Electrolyte balance
Electrolyte balance is the maintenance of the correct concentrations of the body's dissolved salts — principally sodium, chloride and potassium — in the blood and tissue fluids. In the sauna it matters because sweating removes salts as well as water, and because the heart, the nerves and the muscles all depend on small, carefully regulated electrolyte concentrations. For the ordinary bather the balance is rarely disturbed by a single session; it becomes a real concern in very long or very frequent bathing, in hot humid conditions, and in persons whose bodies already handle salts poorly.
Sweat and salt
Sweat is a filtrate of the blood produced by the sweat glands, and it is hypotonic: the salt concentration of sweat is lower than that of the plasma, typically of the order of tens of millimoles per litre for sodium, with chloride a little lower and only small amounts of potassium, calcium and magnesium. Sweating at a high rate produces more dilute sweat than slow sweating, and heat acclimatisation lowers the salt content further — an adaptation that conserves sodium in people regularly exposed to heat (see Heat acclimation). Measurement of the salt and water content of collected sweat is described under Sweat analysis.
The important consequence of sweat being dilute is asymmetry: a litre of sweat costs the body roughly a litre of water but only a fraction of the sodium that the same litre would have contained in the blood. Water is therefore lost faster than salt, and the predominantly watery deficit of sauna bathing is Dehydration rather than a salt shortage. Halving that argument works the other way: because salt is not poured out as quickly as water, replacement of fluid by plain water during a session is almost always sufficient unless the session is extreme.
Balance during and after bathing
Over one ordinary session of 15–20 minutes, with losses of the order of half a litre of sweat, the electrolyte movements are well inside the range that the kidneys and the diet handle routinely. The kidneys, which regulate both water and salts, respond to the reduced fluid volume by conserving water; the small acute reduction in kidney blood flow during heat exposure (see Blood circulation) is part of this response and is harmless in healthy people. After the session, eating and drinking restore both compartments; normal food contains far more salt than the session cost (see Eating after sauna and Food and drink in a wellness facility).
The picture changes when losses accumulate: all-day bathing, repeated rounds with little drinking between them, very humid rooms where sweat drips without evaporating, or coexisting fluid losses such as vomiting or diarrhoea can produce combined water-and-salt deficits. Sport-type drinks, which contain sodium and potassium as well as water, then become a practical way of replacing both, and the same logic underlies rehydration solutions recommended for illness. In such situations, muscle cramps, a sense of weakness, or an odd heart rhythm are warnings that the body's salt economy, not merely its water content, is disturbed; the cardiovascular side of this question is discussed under Cardiovascular effects of sauna and Heart rate in a sauna, and the symptoms should be taken seriously (see Fainting in a sauna and Heat exhaustion).
Who should be careful
Electrolyte handling is not equally robust in everyone. Kidney disease reduces the ability to conserve or excrete salts, so both the acute water loss and the eventual replacement need to be managed deliberately; diabetes alters kidney function and fluid balance; and medication — particularly diuretics, which remove sodium and potassium, and some blood-pressure medicines — changes the whole equation by which a bather should judge fluid intake. Alcohol increases urine production and thereby disturbs the balance from another side. For all these groups, and for anybody with a known electrolyte disorder, the general rule of this encyclopaedia applies: discuss bathing with a clinician and follow the advice in Medical advice before sauna and Sauna contraindications rather than general textbook values.
Two further cautions belong in any account of electrolyte balance. First, thirst is a poor guide to sodium need — the healthy bather who drinks water steadily during a long session is usually doing the right thing, and the addition of salt to the diet should come from food, not from deliberate salt intake in the sauna. Second, the balance is struck over days, not minutes: a single frugal or generous meal can adjust the salt economy after a heavy day of bathing, and there is no physiological basis for the idea that salt must be replaced within the session itself. The direction to watch is the overall one — steady fluid intake during the stay, and ordinary meals, with symptoms treated as warnings (see Hydration during sauna and Sauna safety).
Further reading
References
- Baker LB. "Physiology of sweat gland function: the roles of sweating and sweat composition in human health." Temperature 6(3):211–259, 2019. doi:10.1080/23328940.2019.1632145
- Cheuvront SN, Kenefick RW. "Dehydration: physiology, assessment, and performance effects." Comprehensive Physiology 4(1):257–285, 2014. doi:10.1002/cphy.c130017
- Kukkonen-Harjula K, Kauppinen K. "Health effects and risks of sauna bathing." International Journal of Circumpolar Health 65(3):195–205, 2006. doi:10.3402/ijch.v65i3.18102
- Laukkanen JA, Laukkanen T, Kunutsor SK. "Cardiovascular and other health benefits of sauna bathing: a review of the evidence." Mayo Clinic Proceedings 93(8):1111–1121, 2018. doi:10.1016/j.mayocp.2018.04.008
- Tansey EA, Johnson CD. "Recent advances in thermoregulation." Advances in Physiology Education 39(3):139–148, 2015. doi:10.1152/advan.00126.2014