Sauna & Cold Plunge Hydration: What to Drink Before and After

A sauna dehydrates you by making you sweat. A cold plunge dehydrates you through your kidneys, without sweating at all. Nearly every guide treats these as the same problem. They aren't, and the fix is different for each.

Search this topic and you'll get the same paragraph on every site: "Sauna and cold plunge both dehydrate you, so drink water and electrolytes before and after."

Half right. And the half that's wrong is the interesting half.

These two are dehydrating you through completely different physiology. One is sweat. The other is a kidney response to being underwater. It happens whether the water is cold or not, and it keeps going after you get out. Understanding which one you just did tells you what to actually drink.

Before you read on: this guide covers situations that can become medical emergencies. Read the scope and safety note first.

The Short Answer

Bottom line: sauna costs you fluid through sweat and takes some sodium with it; a cold plunge costs you fluid through increased urine output and takes some sodium with it too. But the one controlled study on what to drink afterward found that replacing the volume matters more than what's dissolved in it.

The three things that actually change your behaviour:

• Sauna losses are smaller than the internet implies. Measured average for a two-round session: about 0.4–0.5 litres. That's a large glass or two, not a crisis.
• Cold plunge fluid loss is real and invisible. It runs through your kidneys, not your skin, so you get no sweat-based feedback that it's happening.
• "Electrolytes beat water" doesn't survive the head-to-head. Electrolyte drinks restored plasma volume better. And produced identical performance. Volume was what mattered.

Sauna: How Much Fluid Do You Actually Lose?

Roughly half a litre for a typical session: a real loss, but far less dramatic than sauna marketing suggests.

This has actually been measured at scale. A study of 674 sedentary young adults ran two 10-minute sessions at 90°C with 35% humidity, weighing everyone before and after:

The useful conversion, stated in that paper: one kilogram of body mass corresponds to about one litre of perspired fluid. So a typical two-round sauna costs somewhere around 0.4 to 0.5 litres, and a larger person doing a longer session might reach 0.8.

Two things follow. First, the "you sweat out litres in the sauna" claim is inflated for ordinary sessions. You'd need considerably longer or hotter exposure to get there. Second, because you can weigh yourself before and after, you can measure your own number instead of guessing. If you want to turn that into a per-session fluid target, our sweat-rate self-assessment walks through the arithmetic and covers the wide person-to-person variation in how much sodium your sweat actually carries.

Sweat isn't just water. It carries sodium, with smaller amounts of potassium and magnesium. That's the part that makes sauna losses genuinely different from simply not drinking for a while.

Cold Plunge: Why You Dehydrate Without Sweating

Immersion pushes blood from your limbs into your chest. Your body misreads that as too much fluid on board and flushes the "excess" out through your kidneys. This is the mechanism essentially no cold-plunge content explains.

Here's the sequence. When you're immersed, water pressure plus cold-driven vasoconstriction squeeze blood out of your arms and legs toward your core. Your heart and central vessels suddenly hold more blood than usual. Stretch receptors there report a surplus.

Your body acts on that report. It increases urine production to shed a fluid excess that doesn't actually exist. Your total fluid hasn't gone up, it's just been redistributed. The Institute of Medicine's review of cold stress and fluid balance names the likely driver as "the central movement of fluid caused by peripheral vasoconstriction," while noting researchers still disagree on the full mechanism.

Two details that make this more interesting than the usual telling:

It isn't purely about cold. A 1984 study in the Journal of Applied Physiology immersed subjects to the neck in thermoneutral 34.5°C water (not cold at all) and still measured 1,061 ml of urine over six hours, with a net water balance of −1,285 ml. Immersion itself drives this. Cold amplifies it by adding vasoconstriction on top.

The simple hormonal story is wrong. Wellness explanations usually say cold suppresses your antidiuretic hormone. That same 1984 study measured it directly and found vasopressin rose, from 0.7 to 3.0 pg/ml. And the authors concluded that "the action of factors other than PVP suppression are necessary to explain the mechanism of immersion diuresis." The body was signalling to retain water and was losing it anyway.

Practically: you get no feedback. Sweat is visible, so heat gives you a cue to drink. A cold plunge gives you nothing. And cold blunts thirst on top of it. You'll notice on the drive home, not in the tub.

One more factor for outdoor plunges in genuinely cold air: respiratory water loss roughly doubles in the cold, from about 0.68 litres per 24 hours at 25°C to 1.02 litres at −20°C, per that same Institute of Medicine review. Minor for a short session, but it compounds across a winter of them.

What to Actually Drink Afterward

Here's where the evidence contradicts the standard advice. And it's the one place this question has been tested head-to-head.

A randomized trial published in Military Medicine compared plain water, a carbohydrate drink, and a carbohydrate-electrolyte drink for recovery after immersion diuresis, then tested exercise performance afterward.

The plasma-volume results favour electrolytes:

But performance didn't follow. Every condition produced the same running distance: 1.4 km across the board, including the no-immersion control (P = .28). The authors' conclusion is worth reading closely:

"fluid replacement following immersion diuresis should focus on restoring volume lost rather than fluid constituents"

Volume, not constituents. The electrolyte drink genuinely restored blood plasma better, and that better number didn't translate into performing better. That's close to the opposite of what electrolyte marketing built on this topic would tell you.

State the caveats plainly, because they're large: twelve men, and a four-hour immersion. A typical cold plunge is two to five minutes. Extrapolating from four hours in the water to three minutes in a barrel is a real stretch, and anyone citing this study as settled proof for a three-minute plunge is overreaching, including in the direction this article is arguing.

What survives the caveats: there's no good evidence that an electrolyte drink outperforms adequate water after a cold plunge, and there is direct evidence that volume is what moved the outcome.

The Protocol

Two different sessions, two different priorities. Front-load for the sauna; watch the invisible loss after the plunge.

On electrolytes, scoped honestly: for a single 20-minute sauna, water is enough. Half a litre of sweat doesn't warrant a supplement. Sodium starts to matter with repeated or long sessions, deliberate heat-acclimation blocks, or if you're someone whose sweat runs salty. The person-to-person range there is roughly tenfold, which our sweat-rate guide covers in detail.

Deliberately no product recommendation for the cold-plunge side. The one controlled study points toward volume rather than composition, and recommending an electrolyte product there would contradict the evidence this page just cited. If you want to compare formats for the sauna use case, our electrolyte format comparison covers the trade-offs.

Common Mistakes

• Treating both sessions the same. Different mechanisms, different timing, different needs. This is the error the entire category makes.
• Waiting for thirst after a plunge. Cold suppresses thirst while immersion diuresis is still running. Thirst is the wrong instrument here.
• Chugging a litre immediately after. Large boluses mostly pass straight through. Steady intake over the following hour retains more.
• Assuming the sauna cost you litres. Measured average is under half a kilogram for a typical session. Over-drinking on that assumption is its own risk.
• Drinking alcohol around either. Alcohol adds a diuretic effect to a body already shedding fluid, on top of the cardiovascular risk that makes it unsafe with heat and cold exposure regardless.
• Skipping the weigh-in. Thirty seconds on a scale, once, replaces every population average on this page with your own number.

FAQ

Do you lose fluid in a cold plunge if you're not sweating? Yes, through your kidneys rather than your skin. Immersion pushes blood from your limbs toward your chest, your body reads that as having too much fluid on board, and it responds by producing more urine. The effect is called immersion diuresis, and it happens even in thermoneutral water, and cold amplifies it by adding vasoconstriction.

How much fluid do you actually lose in a sauna? Less than most people assume. In a study of 674 sedentary young adults doing two 10-minute sessions at 90°C, average body-mass loss was 0.37 kg for women and 0.50 kg for men, roughly 0.4 to 0.5 litres of fluid, since one kilogram of body mass corresponds to about one litre of sweat. Heavier participants lost more, up to about 0.8 kg.

Should you drink electrolytes or plain water after a cold plunge? The one controlled comparison found volume matters more than content. Carbohydrate and electrolyte drinks restored plasma volume better than water, but produced no better exercise performance afterward. The authors concluded that fluid replacement after immersion diuresis should focus on restoring the volume lost rather than on what's in the fluid.

Should you drink water before or after a sauna? Both, but front-loading matters more. Arriving already topped up means you start with reserve rather than chasing a deficit, and it lets you judge the session by how you feel rather than by thirst, which lags well behind actual fluid loss.

Can you do sauna and cold plunge back to back without special preparation? Contrast sessions stack two separate fluid-loss mechanisms (sweating in the heat, then increased urine output from immersion) so the total loss is larger than either alone. The practical adjustment is to drink between rounds rather than only at the end, and to stop on lightheadedness rather than pushing through it.

Keep Reading

• The Sweat-Rate Self-Assessment & Advanced Electrolyte Guide: turn a before-and-after weigh-in into your own per-session fluid number, and work out whether your sweat runs salty.
• Electrolyte Format Comparison: powders, tablets, capsules and ready-to-drink compared, if you've decided you need one.
• Your Real Daily Water Number, Calculated Step by Step: the daily baseline these sessions sit on top of.

A Note on Scope

This article is for informational purposes only and isn't medical advice. Heat and cold exposure are genuine cardiovascular stressors. Check with a clinician before starting either if you have heart disease, high or low blood pressure, are pregnant, or take medication affecting blood pressure or fluid balance. Never use a sauna or cold plunge after drinking alcohol, and never plunge alone. Get out immediately for chest pain, severe dizziness, confusion, or uncontrollable shivering, and treat those as emergencies rather than part of the process.

Sources

• Podstawski R, Boraczyński T, Boraczyński M, Choszcz D, Mańkowski S, Markowski P. "Sauna-Induced Body Mass Loss in Young Sedentary Women and Men." The Scientific World Journal, 2014: measured body-mass loss across 674 participants, the per-BMI ranges, and the one-kilogram-to-one-litre conversion.
• Kravik SE, Keil LC, Silver JE, Wong N, Spaul WA, Greenleaf JE. "Immersion diuresis without expected suppression of vasopressin." Journal of Applied Physiology, 1984: urine output and water balance during six hours of thermoneutral immersion, and the finding that vasopressin rose rather than fell.
• Wheelock CE, Lavoie EM, Stooks J, Schwob J, Hess HW, Pryor RR, Hostler D. "Carbohydrate or Electrolyte Rehydration Recovers Plasma Volume but Not Post-immersion Performance Compared to Water After Immersion Diuresis." Military Medicine, 2024: the head-to-head plasma-volume and performance results, and the volume-over-constituents conclusion.
• Institute of Medicine: "Influence of Cold Stress on Human Fluid Balance," in Nutritional Needs in Cold and in High-Altitude Environments: the peripheral-vasoconstriction mechanism for cold-induced diuresis, the open questions about it, and respiratory water loss in cold versus temperate air.

Not used: sauna and spa market-size projections. Published estimates for that market range from roughly $1.4B to $22B depending on how each firm defines it, and the specific figure circulating in trend coverage appears to conflate one report's 7.2% growth rate with a dollar target, so no market-size number is cited here. Also excluded: sauna and cold-plunge manufacturer blogs and hydration-brand content used only to map the landscape, none of which cite the physiology literature above.