A study intended to support the evidence of causation found that increasing levels of physical activity and reducing the amount of time spent sitting could help reduce the risk of breast cancer.
The results of the Mendelian randomization study were consistent across all stages and types of breast cancer, suggesting a greater emphasis on exercise as a method for breast cancer prevention.
Mendelian randomization is a method that uses genetic variants as a proxy for a specific risk factor to obtain genetic evidence supporting a causal relationship which in this case was lifelong sedentary behavior and physical activity levels.
Although observational studies have shown that sedentary behavior and physical inactivity are associated with a higher risk of breast cancer, they do not prove the cause of breast cancer.
Mendelian randomization was then used to assess whether lifelong sitting time and physical activity may be causally associated with breast cancer risk, particularly for different cancer types.
Data were included from 130,957 women of whom 69,838 had locally invasive cancers, 6667 had in situ cancers that had not yet spread, and 54,452 women without breast cancer as a comparison group.
Data from previously published studies were used to genetically predict how physically inactive or active the participants in this study were.
The overall risk of breast cancer was then estimated, based on whether or not the women were in menopause; and by cancer type (progesterone or estrogen positive, or HER-2, or all 3 hormones negative/positive), cancer stage (extent and extent of tumor spread), and cancer grade (degree of abnormality of cancer cells).
These case groups included 23,999 peri/premenopausal women with invasive breast cancer and 17,686 women without invasive breast cancer; 45,839 postmenopausal women who had breast cancer and 36,766 who did not have breast cancer.
46,528 tumors were estrogen receptor positive with 11,246 controls; 34,891 progesterone receptor-positive tumors with 16,432 controls; 6945 HER2 positive tumors with 33,214 controls; 1974 triple positive cases; and 4964 triple negative cases.
There were 42,223 cases of invasive lobular/ductal carcinoma with 8795 controls, 3510 cases of ductal carcinoma in situ; 17,583 stage 1 cancers, 15,992 stage 2 cancers, and 4553 stage 3 to 4 cancers; 34,647 moderately abnormal cell tumors and 16,432 highly abnormal cell tumors.
Analysis of the data revealed that a genetically higher level of physical activity was linked to a 41 percent reduction in the risk of invasive breast cancer, and this regardless of cancer type, grade, stage or menopausal status.
Genetically predicted physical activity of vigorous intensity for 3 days or more per week has also been linked to a 38% reduction in breast cancer risk, compared with no vigorous activity. These results were consistent for most case groups.
Additionally, a genetically higher level of sitting time has been linked to a 104 percent higher triple-negative breast cancer risk. These results were consistent for all types of hormone-negative cancers.
While reducing sitting time and increasing physical activity are already recommended to prevent cancer, these study findings provide significant evidence that less sitting time and more physical activity will help reduce breast cancer risk.
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