Longitudinal associations of sauna bathing with inflammation and oxidative stress: the KIHD prospective cohort study

Purpose: We sought to determine cross-sectional and longitudinal associations of frequency of sauna bathing with high sensitivity C-reactive protein (hsCRP), fibrinogen, leucocyte count and gamma-glutamyltransferase (GGT).

Design: Baseline sauna bathing habits were assessed in 2269 men aged 42-61 years. Concentrations of hsCRP, fibrinogen, leucocyte count, and GGT were determined at baseline and 11 years later. The associations of sauna bathing frequency with baseline and 11-year hsCRP, fibrinogen, leucocyte count, and GGT levels were examined using robust multivariate regression analyses.

Results: In baseline analysis, 4-7 sauna sessions/week (compared with 1 sauna session/week) was associated with -0.84 mg/l (95% CI, -1.55, -0.14; p = .019) lower hsCRP; -0.07 g/l (95% CI, -0.15, 0.02; p = .112) lower fibrinogen; and -0.28 × 10⁹/l (95% CI, -0.51, -0.06; p = .015) lower leucocyte count, after multivariable adjustment. In longitudinal analysis, the corresponding estimates were -1.66 mg/l (95% CI, -3.13, -0.19; p = .027); -0.16 g/l (95% CI, -0.31, -0.02; p = .031); and -0.49 × 10⁹/l (95% CI, -0.85, -0.14; p = .007) respectively. Sauna bathing frequency was not associated with GGT at baseline and 11 years.

Conclusion: Observational evidence supports the hypothesis that reduction in inflammation may be one of the pathways linking frequent sauna bathing with decreased risk of acute and chronic disease conditions. KEY MESSAGES Cross-sectional evidence or short-term studies suggest Finnish sauna bathing may exert its beneficial health effects via reduction in inflammation and oxidative stress; however, the long-term effects of sauna bathing on these outcomes are uncertain. In this population-based prospective cohort study, frequent sauna sessions significantly decreased levels of inflammatory markers at baseline and 11-year follow-up; but had no effect on oxidative stress. The health benefits of sauna bathing may in part be mediated via reduced systemic inflammation.