Physical activity, metabolic syndrome and insulin resistance in relation to mammographic density

The primary purpose of the present study was to provide insight about modifiable factors that may affect breast cancer risk through involvement of mammographic density (MD). Percent MD, the proportion of total breast tissue area that appears radiologically dense, naturally declines with age but is a strong risk factor for breast cancer.;Chapter 1. Higher levels of physical activity (PA) may protect against breast cancer by increasing the age-related decline in percent MD and thereby lowering the cumulative exposure to growth factors that affect breast cell proliferation. We used longitudinal data (1996-2004) from 722 participants in the MD sub-cohort of the Study of Women's Health Across the Nation (SWAN), a multiethnic cohort of women who were pre- and early perimenopausal at baseline. We computed the total PA score as the sum of PA domains, specifically sports/exercise, household/care-giving and daily routine. We found that a one-unit increase in total PA score was associated with a very small, 0.09% weaker, annual decline in percent MD (SE = 0.03, P = 0.01). Increasing total PA score was associated with less decline in non-dense breast area (P < 0.01), and not associated with the change in dense breast area (P = 0.17).;Chapter 2. Insulin resistance and the metabolic syndrome (MetS) are risk factors for postmenopausal breast cancer, independent of adiposity, and percent MD may be an intermediate marker between these metabolic abnormalities and breast cancer. Examining cross-sectional data from 790 pre- and early perimenopausal women enrolled in the MD sub-cohort of the Study of Women's Health Across the Nation (SWAN), percent MD was not associated with the MetS, independent of body mass index (BMI), or with insulin resistance, as measured by the homeostasis model assessment of insulin resistance (HOMA-IR) index, independent of waist circumference. However, we did observe a modest inverse association of percent MD with the MetS component, central adiposity [beta = -4.9, P = 0.01 for high (defined as ≥ 88 cm for Caucasian and African American women and ≥ 88 cm for Chinese and Japanese women)], independent of BMI. In a subset of 728 women, for whom we had longitudinal data (1996-2004), we did not find that MetS or insulin resistance modified the age-related decline in percent MD after adjustment for adiposity.;Chapter 3. Patterns of leisure time physical activity from adolescence through middle adulthood may play an important role in preventing adult obesity, a modest risk factor for postmenopausal breast cancer. We estimated the average annual MET-hours/week spent in recreational activities over a woman's lifetime, as measured cumulatively since menarche, in a subset of 112 women of the 814 women who participated in the Sacramento Community Health Study (SCHS), a cross-sectional, community-based health study. We found that higher levels of participation in recreational activities from menarche through middle adulthood were associated with lower current body size as measured by BMI. In particular, approximately 2.6 hours/week of moderate or vigorous activity in middle adulthood were associated with a healthy weight. Women who remained active from early through middle adulthood had approximately 5 kg/m2 lower BMI compared to women who remained inactive during the same period (beta = -4.1, SE = 1.3, P < 0.01), independent of total caloric intake. Our findings were consistent with the recommendation for 30 minutes of moderately cardio-intense activity five days/week by the American Heart Association and the American College of Sports Medicine.;Conclusion. Increased PA was not associated with an increase in the age-related decline in percent MD. Also, women with the MetS and insulin resistance did not have higher percent MD than women without the MetS or insulin resistance. However, these results do not exclude the possibility that PA, insulin resistance, and MetS may affect breast cancer risk via a separate mechanism than through an effect on dense breast tissue area. We found PA, MetS and insulin resistance to affect the quantity of non-dense but not dense tissue area. Habitual participation in recreational activities over a woman's lifetime, and especially participation in vigorous activities, favorably influenced body size in adulthood.