Physical activity may protect against breast cancer by modulating breast tissue composition. We evaluated the association of physical activity with two visual assessments of breast tissue composition—percentage of mammographic density (a radiologic observation) and age-related lobular involution (a histologic assessment).
Among 164 premenopausal and postmenopausal women with breast cancer, physical activity (household, occupational, and recreational) performed during the year preceding the diagnosis was evaluated using a validated questionnaire. Percentage of mammographic density was assessed in the contralateral breast by a computer-assisted method. Age-related lobular involution was assessed in normal breast tissue on H&E-stained slides. Multivariate generalized linear models were used to assess associations by quartiles of physical activity.
Overall, we observed no significant association between total physical activity and percentage of mammographic density or degree of lobular involution. However, occupational physical activity was significantly positively associated with the predominant type I/no type III lobules among premenopausal women (last quartile: prevalence ratio [PR], 5.92; Ptrend = 0.04). Although total physical activity was positively associated with the predominant type I/no type III lobules among premenopausal women (last quartile: PR, 2.61; Ptrend = 0.08), an inverse association was observed among postmenopausal women (last quartile: PR, 0.44; Ptrend = 0.01). Higher levels of household physical activity were significantly associated with higher prevalence of lower mammographic density and complete involution among postmenopausal women (last quartile: PR, 1.21; Ptrend = 0.01).
Physical activity may be associated with less dense and more involuted breasts. Physical activity's effect on mammographic density or age-related lobular involution may mediate, in part, its protective effect against breast cancer.