You Know How Much to Drink. Now How Much Sodium Do You Actually Need?

A practical sodium guide for marathon and ultramarathon runners

You've already measured your sweat rate.

Perhaps you've discovered that under race-like conditions you sweat:

1.0 litre per hour

Now another question appears:

How much sodium am I losing, and how much of it should I replace?

This is where endurance hydration gets a little more interesting.

Because two runners can both sweat 1 litre per hour and lose completely different amounts of sodium.

One runner may lose only a few hundred milligrams.

Another may lose well over 1,000 mg.

The research from Sawka et al. 2007, Baker 2017, and Barnes et al. 2019 helps us understand why, and gives us a practical way to build a sodium strategy without turning every long run into a chemistry exam.

First: sweat volume and sweat saltiness are different things

Your sweat rate answers:

How much fluid am I losing?

Your sweat sodium concentration answers:

How salty is that fluid?

You need both if you want to estimate your sodium loss accurately.

Baker's 2017 review found enormous variation in sweat sodium concentration among athletes. Whole-body sweat sodium was roughly:

10–70 mmol/L

That works out to approximately:

230–1,610 mg of sodium per litre of sweat

Localized measurements can range even wider.

That's a remarkable range.

Imagine two runners who each sweat:

1 L/hour

Runner A's sweat contains:

400 mg sodium/L

Runner B's contains:

1,200 mg sodium/L

They have exactly the same sweat rate.

But Runner B is losing three times as much sodium every hour.

So sweat rate alone cannot tell you your sodium requirement.

The three numbers you need

Think of your sodium strategy as three boxes:

1. How much do you sweat?

For example:

1.2 L/hour

2. How salty is your sweat?

For example:

800 mg sodium/L

3. How much sodium does that mean you're losing?

If every litre contains 800 mg sodium and you're losing 1.2 litres each hour:

800 mg + another 20% = approximately 960 mg/hour

So your estimated sodium loss is:

about 960 mg of sodium per hour

No complicated equation needed.

You're simply asking:

"How many litres am I losing, and how many milligrams are hiding in each litre?"

A few examples

If your sweat sodium is 500 mg/L:

• sweating 0.5 L/h means losing about 250 mg sodium/h
• sweating 1.0 L/h means losing about 500 mg/h
• sweating 1.5 L/h means losing about 750 mg/h

If your sweat sodium is 1,000 mg/L:

• 0.5 L/h means about 500 mg/h
• 1.0 L/h means about 1,000 mg/h
• 1.5 L/h means about 1,500 mg/h

And if you're both a heavy sweater and a salty sweater, the numbers can climb quickly.

That's exactly what Barnes and colleagues saw in athletes.

What Barnes 2019 adds

Barnes et al. analysed sweat data from 1,303 athletes.

They didn't just look at how much athletes sweated.

They combined:

whole-body sweat rate

with:

sweat sodium concentration

to estimate:

rate of sweat sodium loss

Endurance athletes averaged approximately:

1.28 L of sweat per hour

and about:

51.7 mmol of sodium loss per hour

which converts to roughly:

1,190 mg sodium per hour

But the variation was enormous. The standard deviation for sodium loss in the endurance group was about 640 mg/hour.

That's the useful lesson.

The average endurance athlete in Barnes's dataset lost around 1.2 grams of sodium every hour.

That does not mean every marathon runner should take 1.2 grams of sodium every hour.

It means population averages are poor substitutes for individual measurements.

Here's the crucial part: loss is not the same as intake

Suppose you've calculated:

I lose about 1,000 mg sodium/hour.

It's tempting to conclude:

"Then I should consume 1,000 mg every hour."

But the research does not give us a universal rule saying that sodium lost through sweat must be replaced 100% during exercise.

That's the same distinction we made with fluid:

Sodium loss ≠ sodium intake

Your body contains a substantial sodium reservoir.

You're also likely consuming sodium in drinks, gels, bars and real food.

And unlike a laboratory experiment, your marathon or ultra eventually ends, after which sodium can continue to be replaced through normal eating and drinking.

Barnes actually makes this distinction in discussing recovery: complete restoration of sodium and fluid balance is important after exercise, particularly when athletes have large losses or little recovery time before another session.

So don't imagine the body as a bank account where every milligram leaving your skin has to be deposited immediately.

What does Sawka 2007 recommend?

Sawka and colleagues took a more practical approach.

Their ACSM position stand recommends individualized fluid and electrolyte strategies because both sweat rate and sweat sodium losses vary substantially between athletes.

For prolonged exercise, the guidance associated with this position stand is commonly expressed as a drink containing roughly:

20–30 mmol sodium per litre

which is approximately:

460–690 mg sodium per litre of drink

This is important.

Notice that the recommendation is expressed as:

mg per litre of fluid

not:

mg per hour

Those are very different numbers.

Sodium concentration versus sodium per hour

Suppose your drink contains:

600 mg sodium/L

If you drink:

500 mL/hour

you're consuming:

300 mg sodium/hour

If you drink:

750 mL/hour

you're consuming:

450 mg/hour

If you drink:

1 L/hour

you're consuming:

600 mg/hour

Same drink.

Very different hourly sodium intake.

This is why a product advertising:

"600 mg sodium"

doesn't tell you much until you know whether that's per tablet, per serving, per bottle or per litre.

For endurance athletes, always ask:

How many mg of sodium am I actually consuming each hour?

Let's put fluid and sodium together

Imagine you have already measured:

Sweat rate: 1.0 L/hour

Your sweat test tells you:

Sweat sodium: 900 mg/L

So you're losing approximately:

900 mg sodium/hour

From the previous hydration article, you've determined that drinking roughly:

600 mL/hour

works well for you.

You choose a drink containing:

600 mg sodium/L

At 600 mL/hour, that drink gives you:

360 mg sodium/hour

You're losing approximately:

900 mg/hour

and consuming:

360 mg/hour

So you're replacing only part of your sodium loss during the run.

And that isn't automatically a problem.

The three papers do not tell us that this athlete should immediately increase sodium intake to exactly 900 mg/hour.

They tell us that knowing the athlete's losses can help individualize the strategy.

There is no scientifically established "replace X%" rule

This deserves its own section because it's easy to turn sweat testing into false precision.

You may see advice saying:

"Replace 50% of sodium losses."

or:

"Replace 70–80%."

or:

"Match your sweat sodium losses exactly."

Those numbers may be used as practical strategies in particular situations, but Sawka 2007, Baker 2017, and Barnes 2019 do not establish a universal percentage that every endurance athlete should replace during exercise.

Barnes tells us what athletes lose.

Baker tells us how to measure those losses and why they vary.

Sawka gives us a practical electrolyte-containing beverage range and argues for individualized replacement.

None provides a magic:

Sodium loss × 0.73 = perfect race intake

Unfortunately, physiology declined to make the spreadsheet that tidy.

So what should you do if you DON'T have a sweat sodium test?

Most runners don't.

That's fine.

You can still build a reasonable plan.

For prolonged endurance exercise, a sensible starting point based on the Sawka framework is a drink containing approximately:

500–700 mg sodium per litre

Then combine that with the drinking range you've already established through sweat-rate testing.

For example, if your hydration plan is approximately:

600 mL/hour

then a drink containing:

500–700 mg/L

would provide approximately:

300–420 mg sodium/hour

If you drink:

800 mL/hour

the same drink provides approximately:

400–560 mg/hour

This gives you a starting point to test in training.

It isn't a claim that everyone *needs* exactly this amount.

It is simply a practical starting concentration supported by established exercise-hydration guidance. Later heat guidelines have continued to recommend roughly 0.5–0.7 g sodium/L during exercise lasting more than an hour.

What if you know you're a salty sweater?

You've probably seen the runner.

Black shirt at the start.

Abstract white salt painting at the finish.

Visible salt deposits can tell you that substantial sodium is leaving the body, but they cannot tell you whether your sweat contains:

700 mg/L

or:

1,400 mg/L

For that, you need an actual measurement.

If you regularly race long events, sweat heavily, train in hot conditions or repeatedly finish sessions covered in salt, a properly performed sweat sodium test can be useful.

Baker's review specifically concludes that sweat testing can help guide individualized fluid and electrolyte strategies, provided that the measurement is collected and interpreted correctly.

If you DO have a sweat sodium test

Let's say your test reports:

1,100 mg sodium/L

And you've measured your race-condition sweat rate at:

1.3 L/hour

Your estimated sodium loss is therefore roughly:

1,100 mg × 1.3

or about:

1,430 mg sodium/hour

Now you know something valuable.

You are probably losing substantially more sodium than someone whose sweat contains 400 mg/L.

That information may justify a more deliberate sodium strategy, particularly during:

• long races
• hot races
• high-volume training
• stage races
• events with short recovery periods

But again:

1,430 mg lost does not automatically mean 1,430 mg must be swallowed each hour.

Think of your measured loss as information that helps define the scale of the problem, not as an automatic prescription.

Be careful with sweat sensors and patches

This is one of Baker's most important practical warnings.

A patch attached to your forearm measures:

forearm sweat

It doesn't directly scoop up every drop produced by your entire body.

Sweat sodium concentration differs between anatomical sites.

Baker reports localized sweat sodium measurements of roughly 10–90 mmol/L, compared with whole-body values closer to 10–70 mmol/L. The result also depends on collection technique, skin cleaning, timing, evaporation, storage and laboratory analysis.

Barnes used regional absorbent patches too, but importantly, those measurements were normalized to estimated whole-body sweat sodium concentration before sodium-loss rates were calculated.

So if a commercial sweat test tells you:

"Your sodium concentration is 1,200 mg/L."

ask:

Is that the local measurement, or has it been validated or corrected to estimate whole-body sweat sodium?

That small methodological detail can make a surprisingly large difference.

Your sodium number also changes

Just as you don't have one permanent sweat rate, you don't necessarily have one immutable sweat sodium concentration.

Baker reviews several factors that can alter sweat rate and/or sweat sodium concentration, including:

• exercise intensity
• environmental conditions
• heat acclimation
• hydration status
• diet
• body characteristics
• training status

So a sweat test performed during an easy indoor run in winter isn't necessarily the final word on what happens halfway through a hot summer ultramarathon.

The goal is not laboratory perfection.

It's to get close enough to make sensible decisions.

The ultramarathon problem

Sodium becomes particularly interesting as race duration increases.

Imagine you're losing:

1,000 mg sodium/hour

During a four-hour marathon, that's approximately:

4 grams sodium lost

During a twelve-hour ultra:

12 grams

During a twenty-four-hour race:

24 grams

Again, you don't necessarily need to replace every gram while racing.

But the longer the event lasts, the harder it becomes to simply ignore large cumulative sodium losses.

This is where an ultrarunner with high sweat rate and high sweat sodium may have very different needs from a smaller athlete racing slowly in cool conditions.

Barnes's findings are particularly relevant here because endurance athletes were among the groups with the highest sweat rates and highest sodium-loss rates in their dataset.

Don't forget the sodium in food

Ultramarathon runners have another advantage over marathon runners:

you eat.

Potentially quite a lot.

Soup.

Broth.

Sandwiches.

Pretzels.

Potatoes with salt.

Sports drink.

Gels.

Bars.

Aid-station food.

All of this contributes sodium.

Suppose your plan contains:

350 mg/hour from sports drink

plus:

200 mg/hour from gels

plus:

300 mg/hour from food

Your total isn't 350 mg.

It's:

850 mg sodium/hour

This matters enormously when evaluating a sodium strategy.

Don't count capsules while ignoring everything you're eating.

Build a sodium budget

For one representative race hour, write down everything you expect to consume.

For example:

600 mL sports drink: 360 mg sodium

2 gels: 200 mg sodium

Small amount of aid-station food: 150 mg sodium

Total:

710 mg sodium/hour

Now compare that with your estimated sweat sodium loss.

Perhaps you're losing:

1,100 mg/hour

So you're replacing a substantial portion, but not all, of what you're losing.

Then test the strategy during long training sessions.

That's considerably more useful than choosing salt tablets first and trying to build the rest of your nutrition around them.

Read labels carefully: sodium is not the same as salt

This causes endless confusion.

Sodium and table salt are not interchangeable numbers.

Table salt is sodium chloride.

Approximately:

1 gram of sodium = 2.5 grams of salt

So:

500 mg sodium ≈ 1.27 g salt

1,000 mg sodium ≈ 2.54 g salt

If your sweat test says you lose:

1,000 mg sodium/hour

that does not mean you lose only 1 gram of table salt.

It's equivalent to roughly:

2.5 grams of salt per hour

When comparing electrolyte products, use the number labelled:

SODIUM

not the total weight of "electrolytes" or "salt."

More sodium does not protect you from drinking too much

This deserves flashing lights around it.

One of the most dangerous misconceptions in ultrarunning is:

"As long as I take enough electrolytes, I can drink as much water as I want."

No.

Exercise-associated hyponatremia is strongly associated with excessive fluid intake.

Later clinical guidelines reviewing the evidence found that sodium supplementation may reduce the fall in blood sodium under some circumstances, but it does not prevent hyponatremia when an athlete continues to overdrink. Fluid intake has a larger effect on blood sodium concentration than sodium supplementation.

This makes sense when you remember that blood sodium is a concentration.

Imagine making soup.

You can add salt.

But if you keep pouring litres and litres of water into the pot, the soup still becomes diluted.

Your body is rather more sophisticated than soup, thankfully, but the principle is useful.

Electrolyte capsules are not permission to overdrink.

Sodium isn't insurance against every muscle cramp either

Salt has acquired a heroic reputation in endurance sport.

Cramp?

Salt.

Tired?

Salt.

Hot?

More salt.

But muscle cramping is complicated, and modern heat-sport guidance recognizes that many exercise-associated cramps are predominantly associated with neuromuscular fatigue rather than simply being caused by sodium deficiency. Sodium depletion may contribute in particular circumstances, especially in athletes with very large losses during prolonged exercise in the heat.

So repeatedly cramping doesn't automatically prove that you need giant doses of sodium.

Look at:

• pacing
• muscular fatigue
• training
• heat
• hydration
• carbohydrate intake
• sodium losses

rather than blaming the salt shaker immediately.

A marathon example

Meet Mark.

Body weight: 72 kg

Marathon sweat rate: 1.0 L/hour

Expected race time: 3 hours 45 minutes

From his hydration testing, Mark plans to drink:

600 mL/hour

He doesn't have a sweat sodium test.

So he begins with a drink containing approximately:

600 mg sodium/L

At 600 mL/hour, that supplies:

360 mg sodium/hour

His gels add another:

150 mg/hour

So his total intake is approximately:

510 mg sodium/hour

He tests this strategy during several long marathon-specific sessions.

No laboratory cosplay required.

A high-sodium ultrarunner example

Now meet Emma.

Her sweat testing suggests:

Sweat rate: 1.2 L/hour

Whole-body-equivalent sweat sodium: 1,000 mg/L

So her estimated loss is:

1,200 mg sodium/hour

Her tested fluid plan averages:

700 mL/hour

Her drink provides:

700 mg sodium/L

At 700 mL/hour that's:

490 mg sodium/hour

Food and gels contribute another:

300 mg/hour

So her approximate total intake is:

790 mg/hour

Her estimated loss is:

1,200 mg/hour

Her intake is:

790 mg/hour

Is that scientifically guaranteed to be perfect?

No.

But now Emma has something useful:

• measured sweat rate
• measured sweat sodium
• estimated sodium loss
• known sodium intake
• a strategy she can test under race-like conditions

That's individualized hydration.

If you don't have testing, start simple

For most marathon runners, you don't need to begin by buying sweat sensors and a sack of electrolyte capsules.

Start with what you already know.

Step 1

Measure your sweat rate.

Step 2

Build your fluid plan.

Step 3

For prolonged exercise, consider a drink around:

500–700 mg sodium/L

as a reasonable research-based starting range.

Step 4

Count sodium from gels, food and other products.

Step 5

Test the entire strategy during long runs.

Step 6

If you appear to have unusually high sodium losses, race very long events, train heavily in heat, or simply want more individualized information, consider properly conducted sweat sodium testing.

That takes you from guessing to measuring without pretending that measurement gives you an exact prescription.

If you have sweat testing, go one step further

If you know both:

your sweat rate

and:

your whole-body-equivalent sweat sodium concentration

calculate your estimated sodium loss per hour.

For example:

Sweat rate: 1.4 L/hour

Sweat sodium: 900 mg/L

Estimated loss:

about 1,260 mg sodium/hour

Now compare that with your actual race nutrition.

If you're consuming only:

200 mg/hour

you know you're replacing relatively little.

If you're consuming:

800 mg/hour

you're replacing considerably more.

If you're consuming:

1,500 mg/hour

you're actually consuming sodium faster than your estimated sweat loss.

The sweat test gives you context.

It doesn't give you a command.

Don't chase an exact number

Your sweat test might say:

932 mg sodium/L

That doesn't mean your race plan needs to contain:

932 mg/L

Your actual sweat sodium concentration varies.

Your sweat rate varies.

Your drinking rate varies.

Your food intake varies.

Your race intensity varies.

And the measurement itself has error.

Baker's entire review is essentially a warning against treating sweat-testing numbers as more precise than they really are.

So think:

700–900 mg/hour works well for me in long, hot races

rather than:

I require exactly 817 mg at minute 60.

The first statement is an endurance plan.

The second is a hostage situation involving a spreadsheet.

What matters most for marathoners?

For many marathon runners, sodium intake is relatively straightforward.

Your race is typically short enough that you don't need to restore your entire sodium balance before crossing the finish line.

Priorities are:

1. Don't overdrink.

2. Have some sodium in your fluid/nutrition during prolonged exercise.

3. If you're a particularly heavy or salty sweater, individualize further.

4. Practice the strategy before race day.

The Sawka guideline of roughly 500–700 mg sodium/L of drink provides a useful starting point for many athletes, but Baker and Barnes tell us why it cannot represent everybody.

What matters most for ultrarunners?

The longer the race, the more useful individual data become.

For an ultra, know:

your approximate sweat rate

your approximate drinking rate

your sweat sodium concentration, if available

your estimated sodium loss/hour

your sodium intake from drinks

your sodium intake from food

Then watch how the strategy behaves over many hours.

Because a small hourly mismatch multiplied by twelve, eighteen or twenty-four hours can become quite large.

But even in ultras, remember the central safety lesson:

Sodium cannot rescue an athlete who is consistently overdrinking.

Avoiding excessive fluid intake remains fundamental to preventing exercise-associated hyponatremia.

The whole system in plain English

After these three articles, your hydration strategy should now look something like this:

1. Find out how much you sweat.

Use body weight before and after representative training runs.

You now know your approximate:

L/hour

2. Decide how much you actually need to drink.

Don't automatically replace every litre of sweat.

Develop a fluid range you tolerate well under race-like conditions.

You now know your approximate:

mL/hour

3. Find out how salty your sweat is.

If you have a properly conducted sweat test, use it.

If you don't, use established beverage recommendations as a reasonable starting point rather than pretending you know your individual sweat concentration.

You now have:

mg sodium/L

4. Estimate how much sodium you're losing.

Combine sweat rate with sweat sodium concentration.

You now have:

mg sodium lost/hour

5. Count what you're actually consuming.

Drink + gels + food + capsules.

You now have:

mg sodium consumed/hour

6. Test everything together.

Fluid.

Sodium.

Carbohydrate.

Temperature.

Intensity.

Stomach tolerance.

Race equipment.

Because that's what race day will do.

It will test all of them at once.

The biggest takeaway

There is no universal sodium number for marathon and ultramarathon runners.

Not:

300 mg/hour.

Not:

500 mg/hour.

Not:

1,000 mg/hour.

And not:

100% of sweat sodium losses.

The research points toward something more useful:

Measure what you can. Understand what the measurement means. Then individualize.

Sawka 2007 gives us the hydration and electrolyte-replacement framework and emphasizes individualized replacement because athletes differ substantially in sweat losses.

Baker 2017 shows just how dramatically sweat sodium concentration can vary and explains why the quality and context of sweat testing matter.

Barnes et al. 2019 puts those principles into a large athletic population, showing that endurance athletes can have substantial fluid and sodium losses but enormous individual variation.

Together, they give us a remarkably practical sequence:

Sweat rate → Drinking rate → Sweat sodium → Sodium loss → Sodium intake

And one final distinction is worth remembering:

Your sweat test tells you what you lose.

Training tells you what works.

The best sodium strategy lives somewhere between those two.