Objective: Although physical activity modulates the hypothalamic-pituitary-ovarian axis, the few studies that have investigated whether physical activity is associated with age at natural menopause have yielded mixed results. We set out to determine whether physical activity is associated with the timing of natural menopause in a large cohort of California women overall and by smoking history.
Methods: We investigated the association between long-term physical activity (h/wk/y) and age at natural menopause among 97,945 women in the California Teachers Study. Multivariable Cox proportional hazards regression methods were used to calculate hazard ratios (HRs) and 95% confidence intervals (CIs). The impact of cigarette smoking (never smoker, former light smoker, former heavy smoker, current light smoker, and current heavy smoker) as an effect modifier was evaluated.
Results: In a multivariable model adjusted for body mass index at age 18 years, age at menarche, race/ethnicity, and age at first full-term pregnancy, increased physical activity was statistically significantly associated with older age at natural menopause (Ptrend = 0.005). Higher body mass index at age 18 years (Ptrend = 0.0003) and older age at menarche (Ptrend = 0.0003) were also associated with older age at natural menopause. Hispanic ethnicity (vs non-Hispanic whites; HR, 1.17; 95% CI, 1.09-1.26), current smokers (vs never smokers; HR, 1.68; 95% CI, 1.60-1.75 for current light smokers; HR, 1.38; 95% CI, 1.33-1.44 for current heavy smokers), and older age at first full-term pregnancy (HR≥29, 2+ full-term pregnancies vs HR<29, 2+ full-term pregnancies, 1.10; 95% CI, 1.06-1.14) were associated with earlier age at natural menopause. Upon stratification by smoking history, increased physical activity was statistically significantly associated with older age at natural menopause among heavy smokers only (HRhighest quartile vs HRlowest quartile, 0.88; 95% CI, 0.81-0.97; Ptrend = 0.02 for former heavy smokers; HRhighest quartile vs HRlowest quartile, 0.89; 95% CI, 0.80-0.99; Ptrend = 0.04 for current heavy smokers).
Conclusions: Age at natural menopause is a complex trait; the determinants of age at natural menopause, including physical activity, may differ by smoking status.
From the 1Department of Population Sciences, City of Hope National Medical Center, Duarte, CA; 2Department of Oncology, Oslo University Hospital, Ulleval, Norway; 3Department of Preventive Medicine, University of Southern California Keck School of Medicine, Los Angeles, CA; 4Cancer Prevention Institute of California, Berkeley, CA; and 5Department of Health Research and Policy, Stanford University School of Medicine, Palo Alto, CA.
Received May 10, 2012; revised and accepted July 26, 2012.
Funding/support: This work was supported by the National Institutes of Health (grants CA 77398 and CA 136967). A.E. was supported by the Norwegian Cancer Society (grant 05087) and the Fulbright Foundation.
Financial disclosure/conflicts of interest: None reported.
Address correspondence to: Leslie Bernstein, PhD, Division of Cancer Etiology, Department of Population Sciences, Beckman Research Institute, City of Hope National Medical Center, 1500 East Duarte Road, Duarte, CA 91010. E-mail: email@example.com