Sedentary Behaviors, TV Viewing Time, and Risk of Young-Onset Colorectal Cancer

BACKGROUND: Colorectal cancer (CRC) diagnosed before age 50 years, or young-onset CRC, is increasing globally with undefined etiology. A sedentary lifestyle is an emerging risk factor for CRC after age 50 years, but its role in young-onset CRC is unknown. METHODS: We prospectively evaluated sedentary behaviors, primarily time watching television (TV), and risk of young-onset CRC among 89 278 women in the Nurses’ Health Study II ages 25–42 years at recruitment (1991–2011). We used Cox proportional hazards modelling to estimate relative risks (RR) and 95% confidence intervals (CIs). Statistical tests were two-sided. RESULTS: We documented 118 young-onset CRCs over 1 262 540 person-years. Sedentary TV viewing time was statistically significantly associated with increased risk of young-onset CRC, after adjusting for putative risk factors, including obesity and physical activity. Compared to no more than 7 hours per week, women with 7.1–14 hours per week of TV time had a multivariable relative risk (RR) of 1.12 (95% confidence interval [CI] = 0.72 to 1.75), further increased for greater than 14 hours per week (RR = 1.69, 95% CI = 1.07 to 2.67, P(trend) = .03). This association was observed among participants without a CRC family history and was more pronounced for rectal cancer (RR for >14 vs ≤7 hours per week 2.44, 95% CI = 1.03 to 5.78, P(trend) = .04). Overweight or obese participants may be more susceptible. CONCLUSION: Independent of exercise and obesity, prolonged sedentary TV viewing time, a surrogate for a more inactive lifestyle, was associated with increased risk of young-onset CRC, particularly of the rectum. These findings provide further evidence on the importance of maintaining an active lifestyle.