LAUSR

Background:Prolonged sitting and lower levels of physical activity have been associated with increased levels of parent oestrogens (oestrone and oestradiol), the key hormones in female cancers, in postmenopausal women. However, it is unknown whether sitting and physical activity are associated with circulating oestrogen metabolite levels.Methods:Among 1804 postmenopausal women enrolled in the Women’s Health Initiative Observational Study, 15 serum oestrogens/oestrogen metabolites were quantified using liquid chromatography–tandem mass spectrometry. Physical activity and sitting were self-reported via questionnaire. Using baseline, cross-sectional data, geometric means (GM) of oestrogens/oestrogen metabolites (pmol l−1) were estimated using inverse probability weighted linear regression, adjusting for potential confounders and stratified on menopausal hormone therapy (MHT) use.Results:Longer time spent sitting (⩾10 vs ⩽5h per day) was associated with higher levels of unconjugated oestrone, independent of moderate- to vigorous-intensity physical activity and body mass index, among both never/former (GM=70.6 vs 57.7) and current MHT users (GM=242 vs 179) (P-trend ⩽0.03). Among never/former MHT users, sitting (⩾10 vs ⩽5h per day) was positively associated with 2-methoxyestradiol (GM=16.4 vs 14.4) and 4-methoxyestradiol (GM=2.36 vs 1.98) (P-trend ⩽0.04), independent of parent oestrogens. Inverse associations between moderate- to vigorous-intensity physical activity (⩾15 vs 0 metabolic equivalent task-hours per week) and parent oestrogens were found as expected. After adjustment for parent oestrogens, physical activity was not associated with oestrogen metabolites.Conclusions:Our data suggest that prolonged sitting and lower moderate- to vigorous-intensity physical activity are associated with higher levels of postmenopausal oestrogens/oestrogen metabolites, the oestrogen metabolism patterns that have previously been associated with higher endometrial and breast cancer risk.