Associations of residential greenness exposures with age at natural menopause in the Nurses' Health Study II Cohort

Abstract

Background Age at natural menopause is an important indicator of reproductive aging for women and has been suggested to be affected by environmental factors. However, there is little evidence of the impact of greenness on menopausal age, or possible effect modification by physical activity. Methods We followed 105,665 premenopausal participants from the Nurses’ Health Study II, an ongoing nationwide female cohort, from age 40. Self-reported menopausal status and physical activity were obtained on follow-up questionnaires. Women who reported a hysterectomy, oophorectomy, cancer diagnosis, or death were excluded at baseline or censored during the follow-up. We calculated physical activity levels using metabolic equivalent task hours per week (MET hour/week). Residential greenness was measured using the Normalized Difference Vegetation Index in July of each year at 1 km spatial resolution, and was calculated as cumulative average from age 40, ages 40 to 45, and in the previous year. A time-varying Cox proportional hazard model was used to obtain the hazard ratio of natural menopause at any age adjusting for calendar year, region, lifestyle and reproductive factors, neighborhood socioeconomic status, and residence-level PM2.5. Results A total of 64,251 participants reported natural menopause during 1,054,109 person years. Exposure to residential greenness was not associated with age at natural menopause in any exposure time window (HR=1.01-1.02 comparing the highest to the lowest quartile). No statistically significant effect modification was observed with physical activity. However, among women with very low physical activity (less than 3 MET hours/week), there was a suggestion of earlier menopause with higher exposure to greenness from age 40 (HR=1.09, 95%CI=1.01, 1.18) and from age 40-45 (HR=1.08, 95%CI=1.01, 1.17). Conclusion Our data showed residential greenness was not associated with menopausal age in general. However, we observed suggestive associations between greenness and earlier menopause among women with low levels of physical activity.