Observational studies show that physical inactivity and sedentary behaviour are linked to higher breast cancer risk, but proving they cause breast cancer is another matter.
Mendelian randomisation analysis of data showed that a higher overall level of genetically predicted physical activity was associated with a 41% lower risk of invasive breast cancer, and this was largely irrespective of menopausal status, tumour type, stage, or grade.
Similarly, genetically predicted vigorous physical activity on three or more days of the week was associated with a 38% lower risk of breast cancer, compared with no self-reported vigorous activity. These findings were consistent across most of the case groups.
Finally, a greater level of genetically predicted sitting time was associated with a 104% higher risk of triple negative breast cancer. These findings were consistent across hormone-negative tumour types.
The findings were unchanged after factoring in the production by a single gene of two or more apparently unrelated effects (pleiotropy), such as smoking and overweight, for example. The findings provide "strong evidence" that more overall physical activity and less sitting time are likely to reduce breast cancer risk
'Physical activity, sedentary time and breast cancer risk: a Mendelian randomisation study' by Brigid M Lynch et al. in British Journal of Sports Medicine