Lifetime Soy Intake and Adult Mammographic Density in Chinese Premenopausal Women.

Background: Soy intake has been proposed as a protective factor for breast cancer, especially when exposure occurs early in life. Mammographic density (MD) is a strong predictor of breast cancer risk, but evidence linking soy intake at specific life stages to adult mammographic density remains limited. Objective: The objective of this study was to evaluate the associations between dietary soy intake at different life stages and MD in premenopausal Chinese women. Methods: Dietary soy intake was assessed using a validated soy food frequency questionnaire for the past 12 months and retrospectively for earlier life stages (childhood: 6-12 years; adolescence: 13-18 years; young adulthood: 20-34 years), with recall aided by a life history calendar. MD was measured from bilateral cranio-caudal mammograms using a standardized computer-assisted method. Multivariable regression models were used to evaluate associations between soy protein and isoflavone intake at different life stages and MD, adjusting for relevant confounders. Results: Among 815 premenopausal women (mean age 40.9 y), mean current soy protein and isoflavone intakes were 10.3 g/day and 22.0 mg/day, respectively. Soy intakes across life stages were moderately correlated (r = 0.33-0.81). After multivariable adjustment, soy protein intake during adolescence (β = -0.067, SE = 0.029, p = 0.023) and childhood (β = -0.071, SE = 0.032, p = 0.028) was significantly and inversely associated with adult MD. Young adult intake showed a non-significant inverse trend (β = -0.052, p = 0.075), and current intake showed no association (p = 0.93). Higher mean early-life (ages 6-18) and life course soy intakes were also inversely associated with MD (β range: -0.077 to -0.082; all p < 0.05). Women with consistently high early-life soy intake had 5.8-6.6% lower adjusted MD than those with consistently low intake. Conclusions: Early-life soy exposure may influence adult breast tissue composition and represents a potentially modifiable protective factor in breast cancer prevention. These findings carry important public health implications, particularly for populations experiencing dietary westernization.