Abstract P5-08-04: Physical activity, sedentary behaviour, and risk of breast cancer: Results from the SUN (‘Seguimiento Universidad de Navarra’) project
Authors: Sánchez-Bayona Rodrigo , Gardeazabal Itziar , Fernandez-Lazaro Cesar Ignacio , Martinez-Gonzalez Miguel Angel , Romanos-Nanclares Andrea , Alvarez-Alvarez Ismael , Gea Alfredo , Santisteban Marta , Toledo Estefanía 
Published: March 2020

Abstract
BACKGROUND: Breast cancer is the leading type of cancer in women worldwide. Some factors, like leisure-time physical activity (LTPA), have been inversely associated with the risk of breast cancer. Nevertheless, evidence on the influence of sedentary lifestyles, LTPA, and their combined exposure on breast cancer risk is still limited. We aimed to assess sedentary behavior and recreational PA in relation to subsequent risk of breast cancer in a Mediterranean cohort.
METHODS: The SUN Project is a Spanish dynamic prospective cohort of university graduates initiated in 1999. The baseline assessment included a validated questionnaire on LTPA and sedentary habits. Participants completed biennial follow-up questionnaires, which inquired about medical diagnosis of breast cancer. Self-reported information was confirmed with medical reports; self-reported cases pending confirmation were added to confirmed cases and together they were labeled as "probable cases". For our analyses, we included 10,812 women, with a median follow-up of 11.8 years. Multivariable Cox regression models were used to estimate the hazard ratio (HR) for breast cancer according to 3 categories of weekly METs-hours of LTPA (low: 0-6, medium: 6.1-16.5, high &gt;16.5), h/d of TV watching (&lt;1h, 1-2 h, &gt;2 h) and an 8-item PA Score (low:&lt;2, medium-low:2-3, medium-high:4-5, high:6-8). We first fitted models for overall breast cancer incidence and then separate analyses according to menopausal status at diagnosis.
RESULTS: Mean baseline age of our participants was 34.7 years (SD: 10.6 years). Among 115,802 women-years of follow-up, we identified 190 probable incident cases of breast cancer, out of which we confirmed 101. In multivariable-adjusted analyses, women in the highest category of LTPA showed a significantly lower risk of breast cancer (HR=0.55; 95% CI: 0.34-0.89) compared to women in the lowest category. Women watching &gt;2 h/d of TV had a significantly higher risk of breast cancer (HR=1.71; 95% CI:1.05-2.78) compared to women who watched &lt;1h/d of TV. Women in the highest category of the PA Score (6-8 points) showed a significant decrease in breast cancer risk compared to the lowest PA Score (&lt;2 points) group (HR=0.34; 95% CI: 0.15-0.79). Results were consistent when we considered probable breast cancer cases.
CONCLUSION: In this Mediterranean cohort, women in the highest leisure-time PA categories showed a significant decrease in breast cancer risk. More than 2 h/day of TV watching was associated with a higher risk of breast cancer development. The harmful effect of TV watching persisted despite the practice of &gt;6 MET-h/week of PA.
Hazard ratio (HR) and 95% confidence interval (95% CI) of breast cancer according to levels of LTPA.MET-h/week categoriesLow (0-6)Medium (6.1-16.5)High (&gt;16.5)Cases/ Women-years31/2359032/3963238/52580Multivar. adjusted1 (ref)0.61 (0.37 - 1.00)0.55 (0.34 - 0.89)Hours/day of TV watching&lt; 1 hour1-2 hours&gt;2 hoursCases/ Women-years30/3546930/4484241/35491Multivar. adjusted1 (ref)0.92 (0.55 - 1.54)1.71 (1.05 - 2.78)Joint classificationMET&gt;6MET&gt;6MET&lt;6MET&lt;6TV&lt;2hTV&gt;2hTV&lt;2hTV&gt;2hCases/ Women-years42/6471728/2749518/1559413/7996Multivar. adjusted1 (ref)1.77 (1.09 - 2.88)1.73 (0.99 - 3.03)3.05(1.61 - 5.77)Physical Activity ScoreLow (&lt;2)Medium-Low (2-3)Medium-High (4-5)High (6-8)Cases/ Women-years17/1276637/4542538/426249/14986Multivar. adjusted1 (ref)0.58 (0.32 - 1.03)0.59 (0.33 - 1.06)0.34 (0.15 - 0.79)
Citation Format: Rodrigo Sánchez-Bayona, Itziar Gardeazabal, Cesar Ignacio Fernandez-Lazaro, Miguel Angel Martinez-Gonzalez, Andrea Romanos-Nanclares, Ismael Alvarez-Alvarez, Alfredo Gea, Marta Santisteban, Estefanía Toledo. Physical activity, sedentary behaviour, and risk of breast cancer: Results from the SUN (‘Seguimiento Universidad de Navarra’) project [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-08-04.