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

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. The objective of this study was to evaluate the associations between dietary soy intake at different life stages and MD in premenopausal Chinese women. 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. 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, = 0.023) and childhood (β = -0.071, SE = 0.032, = 0.028) was significantly and inversely associated with adult MD. Young adult intake showed a non-significant inverse trend (β = -0.052, = 0.075), and current intake showed no association ( = 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 < 0.05). Women with consistently high early-life soy intake had 5.8-6.6% lower adjusted MD than those with consistently low intake. : 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.