Background Body size and physical activity are important modifiable risk factors for breast cancer. However, many previous studies are limited by the use of body mass index (BMI), which is unable to distinguish between fat and lean mass. Questions also remain over the role of vigorous compared to lower intensity physical activity in relation to breast cancer risk. We investigate the associations between BMI, body fat percentage, waist-to-hip ratio, waist-to-height ratio, total and vigorous physical activity and both premenopausal and postmenopausal breast cancer.
Methods We analysed data from 48,713 premenopausal and 127,850 postmenopausal women in UK Biobank followed prospectively from 2006 through 2012. We observed 443 premenopausal and 1,422 postmenopausal incident invasive breast cancers during a mean 3.84 years of follow-up. Body size was measured by trained technicians. Self-reported frequency and duration of walking, moderate, and vigorous physical activity was collected by touchscreen questionnaire and calculated as metabolic equivalent task hours per week (MET-hrs/wk). Multivariable-adjusted Cox regression was used. Current users of hormone replacement therapy (HRT) were excluded. All analyses were stratified by age, region, and socioeconomic status and adjusted for family history of breast cancer, previous HRT use, height, parity, age at first birth, smoking, age at menarche, alcohol intake frequency, and age at menopause (for postmenopausal women only). Stata 13 (StataCorp LP, Texas, USA) was used for all statistical analyses.
Results All measures of adiposity were positively associated with increased breast cancer risk in postmenopausal but not premenopausal women. Compared with postmenopausal women in the lowest quartile of body fat percentage (16.8–32.9%; median, 29.6%), women in the highest quartile (41.9–54.3%; 44.8%) had an increased risk of breast cancer (HR, 1.47; 95% confidence interval, 1.24–1.74; ptrend < 0.001). Vigorous physical activity was associated with lower breast cancer risk in postmenopausal but not premenopausal women. Postmenopausal women who participated in an average of 17.5 MET-h/week of vigorous physical activity had a 15% lower risk of breast cancer (HR 0.85; 0.74–0.98; ptrend = 0.025) which was attenuated after adjusting for body fat percentage (HR 0.88, 0.77–1.02, ptrend = 0.084). Higher levels of total physical activity were associated with non-statistically significant breast cancer risk reductions in all women.
Conclusion Postmenopausal breast cancer risk is strongly associated with greater adiposity. Although greater vigorous physical activity is protective against postmenopausal breast cancer, adjusting for body fat percentage attenuated the association. Longer follow-up is needed to clarify associations, particularly in premenopausal women. In future analyses, we plan to subdivide cancers by hormone receptor status.
- breast cancer
- physical activity
- body size
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