Physical activity and breast cancer risk: results from the UK Biobank prospective cohort

Abstract

Background: Previous 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.

Methods: We 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.

Results: A 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.

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.

Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1. Association between physical activity and risk of invasive breast cancer.

Self-reported physical activity was grouped into quartiles. Relative risk for the association between self-reported physical activity and risk of invasive breast cancer is plotted against the median repeat assessment physical activity value within each quartile of baseline self-reported physical activity. The analysis is stratified by age at recruitment, region of recruitment and socioeconomic status, and is adjusted for family history of breast cancer, height, number of births, age at menarche, age at first birth, oral contraceptive use, ever had breast cancer screening, smoking status, alcohol intake frequency, hormone replacement therapy use, age at menopause and a variable generated by the cross-classification of body fat mass and menopausal status. The figure shows point estimates and 95% group-specific confidence intervals. CI, confidence interval.

Fig. 2. Relative risk of physical activity and breast cancer risk by BMI, ever use of HRT, number of births and alcohol intake.

Analyses are stratified by age at recruitment, region of recruitment and socioeconomic status, and are adjusted for family history of breast cancer, height, number of births, age at menarche, age at first birth, oral contraceptive use, ever had breast cancer screening, smoking status, alcohol intake frequency, hormone replacement therapy use, age at menopause and a variable generated by the cross-classification of body fat mass and menopausal status. Analyses of BMI 2 and 25 kg/m2 or more were adjusted for a BMI group-specific variable generated by the cross-classification of body fat mass and menopausal status instead of the overall cross-classified variable. Analyses stratified by ever use of hormone replacement therapy were restricted to postmenopausal women. The figure shows the linear trend per 50 MET hours/week of self-reported physical activity for each subgroup and 95% group-specific confidence intervals. BMI, body mass index; HRT, hormone replacement therapy; RR, relative risk. Note: The number of cases does not sum to the total, as information on ever use of HRT was missing for 4 postmenopausal cases and information on the number of births was missing for 2 cases.