Associations between the duration of outdoor sunlight exposure and the risk of developing 134 different diseases

Background Existing studies of sunlight exposure and health have mainly focused on specific organs, particularly the skin and eyes, while associations with other organs and systems remain largely unexplored. We aimed to comprehensively examine the associations between outdoor sunlight exposure duration and 134 diseases. Methods Using UK Biobank data, summer and winter outdoor durations were collected by questionnaire, and average daily exposure was calculated as their arithmetic mean. Incident diseases were defined using 525 three-character ICD-10 codes. Cox proportional hazards models estimated associations, and restricted cubic splines assessed potential nonlinearity. Results Among 281,282 participants [mean (SD) age, 55.81 (8.07) years; 55.79% female], 1,115,346 incident cases occurred during a median follow-up of more than 13.4 years. After false discovery rate correction, sunlight exposure was associated with 36 diseases, with hazard ratios ranging from 0.914 to 1.086. For example, each additional hour of average exposure was associated with higher risks of malignant melanoma of the skin (HR = 1.052, 95% CI 1.012–1.094), obesity (1.039, 1.027–1.052), cerebrovascular diseases (1.028, 1.011–1.046), and osteoarthritis (1.042, 1.033–1.052), and lower risks of osteoporosis (0.975, 0.957–0.994), disorders of mineral metabolism (0.968, 0.944–0.992), and pressure ulcers (0.947, 0.916–0.979). Thirty diseases, including depression, anxiety, sleep disorders, arrhythmias, iron deficiency anemia, and kidney failure, showed U-shaped associations. Conclusion Outdoor sunlight exposure duration showed positive, inverse, and nonlinear associations with a broad range of diseases. These findings highlight the potential importance of moderate exposure and appropriate sun protection but require validation in other populations.