Women who spent over an hour a day sitting in front of the TV had an increased risk of developing colorectal cancer before age 50, results of a prospective analysis of the Nurses' Health Study II suggested.
Compared with women with 7 hours per week or less of viewing time, relative risk (RR) of young-onset colorectal cancer was increased by 12% (RR 1.12, 95% CI 0.72-1.75) in those reporting moderate TV time (7.1-14 hours per week), and by almost 70% for those whose viewing time exceeded 14 hours per week (RR 1.69, 95% CI 1.07-2.67, P=0.03 for trend), reported Yin Cao, ScD, MPH, of Washington University School of Medicine in St. Louis, and colleagues.
The increased risk was observed in women with no family history of CRC, and was more pronounced for rectal cancer (RR 1,91, 95% CI 0.86-4.25) for the moderate viewing group and for those reporting >14 hours of TV time per week (RR 2.44 for >14 vs <7 hours per week, 95% ci 1.03-5.78, P=0.04 for trend), the authors wrote in JNCI Cancer Spectrum.
Also, "overweight or obese participants may be more susceptible" to increased CRC risk, they stated.
"This study may help identify those at high risk and who might benefit more from early screening," the authors noted. "The fact that these results were independent of BMI and physical activity suggests that being sedentary may be an altogether distinct risk factor for young-onset colorectal cancer."
"Young-onset CRC, or CRC before age 50 years, is typically diagnosed at a more advanced stage than conventional CRC (mean age 65 years at diagnosis), with more aggressive tumors, different clinicopathological characteristics, and greater years of life lost," they explained.
In marked contrast to decreasing overall CRC incidence, rates of CRC have increased dramatically in people ages 20-49, with distal colon and rectal cancers disproportionately driving these trends in the U.S., the authors stated, adding that "By 2030, each will increase by 124% and 90%, respectively, among individuals ages 20-34 years, and 46% and 28% among those ages 35–49 years."
The current research is one of many that have linked sedentary behavior with a variety of chronic health outcomes, including a 2013 study that found associations between a sedentary lifestyle and an increased CRC risk in men. However, not all studies have identified conclusive or causal associations between prolonged occupational sitting and cancer and other health risks, as noted in a 2010 systematic review.
Cao's group prospectively evaluated sedentary behaviors, primarily time watching TV, and risk of young-onset CRC among 89,278 women in the Nurses' Health Study II, ages 25-42 at recruitment, between 1991 and 2011. Relative risks and 95% confidence intervals were estimated using Cox proportional hazards modeling; statistical tests were two-sided.
The group documented 118 young-onset CRCs (diagnosed at a median age 45) over a median follow-up of 13.9 years (1,262,540 person-years). Women who reported more TV viewing time were more frequently postmenopausal, smoked a greater number of pack-years (among ever smokers), had a higher rate of type 2 diabetes, more frequently used aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs), used less multivitamins, were less physically active, and had generally poorer indices of healthy dietary intake, the group wrote.
Women reporting less physical activity, and those with a prior history of smoking also had elevated risks for young-onset CRC compared with their reference counterparts.
There was no clear increase in risk for other forms of sitting at home, such as meal time or time spent at a desk, or between sitting away from home and risk of young-onset CRC, Cao's group noted. And limited evidence suggests that when compared, TV viewing, but not computer use, was more closely associated with chronic disease risk.
A trend toward reduced risk of young-onset CRC was observed among participants who reported greater light intensity behaviors, such as standing or walking at work or home, the authors added.
Several biological mechanisms support these observations, they stated. Prolonged sitting can result in extended exposure to fecal carcinogens such as secondary bile acids; impaired glucose homeostasis and decreased vitamin D levels; and gut dysbiosis, they wrote, while increased TV viewing specifically may result in lower energy use, higher caloric intake, and less healthy diets, all of which are CRC risk factors.
"These findings suggest that minimizing inactivity may offer protection against young-onset CRC beyond any risk reduction gained from the prevention of other major chronic diseases," the authors stated.
A study limitation included the use of a cohort "comprised mainly of white female nurses, an accurate reflection of the profession's demographics at cohort conception, and a racial group disproportionately burdened by young-onset CRC."
Cao disclosed support from the Raymond P. Lavietes Foundation. Co-authors disclosed support from the NIH, the Crohn's and Colitis Foundation, the National Research Foundation of Korea, and Massachusetts General Hospital.
More Data Tie Prolonged Sitting to Higher
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