Abstract 864: Body size and composition, physical activity and breast cancer risk in UK Biobank
Authors: Guo Wenji , Reeves Gillian K. , Key Timothy J. 
Published: August 2015

Abstract
Background: Body size and physical activity are two important modifiable risk factors for breast cancer. 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 (WHR), waist-to-height ratio (WHtR), total and vigorous physical activity and both premenopausal and postmenopausal breast cancer.
Methods: We analyzed data from 48,713 premenopausal and 127,850 postmenopausal women followed 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. Current users of hormone replacement therapy (HRT) were excluded from the analyses. Multivariable-adjusted Cox regression was used. All analyses were stratified by age, geographical region, and socioeconomic status. Analyses were 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). This research has been conducted using the UK Biobank Resource, a nationwide study of approximately 500,000 UK adults between ages 40-70 at recruitment.
Results: All measures of adiposity were positively associated with increased breast cancer risk in postmenopausal but not in 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 &lt;0.001). Vigorous physical activity was associated with decreased breast cancer risk in postmenopausal but not in premenopausal women. Postmenopausal women who participated in an average of 17.5 MET-h/week of vigorous physical activity had a 15% decreased risk of breast cancer (HR 0.85; 0.74-0.98; ptrend = 0.025). This association was no longer significant 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 both premenopausal and postmenopausal women.
Conclusion: In postmenopausal women, breast cancer risk is strongly associated with greater adiposity while it moderately decreased with greater vigorous physical activity. Adjusting for body fat percentage attenuated the association between vigorous physical activity and breast cancer risk. Longer follow-up is needed to clarify associations, particularly in premenopausal women. We also plan to extend these analyses by subdividing cancers by hormone receptor status.
Citation Format: Wenji Guo, Gillian K. Reeves, Timothy J. Key. Body size and composition, physical activity and breast cancer risk in UK Biobank. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 864. doi:10.1158/1538-7445.AM2015-864