Effects of Bathtub Bathing and Sauna Practices on Cardiovascular and Systemic Health: A Narrative Review.

Thermal bathing practices, including domestic hot-water immersion and sauna use, have been linked to cardiovascular and systemic health outcomes. However, the amount and type of evidence differ between these practices. This review examines current epidemiological and mechanistic findings and discusses their relevance to cardiovascular health. A narrative review was conducted using Ichushi-Web and PubMed. Observational and interventional studies evaluating habitual bathtub bathing or sauna use in relation to physiological or health-related outcomes were included. Studies involving children or pregnant/postpartum women and those without relevant biological endpoints were excluded. In total, 45 studies met the inclusion criteria (17 on bathtub bathing and 28 on sauna use). Studies of habitual bathtub bathing, conducted mainly in Japan, have reported associations with lower arterial stiffness, improved glycemic control, and selected inflammatory or stress-related markers. Experimental work shows that hot-water immersion increases core body temperature, promotes peripheral vasodilation, and induces heat shock protein expression. Sauna bathing, investigated primarily in Finnish prospective cohorts, has been associated with lower cardiovascular and all-cause mortality, as well as favorable inflammatory and endothelial markers. Bathing conditions, age, sex, and cultural context appear to influence these findings. Thermal exposure produces circulatory and molecular responses relevant to cardiovascular regulation. Prospective data support associations between frequent sauna use and long-term outcomes, whereas evidence for domestic bathtub bathing is limited to observational studies of intermediate markers. Further research with clearly defined exposure parameters and clinical endpoints is needed to better understand the long-term cardiovascular implications of habitual thermal practices.