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25th October 2021
Physical inactivity could be the cause of 46,356 cancer cases across the US based on an analysis by researchers from the Department of Surveillance and Health Equity Science, Atlanta, US, on behalf of the American Cancer Society. The importance of undertaking physical activity as a means of reducing the risk of cancer, was highlighted in a 2019 systematic review which found strong evidence for an association between the highest versus lowest physical activity levels and reduced risks of bladder, breast, colon, endometrial, oesophageal adenocarcinoma, renal and gastric cancers. The review also identified how greater amounts of physical activity reduced cancer-specific mortality in those with breast, colorectal, or prostate cancer, with relative risk reductions of between 40 and 50 percent.
But how the risk of cancer due to physical inactivity vary across different states in the US was the question posed in the study by the American Cancer Society. The researchers estimated the proportion of cancer cases attributable to physical inactivity overall and for seven types of cancer (oesophagus, stomach, colon, breast in women, corpus uteri, kidney and bladder). The researchers drew on data from the Behavioural Risk Factor Surveillance System (BRFSS), which collects state-level estimates of various health-related behaviours including information on physical activity. The information on physical activity was drawn from surveys conducted between 2003 and 2006 across a wide age range and the number of incident cancer cases in 2013 – 2016, were obtained from the US Cancer Statistics database, to allow for a lag time between exposure prevalence and the occurrence of cancer.
The levels of physical activity were categorised in terms of metabolic equivalent task (MET) hours/week. A MET is the amount of energy an average adult expends sitting at rest. With the US physical activity guidelines recommending that adults engage in 2.5 to 5 hours/week of moderate intensity activity, this equates to at least 7.5 – 15 MET hours/week. For the purposes of their analysis, the researchers defined optimal physical activity as > 5 hours/week or > 15 MET-hours/week. The team also calculated the population attributable fraction (PAF) which can be used to quantify how a risk factor contributes to the outcome of interest compared, in this case, physical inactivity and cancer.
When optimal physical activity was defined as > 15 MET-hours/week, the overall PAF for both sexes was 3% (95% CI 2.9 – 3%). This amounted to 46,356 incident cancer cases in adults aged > 30 years, that could be attributed to physical inactivity. The overall PAF was higher in women than men (4.1% vs 1.8%), with the result that 32,089 incident cancer cases in women and 14,277 cases in men were attributable physical inactivity.
With respect to the individual cancers, stomach cancer had the highest PAF (16.9%), followed by corpus uteri (11.9%), kidney (11%), colon (9.3%), oesophagus (8.1%), breast (6.5%) and bladder (3.9%). In addition, PAF values varied across the US for the different cancer. For example, the PAF for stomach cancer was 14% in Montana and 21.1% in Kentucky.
The researchers concluded that physical inactivity was a potentially avoidable cause of a large number of cancers and that promoting physical activity could prevent many cases.
Minihan AK et al. Proportion of Cancer Cases Attributable to Physical Inactivity by US State, 2013-2016. Med Sci Sports Exerc 2021