Ultraviolet radiation and age at natural menopause in a nationwide, prospective US cohort

Abstract

BACKGROUND AND AIM: Ultraviolet radiation (UV) is essential for dermal conversion of vitamin D, which is suggested to support reproductive health. However, little is known about the association of UV exposure with reproductive aging. Thus, we aimed to examine the associations between long-term UV exposure and age at natural menopause in a large, nationwide, prospective cohort. METHODS: Women in the Nurses’ Health Study II, a nationwide prospective US cohort enrolled in 1989, were included if they were premenopausal at age 40 (N=105,631) and were followed through 2015 or self-reported menopause, whichever came first. Data on menopause, residence, and confounders were available every two years. Erythemal UV radiation from a high-resolution geospatial model was linked to the participants’ residential histories. Early-life UV was estimated using UV measures in the reported state of residence at birth, age 15, and age 30. We used time-varying Cox proportional hazard models to estimate the hazard ratio (HR) and 95% confidence intervals (CIs) for natural menopause, adjusting for potential confounders and predictors of menopause. RESULTS:A total of 63,801 women reported natural menopause during the 1,051,185 person-years of follow-up. We found weak associations with delayed menopause for long-term UV exposure after age 40 (adjusted HR comparing Q4 to Q1 of cumulative average UV: 0.96, 95% CI: 0.94, 0.99). There was a suggestive association between UV at age 30 with later onset of menopause (adjusted HR comparing Q4 to Q1: 0.97, 95% CI: 0.95, 1.00) but not with UV at birth or age 15. Similar weak associations were found when restricting the outcome to women who reported menopause between age 40 and 45. CONCLUSIONS:UV exposure in adulthood was weakly associated with later onset of menopause. Although consistent with previous findings on vitamin D intake and menopause in the same population, the associations are unlikely to be of clinical relevance. KEYWORDS: reproductive outcomes, long-term exposure, female