Hydration and Electrolytes for Sauna Users: What to Drink Before, During, and After
Overview answer: Sodium is the mineral you lose in the greatest quantity when you sweat (though there is nuance to this as you will see), so it’s your first priority, but it isn’t the only one. Other electrolyte minerals such as potassium, magnesium, and calcium leave in sweat too, but at very low levels.
These total losses still aren’t enough to deplete your body’s reserves but it is still good to replenish to ensure you have baseline adequate levels (especially if you are eating a mineral deficient diet). Drink to hydration, sip rather than chug, and add a little mineral salt with regular sauna use, especially in the beginning. And here’s the counterintuitive part, as you become heat-acclimated, your sweat gets less mineral-rich. Your replacement needs go down over time, not up.
What this article isn’t
I’ve already written about daily hydration here — water quality, home filtration, mineralized water, and the habits that keep you topped up day to day. This is a different piece.
This one is about the session itself: what your body is actually losing when you sweat in a sauna, and what to do about it before, during, and after.
So let’s go to the primary literature and see what’s actually there.
What’s actually in your sweat
Let’s start with the numbers. Here’s the typical composition of human sweat from the physiology literature (1):
| Electrolyte | Average in sweat | Typical range |
| Sodium | ~35 mmol/L | 10–70 mmol/L |
| Potassium | ~5 mmol/L | 3–15 mmol/L |
| Calcium | ~1 mmol/L | 0.3–2 mmol/L |
| Magnesium | ~0.8 mmol/L | 0.2–1.5 mmol/L |
Look at the top row against the bottom. Sodium runs roughly forty times the concentration of magnesium. Sodium dominates sweat losses, and it isn’t close.
That’s the hierarchy, and it’s worth holding onto, because it’s roughly the inverse of how most electrolyte products are formulated and marketed. If a product leads with magnesium, it is not leading with the thing you lose most of.
But notice what this table does not say. It doesn’t say the other minerals are absent.
Is sauna sweat different from exercise sweat?
Almost all sweat composition data comes from people exercising. It’s tempting to assume passive heat produces the same sweat. Mostly it does, but not entirely.
The most direct comparison I found is a study by Verde and colleagues, published in the Journal of Applied Physiology in 1982. They sampled sweat from the same eight men under three conditions: outdoor exercise in the cool, indoor exercise at room temperature, and sauna exposure. The finding:
“Sauna bathing produced sweat with higher magnesium and calcium than exercise did — while sodium, potassium, and chloride were similar across all three conditions.” (2)
There’s a second detail in that paper I find genuinely fascinating. As sweat flow increased, magnesium concentration went down. Meanwhile, we know from other work that sodium concentration goes up at high sweat rates, because the eccrine sweat glands’ (thermoregulatory glands distributed over your body) ability to reabsorb sodium doesn’t scale with flow (3). At a full sauna sweat, reabsorption simply can’t keep pace.
So this is two minerals, moving in opposite directions as you sweat harder. This is the body regulating. Verde’s team concluded exactly this: sweat composition is actively regulated by eccrine sweat glands, not passively filtered. They are “making decisions” about what to keep.
So if you’ve read that saunas draw out minerals differently than exercise does, there is a real, primary-literature basis for that claim where magnesium and calcium are concerned. It isn’t pure supplement marketing.
And I’d underline the conclusion Verde’s own team reached, because other articles I’ve read on this study almost never quote this part: total ionic losses do not appear sufficient to deplete body mineral reserves.
That’s the honest position, and it needs both halves. Sauna sweat does appear to carry more magnesium and calcium than exercise sweat, but it’s still a small amount and does not drain your reserves.
A closer look at potassium
Potassium deserves more precision than the raw numbers suggest.
Your plasma potassium sits around 4–5 mmol/L. Sweat potassium averages about 5 mmol/L. So relative to your blood, sweat is roughly isotonic (solutions that have the same concentration of solutes as another solution) for potassium. In other words, you’re losing it at close to the concentration at which it exists in your plasma.
Compare that to sodium: sweat (~35) is far more dilute than plasma (~140). You lose sodium in much greater absolute quantity, but you lose potassium at a proportionally higher rate relative to blood levels.
That’s a real distinction, and it’s why “you only lose salt during sweating” is wrong.
What keeps it from being alarming is the size of the reservoir. Total body potassium is enormous and overwhelmingly intracellular (inside of a cell) so a session’s worth of sweat barely registers against it. Interestingly, one comparison of running versus passive sauna heating found potassium secretion was actually higher during running, so the sauna isn’t the aggressive potassium drain it’s sometimes made out to be (4).
Potassium is worth replacing through food (such as beet greens, sweet potatoes, and avocados). It is not worth worrying about in a replenishment supplement.
The bigger picture: sweat is hypotonic
Let’s step back from individual minerals for a moment. Taken as a whole, your sweat is hypotonic (lower concentration of solutes) relative to your blood. Blood sodium sits around 135–145 mmol/L; sweat sodium averages about 35 which is roughly a quarter as concentrated. Since sodium and chloride primarily set osmolality (measurement of the concentration of all dissolved solutes), this means that when you sweat, you lose proportionally more water than salt.
Follow that through and you land somewhere surprising: sweating, on its own, tends to raise your blood sodium concentration, not lower it.
So the classic sauna warning that “you’re sweating out your electrolytes, you’ll get depleted!” has the physiology partly backwards. Dehydration from sweating is real and worth managing, but it’s primarily a concentration problem, not a depletion problem.
The acclimation effect: your sodium needs go down
This is my favorite finding in this whole area, and I’ve never seen an electrolyte company mention it.
Heat acclimation substantially improves sodium reabsorption. The classic work here, by Allan and Wilson in 1971, found unacclimated sweat running around 60 mEq/L of sodium and dropping to as low as 10 mEq/L once adapted (5). Later studies have consistently replicated the direction, and the mechanism is reasonably well understood.
As your eccrine sweat glands get better at its job, your body gets better at holding onto sodium. The implication is genuinely counterintuitive: the more consistently you use your sauna, the less sodium you lose per session. Your replacement needs decline as you adapt. The person who most needs to think about sodium replenishment is the beginner, not the veteran.
What acclimation does not do
It does not appear to reduce your sweat magnesium or calcium (6). Which means the “sauna drains your magnesium” story and the “but acclimation protects you from it” reassurance are both resting on the same shaky measurement. Neither half is well supported. That’s tidier than it sounds: if the loss isn’t well established, you don’t need the protection.
There’s a nice through-line here with something I’ve written about before: consistency is where the benefit lives. Regular use doesn’t just build the adaptation you’re after, it also makes your body more efficient at managing the one mineral loss we can actually measure well.
The practical protocol
With all that established, here’s what we actually recommend:
Before a session
Arrive well hydrated. A good way to check your hydration is your urine color. Clear or close to clear is fine. Dark amber means you started behind.
During a session
Bring water. Sip it. The rehydration research favors small doses taken frequently over large volumes at once, your gut absorbs it better and you avoid the sloshing.
Use glass or stainless, not plastic. In a heated cabin, plastic is the last thing I want next to me.
Listen to your body, not your timer. Lightheadedness, nausea, or a headache means get out and cool down. There’s no prize for finishing or pushing it.
After a session
Drink water over the next few hours and consider adding sodium such as from a high quality mineral salt..
Get potassium, magnesium, and calcium from foods such as fruit, potatoes, leafy greens, avocado, nuts, seeds, and dairy. A mineral-rich diet is worth having on general principle. Many Americans are genuinely low in magnesium, and that’s a good reason to address it. It just isn’t a sauna reason. If you supplement magnesium, do it because your diet or your sleep warrants it, not because someone told you you’re sweating it all out.
Bottom line and Recap
- Sodium is the priority — lost at roughly forty times the concentration of magnesium, and the one mineral loss we can measure reliably.
- Potassium is real but modest. It’s lost at close to plasma concentration, a higher proportional rate than sodium — but total body potassium is vast, so it isn’t a concern. Food covers it.
- The magnesium and calcium story doesn’t hold up. The studies behind it were largely measuring skin residue. When researchers washed the skin first, the effect vanished.
- Even the study most often cited to say saunas drain you concluded that total ionic losses aren’t sufficient to deplete your mineral reserves.
- Sweat is more dilute than blood overall, so sweating concentrates your blood sodium rather than depleting it.
- Drink to hydration. Small sips, spread out.
- Add mineral salt to your water after long sessions; get the rest from your diet.
- Acclimation lowers your sodium losses — consistency makes you more efficient, not more depleted.
If you have any questions on this topic, as always, call us and speak with one of our knowledgeable sauna experts (plus all of us are health practitioners!)
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References
- Sawka MN, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr. 2000 Aug;72(2 Suppl):564S-72S. doi: 10.1093/ajcn/72.2.564S. PMID: 10919961.
- Verde T, Shephard RJ, Corey P, Moore R. Sweat composition in exercise and in heat. J Appl Physiol Respir Environ Exerc Physiol. 1982 Dec;53(6):1540-5. doi: 10.1152/jappl.1982.53.6.1540. PMID: 7153150.
- Baker LB, Wolfe AS. Physiological mechanisms determining eccrine sweat composition. Eur J Appl Physiol. 2020 Apr;120(4):719-752. doi: 10.1007/s00421-020-04323-7. Epub 2020 Mar 2. PMID: 32124007; PMCID: PMC7125257.
- Chen YL, Kuan WH, Liu CL. Comparative Study of the Composition of Sweat from Eccrine and Apocrine Sweat Glands during Exercise and in Heat. Int J Environ Res Public Health. 2020 May 12;17(10):3377. doi: 10.3390/ijerph17103377. PMID: 32408694; PMCID: PMC7277079.
- Allan JR, Wilson CG. Influence of acclimatization on sweat sodium concentration. J Appl Physiol. 1971 May ;30(5):708-712. doi: 10.1152/jappl.1971.30.5.708.
- Chinevere TD, Kenefick RW, Cheuvront SN, Lukaski HC, Sawka MN. Effect of heat acclimation on sweat minerals. Med Sci Sports Exerc. 2008 May;40(5):886-91. doi: 10.1249/MSS.0b013e3181641c04. PMID: 18408609.
