Abstract
BackgroundPrevious studies suggest a protective role of physical activity in breast cancer risk, largely based on self-reported activity. We aimed to clarify this association by examining breast cancer risk in relation to self-reported physical activity, informed by accelerometer-based measures in a large subset of participants.MethodsWe analysed data from 47,456 premenopausal and 126,704 postmenopausal women in UK Biobank followed from 2006 to 2014. Physical activity was self-reported at baseline, and at resurvey in a subsample of 6443 participants. Accelerometer data, measured from 2013 to 2015, were available in 20,785 women. Relative risks (RRs) and 95% confidence intervals (CIs) were calculated by using multivariable-adjusted Cox regression.ResultsA total of 3189 cases were diagnosed during follow-up (mean = 5.7 years). Women in the top compared with the bottom quartile of self-reported physical activity had a reduced risk of both premenopausal (RR 0.75; 95% CI 0.60–0.93) and postmenopausal breast cancer (RR 0.87; 95% CI 0.78–0.98), after adjusting for adiposity. In analyses utilising physical activity values assigned from accelerometer measurements, an increase of 5 milli-gravity was associated with a 21% (RR 0.79; 95% CI 0.66–0.95) reduction in premenopausal and a 16% (RR 0.84; 95% CI 0.73–0.96) reduction in postmenopausal breast cancer risk.ConclusionsGreater physical activity is associated with a reduction in breast cancer risk, which appears to be independent of any association it may have on risk through its effects on adiposity.
Highlights
Previous prospective studies have assessed the association between self-reported physical activity and breast cancer, with overall findings that support a protective role for physical activity in breast cancer risk.[1,2,3]
Some studies have reported greater reductions in risk associated with physical activity among postmenopausal women[1] and those with a relatively low body mass index (BMI),[5] but it is unclear as to what extent these observed differences may be attributed to differential reporting bias in self-reported estimates of physical activity by subgroups of menopausal status and BMI, as opposed to genuine effect modification
The most active postmenopausal women reported an average of 94.9 Metabolic equivalents (METs) hours/week compared with 91.8 MET hours/week of physical activity in the most active premenopausal women
Summary
Previous prospective studies have assessed the association between self-reported physical activity and breast cancer, with overall findings that support a protective role for physical activity in breast cancer risk.[1,2,3] self-reported responses from questionnaires are prone to both inaccurate reporting and bias, leading to random and systematic error, respectively.[4]. The purpose of this study was to assess the associations between physical activity and breast cancer risk in over 47,000 premenopausal and 126,000 postmenopausal women, by using baseline self-reported physical activity in conjunction with repeat questionnaires and subsequently measured accelerometer values in large subsamples to better inform the extent to which selfreported estimates may be biased and to reduce misclassification of physical activity level. Women in the top compared with the bottom quartile of self-reported physical activity had a reduced risk of both premenopausal (RR 0.75; 95% CI 0.60–0.93) and postmenopausal breast cancer (RR 0.87; 95% CI 0.78–0.98), after adjusting for adiposity. CONCLUSIONS: Greater physical activity is associated with a reduction in breast cancer risk, which appears to be independent of any association it may have on risk through its effects on adiposity
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