Treatment & Procedure
Long alkaline baths
Biohacking
Hours in warm, alkaline water. The rationale on the packaging, that acid leaves the body through the skin, does not hold up. There is an effect nonetheless, just by a different route: via the circulation and the kidneys.
In short
Anyone who lies in water for a long time measurably excretes water and salt. In one experiment, 13 men passed 758 milliliters of urine and 56 millimoles of sodium during three hours of immersion, around three grams of table salt. The water pressure shifts blood into the chest, the hormone ANP rises, aldosterone falls, and the kidneys let go of water and sodium. That explains the lighter feeling afterwards, as does the reliably reported muscle relaxation. Acid elimination through the skin, by contrast, has no basis: acid is excreted by the kidneys, around 71 millimoles a day. The price of the long duration: in warm water, cardiac output rises by up to 80 percent.
What long alkaline baths are and how they work
In a long alkaline bath you lie not for half an hour but for many hours in warm water to which an alkaline salt has been added. The usual explanation is that acids are drawn out of the body through the skin. There is no evidence for this. The acid balance is regulated by the kidneys: in a metabolic study of 18 postmenopausal women, they excreted around 71 millimoles of acid a day.
What actually happens in the bath is called immersion diuresis. The water pressure on the legs and abdomen pushes blood from the periphery into the chest. The body interprets this as too much volume: the cardiac hormone ANP rises, renin, aldosterone and the water-conserving hormone vasopressin fall, and the kidneys excrete water and sodium. At the same time, vascular resistance drops and the heart pumps more blood per minute.
What is well supported
Immersion diuresis is well measured and reproduced. In three hours, 13 men excreted 758 milliliters of urine and 56 millimoles of sodium; in addition, the rise in ANP and the fall in aldosterone were measured in the blood. In a second experiment lasting five hours, urine flow and sodium excretion rose sixfold.
Muscle relaxation and the feeling of weightlessness set in immediately and are reliably reported. On top of that, the bath is cheap and available without an active ingredient and without a prescription.
What the studies show
Farrow 1992 — water and salt in three hours
13 healthy men with adequate salt intake lay in water for three hours; one half received a sham medication, the other the vasopressin analogue desmopressin. In the placebo group, urine volume was 758 milliliters and sodium excretion 56 millimoles. ANP in the blood rose from 11 to 20 picograms per milliliter, aldosterone fell from 98 to 45. Desmopressin pushed urine volume down to 159 milliliters, an indication that the fall in vasopressin is part of the mechanism.
Nakamitsu 1994 — temperature plays a part
Eight young men each lay in water up to the neck for five hours, at 32, 34.5 and 36 degrees, plus a control day in air. At 32 and 34.5 degrees, urine flow and sodium excretion rose sixfold, at 36 degrees only threefold. Cardiac output increased by about 80 percent at 36 degrees and by about 40 percent at 32 degrees; vascular resistance fell. ANP and urodilatin rose, renin, aldosterone and vasopressin fell at all temperatures. Kidney function, measured by creatinine clearance, remained unchanged.
Sebastian 1994 — where acid leaves the body
18 postmenopausal women received a constant diet. Their kidneys excreted a net 70.9 millimoles of acid a day. Under orally taken potassium bicarbonate, this value fell to 12.8 millimoles; the acid formed in metabolism was almost completely neutralized. The study is the benchmark for the order of magnitude in which the body excretes acid, and through which organ.
Where the data stop
The effect is well measured, but for shorter periods than in practice. The immersion experiments lasted three and five hours, with 13 and 8 men. For considerably longer baths they provide no measurements. None of the experiments tested an alkaline bath; what was measured was immersion itself.
Muscle relaxation, lightness and better sleep are consistently reported, but were not measured in the studies cited here. And the rationale with which alkaline baths are sold, the elimination of acid through the skin, is not covered by any of these papers.
Status, approval and legal
An alkaline bath is a wellness ritual, not a medicine and not a medical procedure. It requires no prescription and contains no active ingredient in the pharmaceutical sense. Claims about deacidification through the skin are not covered by the evidence.
Safety
The deduction for safety concerns the duration. In warm water, cardiac output rises by up to 80 percent, while at the same time the body loses water and sodium. With heart failure, low blood pressure or while taking diuretic medication, immersion lasting hours is not something to try on your own, but something you discuss with a doctor beforehand.
BK-Score Supported, with caveats
| Human evidence | 6 | |
|---|---|---|
| Mechanism | 7 | |
| Safety data | 7 | |
| Hype gap | 6 | |
| Track record of use | 8 |
Rate separately: rationale and effect. The rationale does not hold up – no appreciable acid is excreted through the skin; the benchmark is renal net acid excretion of around 71 mmol a day (Sebastian, NEJM 1994). The effect, by contrast, is well measured: in immersion experiments, 13 men excreted 758 ml of urine and 56 mmol of sodium in three hours, ANP rose, aldosterone fell (Farrow, J Appl Physiol 1992). Temperature plays a part: sixfold excretion at 32 and 34.5 °C, only threefold at 36 °C (Nakamitsu 1994). The safety deduction concerns the duration: cardiac output rises by up to 80 percent – with heart failure, low blood pressure or while taking diuretics, immersion lasting hours is not something to try on your own.
The score rates the state of knowledge, not the substance. “Safety data 9” means well studied – not harmless. “Track record of use 9” means used long and widely – that is not proof of efficacy.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)
Frequently asked questions about long alkaline baths
Does an alkaline bath deacidify the body through the skin?
No, there is no evidence for that. Acid is excreted by the kidneys, around 71 millimoles a day in one metabolic study. The skin plays no appreciable role in this balance.
Why do you feel lighter after a long bath?
Because of immersion diuresis. The water pressure shifts blood into the chest, ANP rises, aldosterone falls, and the kidneys excrete water and salt. In one experiment this amounted to 758 milliliters of urine and around three grams of table salt in three hours.
Does the water temperature matter?
Yes. In an experiment lasting five hours, urine flow and sodium excretion rose sixfold at 32 and 34.5 degrees, but only threefold at 36 degrees. Here, warmer does not mean more effect.
So what does an alkaline bath do?
What is measurable is the excretion of water and salt via the kidneys. On top of that come muscle relaxation and a feeling of weightlessness, which are reliably reported. The bath is cheap and needs neither an active ingredient nor a prescription.
Who should not bathe for hours?
With heart failure, low blood pressure or while taking diuretic medication, immersion lasting hours should be discussed with a doctor beforehand. In warm water, cardiac output rises by up to 80 percent.
The podcast episode (in German)
Episode 88
The alkaline bath – an old ritual, a look into the cell and what warm water can really do
The podcast by Paul Höser (Episode 88) · with Paul & Paula. Alkaline baths have a long tradition. The episode takes the ritual seriously, looks into the cell, where acid and base drive energy metabolism, and shows what warm bathing demonstrably achieves. Information only, not a usage recommendation.
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Sources
Open in the database – with search, filters and comparison (German app)
Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in the hands of a physician. Last updated: 2026-10-05.