Background: Studies of postmenopausal breast cancer have reported positive associations with body size and composition but it is uncertain whether these are due to non-adipose, adipose mass, or central adiposity, and whether they are limited to subgroups defined by age or tumor characteristics.
Methods: In a prospective cohort study of women ages 27 to 75, body measurements were taken directly; fat mass and fat-free mass being estimated by bioelectrical impedance analysis, and central adiposity by waist circumference. Among 13,598 women followed on average for 9.1 years, 357 invasive breast cancers were ascertained via the population cancer registry. Data were obtained on estrogen receptor and progesterone receptor status, grade, and stage.
Results: Estimates of body size such as fat-free mass [hazard ratio per 10 kg increase = 1.45, 95% confidence interval (CI) 1.16-1.82], fat mass (hazard ratio per 10 kg increase = 1.18, 95% CI, 1.06-1.31), and waist circumference (hazard ratio per 10 cm increase = 1.13, 95% CI, 1.03-1.24) were associated with breast cancer risk. There was no association with risk before 15 years postmenopause. About 15 years after menopause, risk increased sharply and remained elevated. There was some evidence that this association might be stronger for estrogen receptor-positive and poorly differentiated tumors but no evidence that it differed by stage.
Conclusion: Given that elements of body size and composition are positively associated with breast cancer risk, although not until 15 or more years postmenopause, it is possible that women could reduce risk by maintaining ideal body weight after menopause.