Inverted "u-shaped" association of cold-water immersion frequency with mental health and upper respiratory tract infection: a cross-sectional study

Evidence for a link between cold-water immersion (CWI) and health benefits remains scarce. The aim of this study was to verify whether CWI and its frequency is associated with mental health indices, duration of upper respiratory tract infection (URTI) and duration of sick leave (SL) taken due to URTI.

Data on N = 732 polar plungers and N = 501 controls was collected via the Internet in June 2022. The following self-reported methods were included: 28-item General Health Questionnaire (GHQ-28) comprising four subscales (somatic symptoms, anxiety and insomnia, social dysfunction, and depressive symptoms), URTI and SL durations were represented as a number of days. The statistical analysis involved analysis of covariance and linear regression controlled for confounding variables: sex, age, multimorbidity, temperament and mindfulness.

Polar plungers declared better mental health status, shorter URTIs and SLs. The increase in frequency of CWI up to twice per week was associated with a decrease in general mental health index (effect size sR = -0.164), shortening of URTIs (sR = -0.144), and SLs (sR = -0.145). Further increase in the frequency was linked to worsening of the results. CWIs twice per week were related to the shortest URTIs (B = -4.431, p < 0.001), shortest SLs (B = -2.606, p < 0.001), and lowest depressive symptoms score (B = -1.057, p < 0.001).

Regular performance of CWI may be related to better mental health, and immunity against URTI in a dose-related manner. These relationships resembled an inverted u-shaped curve and were independent of confounding parameters. Further studies are required to determine whether CWI could be a cost-effective health intervention.