dehydration in runners can depend on the intensity and duration of the run, as well as environmental conditions and other factors. Even just mild dehydration (2% fluid loss) can impair performance and increase the risk of injury.

In general, the body is really good at maintaining hydration, but when exercising, especially endurance exercise (aka sweaty exercise), it becomes more difficult and simply relying on thirst may not be enough, especially during sessions lasting 90 minutes or longer.

Us runners often think we know our bodies well, but some research found that athletes' perception of sweat losses tend to be pretty far off, with most underestimating their volume of sweat lost by ~43%. So we really can't rely on intuition and feeling it out.

So you might be thinking drinking as many fluids as possible is the goal, but hydration isn't just about fluids, and it's possible to overdo it.

Exercise-associated hyponatremia (EAH) can develop when fluids are consumed at a rate greater than the sweat lost and without sufficient sodium, which is the main electrolyte lost through sweat. EAH can lead to symptoms like headaches and nausea, or in severe cases, seizures and even death. Yikes.

This isn't something that only happens in the elite field either. It's often seen in new, less experienced runners and it's more common in female runners.

AND a lot of runners don't know much about it. One study asked 1,000 marathon runners about their knowledge of EAH and found that only 37% were aware of the symptoms and only 25% knew that drinking too much fluid could lead to it.

So, a well-hydrated runner is consuming adequate amounts of both fluids and electrolytes.

How much water do runners really need?

Fluid needs are highly individualized and will depend largely on an individual's sweat rate, which can vary day to day and across intensities, environment, body size, etc.

When it comes to overall, day-to-day hydration, monitoring urine color will give you an idea of how you're doing. Ideally, urine should be pale yellow. Anything darker than that, it's likely time to up the fluids and if it's completely clear, maybe back off a bit.

There are some good general guidelines, like 3L per day for active women and 4L per day for active men, but it's best to have an idea of your sweat rate if you really want to nail down hydration.

Remember, hydration is not just fluids, it's electrolytes (mainly sodium) too.

The classic method for determining your sweat rate would be:

Weigh yourself completely naked and note the number.
Head out for your run and keep track of the ounces of fluids you consume, if any.
When you're done, strip back down naked and weigh yourself again.
Determine the loss by subtracting your second weight from the first and factor in any ounces you consumed on the run.
Then consume at 16-24 ounces of fluids per pound lost.

No one is realistically going to do this, but it's not a bad idea to give it a go once or twice so you have an idea and can estimate your fluid needs in the future.

Keep in mind, this number will vary day to day too, based on your intensity level, duration of exercise, conditions, etc. So if you really want to be on top of your sweat rate, test it in various conditions.

You can also opt for a sweat monitor like the Nix Hydration Biosensor,hDrop Sensor

or the Gx Sweat Patch.

On average, an athlete's sweat rate is 0.5–2.0 L per hour and 900mg of sodium per L.

What if you're a salty sweater?

If you're what we'd consider a "salty sweater", this means you lose more sodium through sweat than someone who is a less salty sweater.

You can determine this by a sweat test or using a sweat patch if you're into the numbers, but there are a few telltale signs as well.

You're likely a salty sweater if:

Your sweat stings your eyes
You clothes and skin have a white cast on them after your run
Your sweat tastes really salty (obviously, right)
You often feel nauseous after a sweaty run
Heat intolerance
Feeling lightheaded or dizzy on the run

We do lose small amounts of other electrolytes through sweat as well, but these losses don't have such a profound effect on performance and they are much smaller than that of sodium.

So if you're a salty sweater, you will need to be consuming more sodium along with your fluids. You might also want to try hyperhydrating by consuming an electrolyte drink (or simply adding more electrolytes to your diet) along with fluids the night before a long run, race, or intense sweaty run.

Hydration strategies for runners

Hydration starts long before the run. This means drinking plenty of fluids regularly throughout the day.

Here are some general guidelines that work for most runners, but remember, needs vary based on individual sweat rates and composition.

1-2 hours before your run: aim to drink 8-24 ounces of fluid, adding in electrolytes if you know yourself to be a salty sweater.
During the run: at least 4 ounces of fluids every 15 minutes (this equates to 16-24 ounces per hour) for runs in hot and humid weather and/or lasting over 1 hour. Include at least 300mg of sodium per hour in humid conditions and runs lasting over an hour. Sodium needs on the run may be upwards of 800mg if you're a very salty sweater.
After the run: aim to replace fluid losses, starting with 16-24 ounces of water. Including sodium and carbohydrates will help speed up rehydration (plus the carbs are important for replenishing glycogen lost during training). Continue drinking fluids to replace the total amount lost on the run.