The Association between Habitual Tub Bathing and Risk of Breast Cancer: The Japan Public Health Center-Based...

Several studies reported that hot tub bathing, a unique Japanese culture, may benefit breast cancer risk factors. It is, however, uncertain whether moderate heat exposure through hot tub bathing could potentially reduce the risk of breast cancer. In this study, we aimed to investigate this association through a long-term prospective cohort study conducted in Japan.

A total of 15,927 Japanese women aged 40-59 years with no history of breast cancer were followed up from 1990 to 2015. The frequency of tub bathing was categorized as 0-2 days/week, 3-4 days/week, and almost every day. The hazard ratios (HRs) and 95% confidence intervals (CIs) for the incident breast cancer were estimated using Cox proportional hazards models after adjusting for potential risk factors for breast cancer, including overweight.

During 367,950 person-years of the follow-up period, 370 breast cancer cases were identified. After adjusting for known confounders for breast cancer risk, the multivariable HR (95% CI) of incident breast cancer was 1.23 (0.93-1.62) for 3-4 days/week and 0.72 (0.40-1.31) for ≤2 days/week (trend p=0.90), compared to almost every day of tub bathing (p for trend = 0.90). No association was also seen for pre-menopausal or post-menopausal women and subjective lukewarm, warm, or hot bath temperatures.

We found no reduced risk of breast cancer for women associated with frequent tub bathing. Further research is needed to obtain objective information on the frequency, duration, and water temperature of tub bathing.