Measures of energy balance and mammographic density in the Nurses' Health Study.

Mammographic density is a strong risk factor for breast cancer; however the mechanism that underlies this association is unclear. We hypothesized that measures of energy balance early in life and in adulthood may be associated with mammographic density. We conducted a cross-sectional analysis of 1,398 women in the Nurses' Health Study to examine associations between physical activity, childhood and current body fatness, weight gain from age 18 years to present and mammographic density. Percent mammographic density was measured from digitized mammograms by a computer-assisted method. Demographic and lifestyle data were obtained from prospectively collected questionnaires. For all analyses, subjects were stratified into three groups: premenopausal women, postmenopausal women not currently taking hormones, and postmenopausal women currently taking hormones. Childhood body fatness was inversely associated with mammographic density. The correlations ranged from -0.15 to -0.19 in the three strata of women (P<or=0.001). The difference in mean percent mammographic density between the leanest and heaviest body types ranged from 6.2 to 9.9%. Similarly, adult body fatness was inversely associated with percent mammographic density. The correlations ranged from -0.41 to -0.48 in the three strata of women (P<0.0001). The difference in mean percent mammographic density between the leanest and heaviest body types ranged from 22.3 to 35.1%. Weight gain from age 18 was also inversely associated with mammographic density. There was no association between recent physical activity and mammographic density. These findings indicate that childhood and adult body fatness and weight change from age 18 are inversely associated with mammographic density.