The 488,720 participants contributed 3,355,603 person-years of follow-up during a mean follow-up of 6.9 years. Among men, a total of 2,298 colon and 942 rectal cancers were identified and among women 1,112 colon and 370 rectal cancers were identified. Baseline participant characteristics by frequency of current exercise/sports activity are shown in . Men and women who were more physically active tended to have a lower BMI, they were less likely to smoke, and they reported consuming less red meat than those who were less physically active. In addition, physically active participants had a higher level of education and they consumed more fruits, vegetables, calcium, and folate than less physically active subjects. Women with greater physical activity were more likely to be currently taking menopausal hormone therapy than their less active counterparts.
Baseline characteristics of the NIH-AARP study population according to frequency of current exercise and/or sports
Among men and women, participants who engaged in exercise/sports 5 or more times per week had an 18% reduced risk of colon cancer (RR=0.82; 95% CI=0.73-0.92) compared to those who reported never or rarely exercising. In addition, colon cancer risk was inversely related to increasing level of daily routine activity (p=0.003), with the lowest risk observed for participants reporting climbing stairs or lifting light loads as part of their daily routine (RR=0.73; 95% CI=0.63–0.85). We next stratified the cohort according to gender in order to examine more closely the effect of physical activity in men and women, although the formal test for interaction was not statistically significant (p=0.322). In men, increasing frequency of current exercise/sports participation was associated with a graded, statistically significant decrease in colon cancer risk in both age and multivariable-adjusted models (). Men who reported engaging in exercise/sports 5 or more times per week had a 21% lower risk of colon cancer than men who reported never or rarely exercising (RR=0.79; 95% CI=0.68–0.91). Additional adjustment for BMI had limited impact, thus subsequent results in the text are presented for multivariable-adjusted models without BMI unless otherwise stated. Among women, current exercise/sports was inversely associated with colon cancer in age-adjusted analyses, but showed no statistically significant trend with colon cancer after adjustment for covariates. However, the multivariable-adjusted relative risks were decreased for women who reported exercising 1–3 times per month or 3–4 times per week compared to women who never or rarely exercised.
Relative risk of colon cancer according to frequency of current exercise/sports and daily routine activity
We next investigated daily routine activity in relation to colon cancer risk. As compared with men who reported mostly sitting throughout the day, men who reported engaging in a combination of sitting and walking, walking and standing, and climbing stairs, or hills, or lifting, or carrying light loads had a statistically significant decreased risk of colon cancer (). Risk did not further decrease among men in the top category of daily routine activity which comprised heavy work or carrying heavy loads, although case numbers were small in that category (n=82 cases). Similarly, among women all but the highest level of daily routine activity were statistically significantly associated with reduced risk of colon cancer in age-adjusted analyses. Associations inwomen became statistically nonsignificant in multivariable-adjusted models.
We next evaluated the relation of physical activity to colorectal cancer according to anatomic sub-site (). Among men, inverse associations with current exercise/sports activity were seen for proximal and distal colon cancers and for rectal cancers. Sub-site results showed some similarities among women, with statistically significant age-adjusted associations for both proximal and distal colon cancers and a significant test for trend for distal colon cancers and rectal cancers, all of which, however, became nonsignificant after multivariable adjustment. Among men, inverse associations were found between daily routine activity and proximal (p=0.028) and distal colon cancers (p=0.008) but not rectal cancers (p=0.163). Among women, a suggestive inverse relation was noted between increasing level of daily routine activity and proximal cancers. Associations with distal colon cancers and rectal cancers showed no consistent patterns among women. For both men and women, risk estimates for rectal cancer did not differ appreciably from cancers of the rectosigmoid junction (data not shown).
Relative risk of colorectal cancer by anatomic site according to frequency of current exercise/sports and daily routine activity
We assessed the time spent at low intensity activity, moderate to vigorous intensity activity, and sedentary behavior in relation to risk of colon cancer (). As compared with men never or rarely engaging in low intensity activity (such as walking), men reporting more than 7 h per week of low intensity activity had a RR of colon cancer of 0.81 (95% CI=0.65–1.00), even after adjustment for moderate to vigorous intensity activity. Similarly, our assessment of moderate to vigorous activity demonstrated an inverse dose-response relationship for colon cancer in men (p=0.037), independent of time spent in low intensity activity, although the relation appeared to level off in the highest category of moderate to vigorous activity. When we examined the combination of low intensity activity and moderate to vigorous activity (i.e., total activity) in men, the RR comparing extreme quintiles was 0.76 (95% CI=0.63–0.90). With regards to sedentary behavior in men, 9 or more hours versus less than 3 h per day of watching television or videos was associated with a RR of colon cancer of 1.61 (95% CI=1.14–2.27). Overall time spent sitting was also suggestively associated with colon cancer in men (p=0.050). Among women, no statistically significant relations with colon cancer were observed for low intensity activity, moderate to vigorous activity, and the combination of low intensity activity and moderate to vigorous activity, or time spent sitting. However, increasing time spent watching television was statistically significantly associated with increased colon cancer risk in age-adjusted models among women. Multivariable risks for sedentary behavior among men and women were similar with or without adjustment for physical activity.
Relative risk of colon cancer according to time spent in sedentary behavior and time spent in low intensity and moderate to vigorous intensity activitya
We investigated colon cancer risk according to total physical activity during different age periods of life. Physical activity variables for different age periods were positively correlated with one another; the correlation coefficients ranged from 0.30 to 0.91 and values decreased with increasing time between age periods. Among men, total physical activity at ages 15–18 and ages 19–29 years was not associated with colon cancer, whereas a decrease in colon cancer risk was observed with increasing levels of total physical activity at ages 35–39 years and increasing levels of total lifetime physical activity (). The RR comparing extreme quintiles of total lifetime activity was 0.82 (95% CI=0.69–0.98). In contrast, among women no relations of total physical activity at ages 15–18, ages 19–29, ages 35–39, or total lifetime physical activity to colon cancer were observed. In further analyses of past activity, we in addition adjusted for total current activity. No appreciable differences in risk estimates emerged, indicating that observed relations with past activity were independent of current activity (data not shown).
Relative risk of colon cancer in relation to total physical activity at ages 15–18, 19–29, 35–39, and over the lifetimea
The associations of physical activity and sedentary behavior to colon cancer risk were not modified by age, body mass, education, race, family history of colon cancer, history of colorectal cancer screening, alcohol consumption, smoking status, aspirin/NSAID use, or menopausal hormone therapy (women only).