National Institutes of Health Study:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4049052/

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The use of water for various treatments (hydrotherapy) is probably as old as mankind. Hydrotherapy is one of the basic methods of treatment widely used in the system of natural medicine, which is also called as water therapy, aquatic therapy, pool therapy, and balneotherapy. Use of water in various forms and in various temperatures can produce different effects on different system of the body. 

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Antidepressive effect of cold shower attributed to presence of high density of cold receptors in skin expected to send an overwhelming amount of electrical impulses from peripheral nerve endings to the brain. It has significant analgesic effect and it does not cause dependence or noticeable side effects.[33]

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One hour head-out water immersions (WI) in various temperatures (32°C, 20°C, and 14°C) produced various effects. Immersion at 32°C did not change metabolic rate (MR) and rectal temperature (Tre), but it lowered the heart rate (HR) by 15%, systolic blood pressure (SBP) and diastolic blood pressure (DBP) by 11% and 12%, respectively, compared, with controls at ambient air temperature. Along with HR and blood pressure (BP), the plasma renin activity, plasma cortisol, and aldosterone concentrations were also lowered by 46%, 34%, and 17%, respectively, while diuresis was increased by 107%.[3]

Immersion at 20°C produced similar decrease in plasma renin activity, HR, SBP, and DBP, in spite of lowered Tre and increased MR by 93%. Plasma cortisol concentrations tended to decrease, while plasma aldosterone concentration was unchanged. Diuresis was increased by 89%. No significant differences in changes in plasma renin activity, aldosterone concentration, and diuresis compared with subjects immersed in 32°C.[3]

Immersion at 14°C lowered Tre and increased MR by 350%, HR, SBP, and DBP by 5%, 7%, and 8%, respectively. Plasma noradrenaline and dopamine concentrations were increased by 530% and by 250%, respectively, while diuresis increased by 163%, which was more than at 32°C. Plasma aldosterone concentrations increased by 23%. Plasma renin activity was reduced. Cortisol concentrations tended to decrease. Plasma adrenaline concentrations remained unchanged. Changes in plasma renin activity were not related to changes in aldosterone concentrations.[3]

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Regular winter swimming significantly decreased tension, fatigue, memory, and mood negative state points with the duration of swimming period; significantly increased vigor-activity scores; relieved pain who suffered from rheumatism, fibromyalgia, or asthma; and improved general well-being in swimmers.[4]

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In patient with chronic heart failure (CHF), thermal vasodilatation following warm-water bathing and low-temperature sauna bathing (LTSB) at 60°C for 15 min improves cardiac function;[5] repeated sauna-therapy (ST) increased left ventricular ejection fraction; increased 6-min walk distance in association with improvement in flow-mediated dilation and increase in number of circulating CD34 (+) cells; reduced plasma levels of norepinephrine and brain natriuretic peptide. These indicates that ST improves exercise tolerance in association with improvement in endothelial function.[6] LTSB improves peripheral circulation in cerebral palsy (CP).[5]

After ST reduced level of total and low density lipoprotein (LDL)-cholesterol concentration, while increased level of high density lipoprotein (HDL)-cholesterol was observed. These changes are good prognoses for the prevention of ischemic heart disease.[7] ST increases endothelial nitric oxide synthase (eNOS) activity and improves cardiac function in heart failure and improve peripheral blood flow in ischemic limbs. In myocardial infarction (MI)-induced Wistar rats ST increases myocardial eNOS, vascular endothelial growth factor mRNA levels.

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Respiratory System: CWI was associated with increase in respiratory minute volume and decrease in end tidal CO2 partial pressure.[11] Repeated cold water stimulations reduced frequency of infections; increased peak expiratory flow, lymphocyte counts, and expression of gamma-interferon; modulated interleukin expression; and improved quality of life (QOL) in patients with chronic obstructive pulmonary disease.[21]

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Based on available literature, this review suggests that hydrotherapy was widely used to improve immunity and for the management of pain, CHF, MI, chronic obstructive pulmonary diseases, asthma, PD, AS, RA, OAK, FMS, anorectal disorders, fatigue, anxiety, obesity, hypercholesterolemia, hyperthermia, labor, etc. It produces different effects on various systems of the body depending on the temperature of water and though these effects are scientifically evidence based, there is lack of evidences for the mechanism on how hydrotherapy improves these diseases, which is one of the limitations of hydrotherapy, and further studies are required to find the mechanism of hydrotherapy on various diseases.

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