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Ekelund and colleagues should be commended for developing this valuable pooled resource1 to estimate the amount of physical activity needed to minimise excess mortality risk from sedentary time. Here we comment primarily on the cancer outcomes for the report published in BJSM2 and more broadly on the public health implications of this research.
Research examining the adverse effects of sitting time on cancer risk and mortality is relatively recent. This study examines the potential for physical activity to modify adverse effects of too much sitting on cancer mortality and demonstrates that high TV-viewing time may be associated with increased cancer mortality unless very high activity levels are achieved (≥35.5 Metabolic Equivalent of Task (MET)-hour/week). The authors rightly note that studying overall cancer mortality may obscure differences in associations by specific cancer sites. Cancer includes hundreds of diseases with distinct aetiologies, and investigation of total cancer mortality is expected to produce relatively weak associations. For instance, we previously reported that high physical activity was associated with 11% lower overall cancer mortality, but risk estimates were 2–3 times stronger for site-specific cancer death.3
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