Diabetes as a risk factor for heatstroke: insights from a nationwide Japanese health insurance database.

Climate change has intensified heat-related illnesses, particularly in Japan, where summer temperatures frequently exceed 35 °C, posing heightened risk to vulnerable populations. Previous studies have primarily assessed heatstroke risks during heatwaves rather than analyzing the ambient temperature at onset, and most have been limited by small sample sizes. This study examined heatstroke incidence across different ambient temperatures, regions, and sexes in individuals with and without diabetes. Using the REZULT database, a nationwide health insurance dataset covering 2.4% of Japan's population, we analyzed 187,569 diabetes patients matched 1:4 to 747,412 non-diabetic controls by age, sex, and region. Survival analyses were performed to estimate the incidence of heatstroke. Diabetic individuals had a significantly higher heatstroke risk, particularly middle-aged adults (30-59 years; hazard ratio [HR] = 1.52, 95% confidence interval [CI]: 1.20-1.94). Males had a slightly higher risk than females (HR = 1.43 vs. HR = 1.38). While the temperature thresholds for heatstroke onset were similar, patients with diabetes showed higher incidence rates. Regional differences were noted, with Kanto (Tokyo area), Chubu (Nagoya Area), and Kinki (Osaka Area) showing lower risks, whereas Kinki showed an increasing trend. In Tokyo, the incidence rates were higher even at lower daily maximum temperatures, except for those aged 40-49 years; a similar trend was observed in those aged 30-39 years in Osaka. These findings highlight the importance of targeted heatstroke prevention strategies for individuals with diabetes, especially middle-aged workers and residents of colder regions. Future research should integrate clinical and environmental factors to develop personalized heat-adaptation measures.