Longitudinal Association of Anthropometry with Mammographic Breast Density

Objective: In cross-sectional studies body mass index (BMI) is negatively associated with percent breast density, a strong risk factor for breast cancer. We sought to evaluate longitudinal associations between BMI and mammographic breast density.

Methods: We studied a prospective cohort of 834 women enrolled in an ancillary study to the Study of Women's Health Across the Nation (SWAN). Height and weight were measured at annual clinic visits. Routine screening mammograms were collected and read for breast density using manual planimetry. Longitudinal associations between BMI and changes in dense breast area and percent density were evaluated with random effects regression models.

Results: Mean follow-up was 4.8 years (SD 1.8), and the mean number of observations per participant was 5.6 (range 1-8). Mean annual weight change was +0.22 kg/year. In fully adjusted models, BMI and weight were not associated with changes in dense breast area (β=-0.0105, p=0.34 and β=-0.0055, p=0.20, respectively), but were strongly negatively associated with changes in percent density (β=-1.18, p<0.001 and β=-0.44, p<0.001, respectively).

Conclusions: This longitudinal study demonstrates that BMI and weight are not associated with the dense area, yet are negatively associated with percent density. Effects of anthropometry on percent breast density may reflect effects on non-dense tissue, rather than on the dense tissue where cancers arise.

Public Health Implications: These results improve our understanding of how increased BMI acts to promote breast cancer, and may lead to opportunities for disease prevention and early detection in the future.