Latitude and ultraviolet radiation dose in the birthplace in relation to menarcheal age in a large cohort of French women

Background Age at menarche is an important determinant of hormonal-related neoplasia and other chronic diseases. Spatial and temporal variations in age at menarche have been observed in industrialised countries and several environmental factors were reported to have an influence. Method We examined geographical variations in self-reported age at menarche and explored the effects of both latitude and ultraviolet radiation (UVR) dose on the onset of menarche in 88 278 women from the French E3N cohort (aged 40-65 years at inclusion). Results The mean age at menarche was 12.8 years. After adjustment for potential confounders (birth cohort, prematurity, birth weight and length, father's income index, body silhouette in childhood, food deprivation during World War II, population of birthplace, number of siblings, breastfeeding exposure and indoor exposure to passive smoking during childhood), latitude and UVR dose (annual or spring/summer) in county of birth were significantly associated with age at menarche (P-trend < 0.0001). Women born at lower latitudes or in regions with higher annual or spring/summer UVR dose had a 3- to 4-month earlier menarche than women born at higher latitudes or in regions with lower UVR. On a continuous scale, a 18 increment in latitude resulted in a 0.04-year older age at menarche [95% confidence interval (CI): 0.03, 0.05], whereas a 1-kJ/m(2) increment in annual UVR dose resulted in a 0.42-year younger age at menarche (95% CI: -0.55, -0.29). Conclusion These data further suggest that light exposure in childhood may influence sexual maturation in women.