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Description of the Evidence

    continued...

    A meta-analysis of 106 observational studies found a RR (ever smokers compared with nonsmokers) for CRC incidence of 1.18 (95% CI, 1.11–1.25), with an absolute risk increase of 10.8 cases per 100,000 person-years (95% CI, 7.9–13.6). There was a statistically significant dose-response effect. In 17 studies with data on CRC mortality, cigarette smoking was associated with CRC death, with a RR (ever smokers vs. never smokers) of 1.25 (95% CI, 1.14–1.37), and an absolute increase in the death rate of 6.0 deaths per 100,000 person-years. For both incidence and mortality, the association was stronger for rectal cancer than for colon cancer.[28]

    Obesity

    At least three large cohort studies have found an association between obesity and CRC incidence or mortality.[29,30,31] The Nurses' Health Study found that women with a body mass index (BMI) of more than 29, compared with women with a BMI of less than 21, had an adjusted RR for CRC incidence of 1.45 (95% CI ,1.02–2.07).[29] In the CPS II [31], men and women with a BMI of 30 to 34.9 had an adjusted RR for CRC mortality (compared with people with a BMI of 18.5–24.9) of 1.47 (95% CI, 1.30–1.66), with a statistically significant dose-response effect.[31] The effects were similar in men and women.

    Factors associated with a decreased risk of colorectal cancer

    Physical activity

    A sedentary lifestyle has been associated in some [32,33] but not all [34] studies with an increased risk of CRC. Numerous observational studies that have examined the relationship between physical activity and colon cancer risk.[35] Most of these studies have shown an inverse relationship between level of physical activity and colon cancer incidence. The average RR reduction is reportedly 40% to 50%. Large U.S. cohort studies have found statistically significant adjusted RR of 0.54 (95% CI, 0.33–0.90) [29] and 0.53 (95% CI, 0.32–0.88) [30] when comparing people with high versus low average energy expenditure. A meta-analysis of 52 observational studies found an overall adjusted RR of 0.76 (95% CI, 0.72–0.81), with similar results for men and women.[36]

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