Background: While the breast cancer risk associated with increasing adult BMI in postmenopausal women can be explained by increases in concentrations of endogenous estrogens the biologic mechanisms behind the inverse association between adolescent BMI and breast cancer risk are still a subject of controversial debate.
Methods: We investigated the association of breast cancer with body size and changes in body size across life time estimated by age-specific BMI Z scores and changes in BMI Z scores from teenage years to middle age in an age-matched population-based case-control study of 2994 Australian women. Logistic regression adjusted for the matching factor age and further potential confounders was used.
Results: Adolescent body leanness in postmenopausal women and excess adult weight gain in all study participants were associated with an increased breast cancer risk with an odds ratio [95% confidence interval] of 1.29 [1.08,1.54] and 1.31 [1.09,1.59], respectively. Interaction analyses restricted to postmenopausal women revealed an increased risk of breast cancer in those who were lean during adolescence and gained excess weight during adulthood (odds ratio [95% confidence interval]: 1.52 [1.19,1.95]) but not in women who were lean during adolescence and did not gain excess weight during adulthood (1.20 [0.97,1.48]) and not in women who were not lean during adolescence and but gained excess weight during adulthood (1.10 [0.95,1.27]) compared to postmenopausal women who were neither lean during adolescence nor gained excess weight.
Conclusion: In postmenopausal women adolescent leanness was only associated with increased breast cancer risk when excess weight was gained during adulthood.
Keywords: Adult; Body mass index; Breast cancer; Childhood; Risk; Weight gain.
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