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 7 hours, 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 than14 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.