A large-scale international study involving more than a hundred thousand women shows that increasing daily physical activity and cutting down time spent sitting can lower the risk of breast cancer. Researchers from Australia, the United Kingdom, and the United States used genetic analysis to explore whether overall activity levels have a true causal effect on cancer risk, rather than simply being associated with other healthy behaviors.
The British Journal of Sports Medicine published the findings, drawing on data from 130,957 women, including 76,505 who had breast cancer. While earlier work identified a link between being physically active and a reduced cancer risk, proving a direct cause-and-effect relationship has been challenging, and results across studies have varied. This new work attempts to address that gap by applying a method that can suggest causation more robustly than observational data alone.
Bridget Lynch, the senior author, explained that the question has long puzzled scientists: whether activity itself lowers risk or whether physically active women simply share other protective habits. The current study, she noted, provides evidence consistent with a direct protective effect of physical activity on breast cancer risk. It points to a measurable reduction in risk across different breast cancer subtypes, suggesting a broad benefit from staying active.
The research team employed Mendelian randomization, a genetic approach that uses inherited variants as proxies for traits like physical activity and sedentary behavior. This method helps to disentangle cause and correlation by looking at how genetic predispositions related to activity influence cancer outcomes, independent of many confounding factors. The results indicate that higher overall activity and more movement are associated with a markedly lower chance of developing invasive breast cancer.
In analyses of both premenopausal and postmenopausal women, engaging in vigorous physical activity on three or more days each week correlated with a substantial reduction in breast cancer risk—about 38 percent—compared with no active exercise. Those gains persisted even when accounting for other lifestyle variables, underscoring the potential importance of regular, vigorous movement as a public health strategy.
Regarding sedentary time, the researchers found that for every additional 100 minutes spent sitting or lying down each day, there was an estimated 20 percent increase in overall breast cancer risk. The risk appeared to rise even more for the triple-negative subtype, a form known for being more aggressive and harder to treat. This highlights the potential harm of prolonged sitting in relation to breast cancer outcomes, independent of routine exercise levels.
Biologically, the team notes that physical activity may reduce cancer risk by lowering circulating levels of estrogen and androgens, hormones implicated in several breast cancer pathways. Exercise can also modulate inflammation, another process tied to cancer development, which could contribute to the protective effect observed in the study population. These mechanisms help explain why movement appears to exert a direct influence on cancer risk, beyond associated healthy behaviors.
The study contributes to a growing evidence base that supports practical recommendations for risk reduction. For individuals in Canada and the United States, increasing daily movement—whether through structured workouts or simply more active routines—combined with measures to interrupt long periods of sitting, could be a meaningful component of breast cancer prevention. Public health messages may gain clarity from these findings, encouraging people to integrate more activity into daily life and to break up sedentary stretches throughout the day, even in busy schedules. The researchers emphasize that every step or minute of movement matters, and that cumulative activity across weeks and months appears to translate into measurable cancer risk reductions. These results align with broader guidelines promoting physical activity for overall health and well-being, now reinforced by cancer prevention data. [citation attribution]