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I have encountered anecdotal claims that the act of taking a cold shower fairly frequently (e.g. once a day) improves health, or at least, reduces susceptibility to illness.
Is there any scientific evidence that this does indeed work, even if only for a narrow and specifically defined segment of the human population?
According to The Effect of Cold Showering on Health and Work, 2016, taking a hot shower followed by 30-90 seconds of cold for 30 days reduced days off work due to illness (but not days of illness) by 29%. The authors propose that this is because cold showers reduce the intensity, but not the duration of illness (see Discussion).
Our findings show that routinely showering (hot-to-) cold for at least 30 days resulted in a reduction of self-reported sick leave from work but not illness days in adults without severe comorbidity. Further research using objective parameters is necessary to determine whether these were causal or associational findings.
Hot-to-cold showers also had some short-term, small, but statistically significant, quality-of-life improvements as measured by SF-36:
Median quality of life MCS after 30 days was slightly higher for all intervention groups (84.7, interquartile range 76.4–90.2 v 85.1, interquartile range 76.7–90.6 v 85.7, interquartile range 78–90.8) compared to the control group (83.9, interquartile range 72.9–89.4) (Table 3). However, after 90 days significant differences were not observed anymore (Table 4). None of the other secondary outcomes were significantly different between groups at 30 and 90 days follow-up (Tables (Tables33 and and44).
I couldn't find any peer-reviewed research that would indicate any specific health benefits, but your claim as stated isn't really possible to answer scientifically. Perhaps a multidecade general morbidity study with cold and hot shower groups could be conducted, but as far as I can tell no such study has been published.
The correlation of coronary heart disease incidence with cold weather is well-supported [1, 2], so it's likely that cold showers could have a similar effect, especially in susceptible groups mentioned in the paper such as the elderly. Cold showers have been shown to have significant effects on systolic and diastolic blood pressure and pulse rate [3], and there have been case studies implicating exposure to cold water in heart failure [4].
One could argue that regular cold showers could condition the body so temperature shocks do not have such a drastic effect, but [1] and [2] show that even the weather, which fluctuates quite a lot, has no such insulating effect.
References:
Repeated exposure to cold stimuli results in an increased tolerance to cold, through a variety of adaptive mechanisms. A significant body of evidence raises the hypothesis that these mechanisms may also confer protection against several diseases and call for a large, prospective epidemiological study.1
The effects found have been:
- Improved metabolic profile (decrease in insulin levels)
- Activation of the immune system
- Improved antioxidant protection1
The following evidence appears to support the hypothesis: Exposure to cold is known to activate the sympathetic nervous system and increase the blood level of beta-endorphin and noradrenaline and to increase synaptic release of noradrenaline in the brain as well. Additionally, due to the high density of cold receptors in the skin, a cold shower is expected to send an overwhelming amount of electrical impulses from peripheral nerve endings to the brain, which could result in an anti-depressive effect. Practical testing by a statistically insignificant number of people, who did not have sufficient symptoms to be diagnosed with depression, showed that the cold hydrotherapy can relieve depressive symptoms rather effectively. The therapy was also found to have a significant analgesic effect and it does not appear to have noticeable side effects or cause dependence. In conclusion, wider and more rigorous studies would be needed to test the validity of the hypothesis.2
How does it cause vasodilation? Well, since the blood becomes shunted, the tissues of the vasoconstricted vasculature have no nutrients and start to accumulate metabolic waste. The increase in metabolic waste and lack of nutrients intrinsically causes vasodilation again. This mechanism is a physiological response, and scientists debate which factor of the two causes the dilation. There are therefore two mechanisms proposed, known as the "Vasodilator Theory" and the "Oxygen Lack Theory" AKA "Nutrient Lack Theory," respectively 3.
There are many different tools and methods of Hydrotherapy including neutral baths, peat baths, fomentations, russian baths, enemas, wet sheet packs, cold mitten friction, constitutionals, etc. which use these basic principles as its guiding force, but we will focus on the cold shower:
"Hydrotherapy has a great role to play in the enhancement of athletic performance and the prevention and treatment of althetic injuries. Kellogg did many experiments which showed that brief hot showers followed by brief cold showers greatly enhanced work performance if give before the activity and, if given after, significantly speeded recovery in persons exhausted by prolonged exercise." (p.201-202)
The main findings of this study were as follows: (1) CWI alleviated symptoms of DOMS at 24, 48, 72 and 96 h post exercise and was effective at 24 and 48 h following high intensity exercise. (2) CWI had a small but significant effect in reducing efflux of CK post exercise. (3) CWI had no effect on recovery of muscle strength but was effective in improving recovery of muscle power.4
More to the point (emphasis my own),
"...correct use of hydrotherapy has an important role to play in the prevention of disease, both chronic and acute. On one hand prolonged hot applications in the form of warm baths, showers, hot tubs, etc., should be avoided because of their depressing effect on circulation, tissue tone and the immune and nervous systems which can lead to varicosities, premature aging, frequent colds, fatigue, and mental sluggishness. On the other hand cold applications in the form of cool showers, cold friction rubs, barefoot walking in the morning dew or fresh snow, and wet socks at night are bracing treatments which can enhance immunity against colds and flu, increase energy and stress tolerance and improve mental and physical performance." (p.209)
Boyle, Wade, and André Saine. Lectures in naturopathic hydrotherapy. East Palestine, Ohio: Buckeye Naturopathic Press, 1988. Print.
(1) T.M Kolettis, M.T Kolettis, Winter swimming: healthy or hazardous?: Evidence and hypotheses, Medical Hypotheses, Volume 61, Issues 5–6, November–December 2003, Pages 654-656, ISSN 0306-9877, 10.1016/S0306-9877(03)00270-6. (http://www.sciencedirect.com/science/article/pii/S0306987703002706)
(2) Nikolai A. Shevchuk, Adapted cold shower as a potential treatment for depression, Medical Hypotheses, Volume 70, Issue 5, 2008, Pages 995-1001, ISSN 0306-9877, 10.1016/j.mehy.2007.04.052. (http://www.sciencedirect.com/science/article/pii/S030698770700566X)
(3) Hall, John E., and Arthur C. Guyton. Guyton and Hall textbook of medical physiology. 12th ed. Philadelphia, Pa.: Saunders/Elsevier, 2011. Print. pp.192-193
(4) Jonathan Leeder, Conor Gissane, Ken van Someren, Warren Gregson, Glyn Howatson . Cold water immersion and recovery from strenuous exercise: a meta-analysis. British Journal of Sports Medicine, Volume 46, Number 4 (March 2012), pp. 233-240, http://ejournals.ebsco.com/direct.asp?ArticleID=4606A5D29A0DA7DFE6DF
For those without access to hot water, for the sake of personal hygiene:
Microbiology: Aerobic Bacterial Counts On Human Skin After Bathing
Head lice infestation in schoolchildren and related factors in Mafraq governorate, Jordan
Estimating the Burden of Disease from Water, Sanitation, and Hygiene at a Global Level
