Hydration and the Cold Plunge: Why You Lose Water in Cold Water

Yes — cold plunges cost you fluid, which surprises almost everyone who does them. Cold-water immersion triggers a well-documented response called cold-induced diuresis: your body pulls blood toward its core, reads the shift as a fluid surplus, and tells your kidneys to start dumping water. Research reviewed by the National Academies found plasma volume drops around 17% during cold-water immersion — more than in cold air — while the cold simultaneously blunts your sense of thirst. So you exit the plunge needing water and not feeling like you do. The fix is simple: treat a plunge like the fluid event it quietly is.

Why cold water makes you lose water

The chain starts at your skin. Cold constricts the blood vessels near the surface to protect your core temperature, which squeezes blood from your limbs and skin into your chest and torso. Your body's pressure sensors read that central crowding as "too much fluid on board" and respond the way they always do — by making urine. The effect has been observed since the 1700s, when physicians first noted that cold bathing increased urine flow, and modern studies have mapped the mechanism through pressure sensing and the hormones that regulate water retention. The hydrostatic squeeze of being immersed adds to it, which is why water outdoes cold air alone.

This is also why the post-plunge bathroom trip is so universal. It isn't the cold "flushing toxins" — it's your kidneys responding, quite reasonably, to misleading information.

The part that makes it sneaky: blunted thirst

Exercise announces its fluid cost — you sweat, you feel hot, you get thirsty. Cold immersion hides all three cues. You don't sweat noticeably, you feel cold rather than worked, and cold exposure dampens thirst signaling, so the usual prompt to drink never arrives. Someone who plunges after a sauna session or a hard workout is stacking fluid losses from two directions while feeling refreshed the whole time. Refreshed and under-hydrated are not mutually exclusive.

A practical plan

Arrive hydrated. Drink normally through the day rather than gulping at the edge of the tub — a plunge on an already-dry day starts you in a hole.

Drink afterward, on schedule rather than on thirst. Since thirst is muted, have a real drink of water after you're warm and dry, whether or not you feel like it.

Count the whole session. If your routine is sauna-then-plunge, or a plunge after training, the sweat losses came first and the diuresis lands on top. The bigger the session, the more deliberate the rehydration should be.

Warm drinks count too. After cold exposure, warm water or plain tea rehydrates just as well and helps you rewarm.

What hydration will not do

Being well-hydrated does not make cold immersion safe, longer plunges tolerable, or the claimed benefits bigger. The evidence on cold plunging for recovery and mood is still mixed and much of it is small-scale — this article takes no position beyond the fluid math. And the safety notes matter more than anything above: sudden cold immersion triggers a cold shock response — an involuntary gasp and rapid breathing — and it puts a sudden load on the heart and blood vessels. The American Heart Association cautions that cold plunges carry real risk for people with high blood pressure or heart conditions, who should talk with their clinician before trying one. Enter gradually, never plunge alone, keep sessions short, and if anyone shows confusion, uncontrolled shivering that stops, or drowsiness in or after cold water, treat it as an emergency and call 911.

The warm-up kit

The easiest way to make post-plunge rehydration automatic is to have the water waiting where you towel off. The NuRich 32 oz Insulated Water Bottle ($29.99) holds enough to cover the session and the hour after, and because it's vacuum-insulated it can just as easily hold warm tea for the rewarming stretch. The NuRich 18 oz Insulated Water Bottle — The Original ($14.99) does the same for shorter routines. Find both at the NuRich collection.

The bottom line

Cold plunges pull fluid out of you through a mechanism most plungers have never heard of, and they silence the thirst cue that would normally tell you to fix it. Hydrate before, drink deliberately after, count your sauna and training losses in the same ledger — and treat the cardiovascular cautions as the headline, because they are.

This article is general health information, not medical advice. Cold-water immersion carries real cardiovascular risks — talk with your clinician before starting a cold plunge practice, especially if you have high blood pressure, heart disease, or any chronic condition. Never plunge alone.

Sources: National Academies Press, "Nutritional Needs in Cold and High-Altitude Environments," Chapter 9: Influence of Cold Stress on Human Fluid Balance (nationalacademies.org/read/5197/chapter/14 — cold-induced diuresis, plasma volume reduction ~17% in cold water vs ~12% in cold air, blunted thirst); American Heart Association News, "You're not a polar bear: The plunge into cold water comes with risks" (heart.org/en/news/2022/12/09 — cold shock response, cardiovascular cautions); NuRich product information.