Estimated read time: 4.8 minutes, about as long as it takes you to find the gel you definitely packed. 🤔

Hey Performance Nerds! Jonah here. 🤓

A sweat sodium test gives you a number that might not matter. By itself, that number can't tell you whether more sodium would make you faster.

Here's what you'll learn today.

This issue comes from Scott Tindal's chat with Dr Alan McCubbin, my favorite electrolyte researcher. On Fuelin Sessions, they talked about when to use sodium during exercise.

The gel I race with and keep recommending because the formula makes sense: BETA FUEL delivers 40g of carbs per gel through a 1:0.8 maltodextrin-to-fructose ratio.

That ratio is the point. It helps you use multiple carb transporters, push higher intake, and keep the gut calmer when fueling gets serious.

A sweat sodium test measures how much sodium is in each liter of sweat. It doesn't tell you how much fluid you'll lose or drink.

A salty sweater loses more sodium in each liter. A heavy sweater loses more fluid, and the longer they run, the more those losses can add up.

The result can also change with exercise intensity, heat and how used to that heat you are. A cool indoor test may not reflect your sweat in a hot race, so testing conditions matter.

Blood sodium is a different measurement: how much sodium is in your blood relative to water.

Sweat removes both water and sodium, but you can lose a greater share of water.

That leaves less water relative to the sodium still in your body. So blood sodium can rise even while you're losing sodium in sweat!

(You don’t need extra salt then!)

Drinking puts some of that water back. What matters is how much you replace compared with how much you lose.

Replace only a small share with plain water, and blood sodium may still rise. Replace most of it with plain water, and blood sodium may instead start to fall.

In McCubbin’s framework, adding sodium may help limit that fall when replacing a large share of fluid losses is useful.

That doesn’t mean every drink needs sodium, or that sodium makes drinking more than you lose safe.

Sodium loss alone can’t set your sodium intake. How much fluid you lose and drink helps determine whether extra sodium would be useful.

McCubbin’s 2025 review explains how sodium can help your body keep some of the water you drink.

If you drink a large amount of plain water quickly, it can dilute your blood. Your kidneys may respond by sending more water into your urine. Adding sodium can reduce that response.

But most evidence for this comes from before exercise or during recovery. Adding sodium doesn’t automatically help you hold onto more water during a run.

During a race, his framework asks how much of your lost fluid you’re drinking back. If you replace a large share with plain water, adding sodium may help keep blood sodium from falling.

Whether that helps you retain more water or race faster is less clear.

Start with how much fluid you’ll lose and drink. Then consider whether extra sodium would help.

McCubbin starts with a practical question: would replacing a large share of your fluid losses help you? How much you sweat and how long you run shape those losses.

During a shorter race, you may not need to replace most of the fluid you lose. Over a longer event, the water you haven’t replaced can add up and hurt performance, particularly in the heat.

That helps explain why McCubbin often skips the sodium math below roughly four hours. It’s a planning guide, not a point when your sodium needs suddenly change.

Heavy sweating alone doesn’t settle it, either. A very heavy sweater may lose fluid faster than they can comfortably drink it.

They might drink a lot but still replace only a small share of their losses. Blood sodium may still rise in that situation.

Even when replacing more fluid would help, you need to be able to stomach it. You also need enough water available on course.

When replacing a large share of your fluid losses is useful and manageable, a sweat sodium test can help guide how much sodium to include.

You can still choose an electrolyte drink because you like the taste. Some also contain carbs to help fuel your run.

Those benefits don’t require an exact sodium target.

McCubbin says his 2018 review found five studies. In four, there was no significant gain in performance from taking sodium during exercise.

The positive study also had the sodium group drinking more fluid. He thinks any benefit may have come from better fluid intake.

A small 2023 study from his group asked what changes when people replace all expected sodium losses. Nine people each completed two five-hour treadmill runs in the heat.

They took sodium capsules on one run and placebo on the other.

Heart rate and body temperature showed no significant difference between the two runs. Neither did how hard the run felt.

The study didn't test race times or the value of using a sweat test.

We don't have strong evidence that taking sodium during a race makes you faster.

Sodium may limit a fall in blood sodium, but it doesn’t remove the extra water if you drink more than you lose.

McCubbin says there’s still a lot we don’t know about cramps. Extra sodium isn’t a guaranteed fix. Source

Want help putting your fueling plan into practice? Fuelin guides you through testing how much you sweat and how much carbohydrate your gut can handle. You can log those sessions in the app.

Try Fuelin free for two weeks and start testing your plan in training.

Jonah

Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.

Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.

A runner holds the same pace at the same temperature, but humidity is much higher. Why does the run usually feel harder?

Same pace, same temperature, but a harder run when humidity rises. The difference is how easily your sweat can evaporate.

The correct answer?

A. Sweat evaporates less efficiently, so the body sheds less heat even while the skin gets wetter.

When sweat turns from liquid into vapor, it takes heat away from your skin. Humid air already contains more water vapor, making that process harder.

More sweat can stay on your skin or drip off without providing that cooling. You can be dripping with sweat and still struggle to lose heat.

Here's how the votes shook out:

A. Sweat evaporates less efficiently, so the body sheds less heat even while the skin gets wetter - 230 votes (about 85%)
B. Humidity slows sweat production, so the body cannot put enough sweat on the skin - 11 votes (about 4%)
C. Humid air contains much less oxygen, so the muscles receive less oxygen at the same pace - 23 votes (about 8%)
D. Sweat becomes saltier in humid air, so electrolyte loss becomes the main limit - 8 votes (about 3%)

272 responses.

Why C was tempting:

A humid run can leave you breathing harder, so the oxygen explanation sounds plausible. But the main issue here is cooling: your body has a harder time getting rid of the heat you produce.

Bottom line?

The same temperature can feel very different when humidity rises. Your usual pace may take more effort because less of your sweat is helping you cool down.