Background: Low circulating levels of coenzyme Q10 (CoQ 10) have been associated with increased cancer incidence and poor prognosis for a number of cancer types, while a recent prospective study observed a positive association for CoQ10 with breast cancer risk. Methods: We prospectively examined the association of plasma CoQ10 with breast cancer risk in a nested case-control study of Chinese women within the Shanghai Women's Health Study (SWHS). Prediagnostic plasma samples were obtained from 340 cases and 653 age-matched controls and analyzed for total CoQ10. Results: A borderline significant inverse association for breast cancer incidence with plasma CoQ10 level was observed by a conditional logistic regression model adjusted for age and age at first live birth, which became significant after elimination of cases diagnosed within 1 year of blood draw (Ptrend = 0.03). This association was independent of menopausal status. Plasma CoQ10 levels were also observed to be significantly associated with circulating γ-tocopherol (r = 0.50; P < 0.0001) and α-tocopherol (r = 0.38; P < 0.0001) levels. Conclusions: Circulating levels of CoQ10 were generally low in this population and the observed association with breast cancer risk may be limited to those women with exceptionally low values. Impact: This study reports an inverse relationship between circulating CoQ10 and breast cancer risk, while the only other prospective study of CoQ10 and breast cancer to date found a positive association. Lower levels of CoQ10 in the SWHS population suggest that the 2 studies may not be contradictory and indicate a possible nonlinear (U-shaped) association of CoQ10 with risk.
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