Abstract
INTRODUCTION: Colorectal cancer (CRC) remains the second most common cancer in women worldwide. Physical activity (PA) has been associated with reduced risk of CRC; however, this has been demonstrated more consistently in men, while results of studies in women have been largely equivocal. We aimed to further examine the relationship between PA patterns and the risk of CRC in women, using repeated measurements. METHODS: We followed participants of the Norwegian Women and Cancer (NOWAC) Study - a nationally representative cohort. Baseline information was available for 79,184 women, and we used this information in addition to follow-up information collected 6-8 years later, for repeated measurement analysis. At enrollment, participants were cancer-free and aged 30-70 years, with a median age of 51 years. We used Cox proportional hazards regression to compute hazard ratios (HRs) and 95% confidence intervals (CIs). RESULTS: During an average of 14.6 years of follow-up and 1.16 million person-years, 885 cases of colon and 426 cases of rectal cancer were identified through linkage to the Norwegian Cancer Registry (median age at diagnosis: 65 years). We found no association between PA level and the risk of colon cancer in baseline or repeated measurements analyses when comparing women with PA level 1-2 to those with PA level 5-6 (reference) (baseline: HR = 0.90, 95% CI 0.66-1.23, p-trend = 0.76; repeated measurements: HR = 0.78, 95% CI 0.55-1.10, p-trend = 0.27). Results were the same when comparing PA level 9-10 to the reference level (baseline: HR = 0.80, 95% CI 0.56-1.12, p-trend = 0.76; repeated measurements: HR = 0.82, 95% CI 0.58-1.16, p-trend = 0.27). Similarly, we found no association between PA levels and the risk of rectal cancer. CONCLUSIONS: Women may need to look beyond PA in order to reduce their risk of CRC.