Breast cancerExposure to sunlight

 There is evidence that sunlight affects risk of breast cancer by affecting melatonin levels.

More recent ecological studies provided more supporting evidence and included indices for other breast cancer risk-modifying factors in order to more accurately assess the role of UVB and vitamin D in the United States123 and Spain4 and France56.

A cross-sectional study using data from National Health and Nutrition Examination Survey Epidemiologic Follow-up Study [NHANES Follow-up Study] found the risk reductions [RRs] were highest for women who lived in United States regions of high solar radiation, with RRs ranging from 0.35-0.75 7.

A death certificate-based case-control study of mortality found significant negative association with jobs with the highest occupational exposure to sunlight (odds ratio (OR) 0.82 (95% confidence interval (CI) 0.70-0.97) for female breast cancer8

A case-control study from Ontario, Canada found: “Time spent outdoors was associated with reduced breast cancer risk during 4 periods of life (>21 vs. ≤6 hours/week age-adjusted odds ratio (OR) = 0.71, 95% confidence interval (CI): 0.60, 0.85 in the teenage years; OR = 0.64, 95% CI: 0.53, 0.76 in the 20s-30s; OR = 0.74, 95% CI: 0.61, 0.88 in the 40s-50s; and OR = 0.50, 95% CI: 0.37, 0.66 in the 60s-74 years).”9

Note that ecological studies integrate the effect of risk-modifying factors such as solar UVB irradiance and vitamin D throughout the entire lifetime while observational studies generally look at a period of time 3-15 after drawing blood for a serum 25(OH)D value.

There is also evidence that sunlight affects risk of breast cancer by affecting melatonin levels. During the day when there is sufficient blue sunlight, no melatonin is produced. At night, when there is no bright blue light, melatonin is produced and helps induce sleep.

Several studies had reported that melatonin reduces the risk of breast cancer1011. A recent paper summarized the global data for breast cancer incidence seasonality, finding peaks in spring and fall at mid- and high latitudes12. It was suggested that vitamin D reduces growth of breast cancer in summer and melatonin does so in winter.

Page last edited: 22 August 2011


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