It can. While exercise can decrease the amount of fat in the breast, the glandular or dense breast tissue is not affected by exercise. So, if a woman loses a lot of weight due to exercise, her breasts can appear more dense due to loss of fat (the amount of fat decreases while the amount of dense tissue remains the same) .
While exercise does not decrease the actual amount of dense tissue in the breasts, it does decrease the overall risk of developing breast cancer .
1. Peters TM, Ekelund U, Leitzmann M, et al. Physical activity and mammographic breast density in the EPIC-Norfolk cohort study. Am J Epidemiol 2008; 167:579-585
2. Azam S, Kemp Jacobsen K, Aro AR, et al. Regular physical activity and mammographic density: a cohort study. Cancer Causes & Control 2018; 29:1015-1025