Sunlight Exposure and Breast Cancer Risk: A Systematic Review and Meta-analysis.

Sunlight exposure may influence breast cancer risk through cutaneous vitamin D synthesis, circadian regulation, and immune modulation, yet prior meta-analyses were limited and did not examine hormone-receptor subtypes. In this systematic review and random-effects meta-analysis of 31 epidemiologic studies, higher sunlight exposure was associated with lower breast cancer incidence (pooled odds ratio 0.80; 95% confidence interval 0.74-0.86), with the strongest associations in case-control studies using personal exposure measures and weaker associations in cohorts relying on ambient-ultraviolet proxies. Protection was confined to estrogen-receptor-negative (0.71; 0.58-0.87) and triple-negative tumours (0.55; 0.37-0.81), with no association for estrogen-receptor-positive disease, and survival was improved for diagnoses in sunnier seasons (hazard ratio 0.84; 0.80-0.88), while a dose-response analysis indicated an 8% lower risk per 10 relative exposure units. Because the evidence is observational and subject to confounding and exposure misclassification, these hypothesis-generating findings nonetheless support further study of the light environment in breast cancer aetiology and prevention, balanced against skin-cancer risk.