Every week, I filter through the latest sports science research. The goal is to find the most interesting new research, break down the findings, and turn them into actionable insights you can use in your training.
If you've ever had your sweat tested to find out how much sodium you lose during exercise, here's a question: what happens to that number when you get used to training in the heat? Do you need to test again?
A new mini review looks at how heat acclimation changes sweat sodium losses, and why measuring those changes isn't as straightforward as you might think.
The study
In this review, researchers examined whether athletes should repeat sweat sodium testing after heat acclimation.
The short answer: heat acclimation does change your sweat sodium, but retesting comes with some important caveats.
Across studies, heat acclimation reduced sweat sodium concentration by approximately 9–23 mmol/L. The body can start conserving sodium within just three days, with changes often levelling off around day six in field conditions. In laboratory settings, sodium conservation continued to increase through day ten.
So, if you spend several weeks training in hot conditions, your sweat sodium concentration may be lower than it was before you acclimated.
But here's the problem: the change you're trying to measure can be similar in size to the variation in the test itself.
For example, increasing exercise intensity from 45% to 65% of VO₂ max can change whole-body sweat sodium concentration by around 20 mmol/L. The collection site, testing technique, sample handling, and dietary sodium intake can also affect the results.
That makes it difficult to know whether a difference between two tests reflects genuine adaptation or simply different testing conditions.
The authors recommend repeating tests only under carefully reproduced heat and exercise conditions. They also suggest reporting whole-body sodium loss rates rather than relying on sweat sodium concentration alone. However, the smallest change that can reliably be detected has not yet been established.
Why it matters
If you're a runner, cyclist, or HYROX athlete training through the summer, heat acclimation can change how your body handles fluid and electrolytes.
But a sweat sodium concentration reading doesn't tell you how much you sweat overall, how much fluid you need to replace, or whether you're adequately hydrated.
You need to consider the bigger picture.
This matters because sodium supplementation is often presented as essential for athletic performance, with athletes encouraged to take in large amounts of electrolytes to replace everything they lose in sweat.
But more sodium isn't automatically better. Your needs depend on your sweat rate, the duration and intensity of your session, the environmental conditions, and what you're eating and drinking. For many athletes doing shorter sessions in moderate conditions, elaborate electrolyte strategies may be unnecessary. Simply eating some salty foods after exercise is typically enough to "replace what you lost."
For longer sessions in the heat, replacing some fluid and sodium can be useful. The goal is to match your hydration strategy to your actual needs, not to chase a high sodium intake simply because you're an athlete.
My thoughts
I find this review interesting because it highlights two things: your body adapts to heat, and sweat testing isn't as precise as the numbers might suggest.
I'm also not convinced that athletes need to obsess over sodium intake to the extent that some sports nutrition advice suggests. The body has effective mechanisms for regulating sodium and fluid balance, and not every athlete needs an elaborate electrolyte plan.
That doesn't mean sodium never matters. Significant sweat losses during prolonged exercise, particularly in hot conditions, can make fluid and sodium replacement important. Drinking excessive amounts of water relative to what you lose can also cause dangerous hyponatremia, so hydration isn't simply about drinking as much as possible.
My takeaway? Start by understanding how much fluid you lose, rather than assuming you need more salt.
Weigh yourself immediately before and after key training sessions, accounting for fluid consumed and any urine produced, to estimate your sweat rate. As a practical guide, try to avoid losing more than around 2% of your body weight during demanding sessions, particularly when performance matters. You also want to avoid drinking so much that you gain weight during exercise.
For longer sessions or hot conditions, something like Hydra+ provides fluid, sodium, and carbohydrates in one bottle. Its sodium and carbohydrate content can support your hydration strategy without requiring you to build one around a single sweat test result.
If you've heat-acclimated and want to retest your sweat sodium, make sure the testing conditions are comparable. Otherwise, you may be reacting to measurement noise rather than a real change in your physiology.
The goal isn't to consume the most sodium. It's to understand your losses, replace what you need, and avoid making hydration more complicated than it has to be.