Evidence suggests that recreational physical activity (RPA) during adolescence is associated with lower breast cancer (BC) risk, but the biological mechanisms underlying this relationship remain unclear. We examined the associations between RPA and three BC biomarkers–breast tissue composition (BTC), oxidative stress, and inflammation–in adolescent girls. We also investigated how oxidative stress and inflammation relate to BTC during this developmental stage.

We analyzed data from a population-based urban cohort of 191 Black/African American and Hispanic (Dominican) adolescent girls (ages 11–20 years). Participants reported the number of hours they participated in organized and unorganized RPA in the past week, categorized as none, < 2 h, and ≥ 2 h. We used optical spectroscopy to measure BTC: specifically, chromophores that are positively (percent water content and percent collagen content) or negatively (percent lipid content) correlated with mammographic breast density. We measured urinary concentrations of 15-F_2t-isoprostane, a sensitive and specific marker of oxidative stress, and blood biomarkers of inflammation, including TNF-α, IL-6, and high-sensitivity C-reactive protein. We used multivariable linear regression models to examine associations, adjusting for age, race, ethnicity, and percent body fat.

51% of adolescent girls reported no past-week engagement in any type of RPA; 73% reported no participation in organized activities, and 66% reported no participation in unorganized activities. Girls who engaged in ≥ 2 h of organized RPA in the past week, compared to none, had lower percent water content in the breast (β: − 0.41, 95% CI: − 0.77, − 0.05), and this association was not modified by percent body fat; they also had lower urinary concentrations of 15-F_2t-isoprostane (β: − 0.50, 95% CI: − 0.95, − 0.05). An association was observed between higher urinary concentrations of 15-F_2t-isoprostane and higher percent collagen content in the breast (β: 0.15, 95% CI: 0.00, 0.31). RPA was not associated with the measured inflammatory biomarkers, and these biomarkers were not associated with BTC after adjusting for percent body fat.

These findings support that RPA is associated with BTC and oxidative stress in adolescent girls, independent of body fat. Additional longitudinal research is needed to understand the implications of these findings regarding subsequent BC risk.