A large meta-analysis of case-control and cohort studies reported that total sitting time, television viewing time, and occupational sitting time were all positively associated with colon cancer development.
Colorectal cancer (CRC) is the third most commonly diagnosed cancer in men and the second in women worldwide.
A large body of evidence has shown that greater levels of physical activity are associated with a lower risk of colorectal cancer, and sedentary behaviors increase colorectal cancer risk
The current study by British scientists investigated how total physical activity levels and sedentary behaviours, such as television watching and time spent using computers, are related to colorectal cancer risk.
This analytical study represents one of the largest single studies to date, which investigated the relationships between physical activity levels and sedentary behaviors with colorectal cancer risk.
The study also examined risks for proximal and distal colon cancers in relation to physical activity and sedentary behaviors. Study findings are reported in the British Journal of Cancer.
For colon cancer, in the sexes-combined model, the highest physical activity category had a 16% lower risk compared to those in the lowest category.
No heterogeneity by colonic subsite was found, with a statistically significant inverse association observed between total physical activity and proximal colon cancer risk, and a non-significant inverse association for distal colon cancer risk.
A statistically significant inverse association between physical activity and colon cancer risk was found only among men, but not women.
Among men, a statistically significant inverse association was found between total physical activity and proximal colon cancer risk, but no relationship was observed for distal colon cancer risk.
Among women, no significant associations were observed between total physical activity and risks in the proximal colon and distal colon.
Higher levels of television watching time were associated with a greater colon cancer risk. This positive relationship was only present among men, and not women.
The positive relationships between television watching time and proximal and distal colon cancer were stronger and statistically significant for men than for women.
Time spent using computers was not associated with risk of colon cancer, proximal colon cancers, and distal colon cancers.
These results are generally consistent with those reported by other large prospective studies, which did not observe heterogeneous physical activity relationships across the proximal and distal colon regions.
The results of current study contribute to a growing body of evidence, which has reported sedentary behaviour as a risk factor for the development of colon cancer, independent of physical activity levels.
In conclusion, in this prospective analysis of UK Biobank participants, total physical activity levels were inversely associated with colon cancer risk, with no heterogeneity found between tumours located in the proximal colon and distal colon.
Sedentary behaviour, as indicated by television watching time, was associated with an elevated colon cancer risk. These findings add to the large body of evidence, which supports the promotion of physical activity in population-wide cancer prevention programs.
The evidence taken together provides strong support for lack of physical activity being causally related to colon cancer.
It has been estimated that 12-14% of colon cancer could be attributed to lack of frequent involvement in vigorous physical activity.
Compared to those who watched one hour or less of TV per day, watching more than 4 hours a day was associated with a 35% greater risk of developing bowel cancer in men. Compared to the low physical activity group, a 23% reduced risk of developing colon cancer was observed for the high physical activity group in men.
Why and how
Several biological mechanisms have been proposed to explain the association between physical activity and colon cancer; many of these mechanisms also support the observation that intense activities are most protective.
These proposed mechanisms suggest that physical activity increases gut motility, enhances the immune system, decreases insulin and insulin-like growth factor levels, reduces risk of obesity, enhances free radical scavenger systems, and influences prostaglandin levels.