Physical activity, body mass index, and prostaglandin E₂ levels in rectal mucosa

Background: Evidence suggests a relationship between prostaglandin levels in colonic mucosa and risk of colon cancer. Physical inactivity and a higher body mass index (BMI; weight in kilograms divided by [height in meters]²) have been consistently shown to increase risk of this cancer. We investigated whether higher levels of leisure-time physical activity or a lower BMI was associated with lower concentrations of prostaglandin E₂ (PGE₂) in rectal mucosa. Methods: This study was conducted in 41 men and 22 women, 42-78 years of age, with a history of polyps, who participated in a randomized clinical trial testing the effects of piroxicam on rectal mucosal PGE₂ levels. An [¹²⁵I]PGE₂ radioimmunoassay kit was used to determine PGE₂ levels in samples of extracted rectal mucosa collected before randomization. Leisure-time physical activity was assessed through a self- administered questionnaire collected at baseline. The reported time spent at each activity per week was multiplied by its typical energy expenditure, expressed in metabolic equivalents (METs), to yield a MET-hours per week score. A repeated measures model was used to assess the effect of BMI and physical activity as predictors of PGE₂ concentration. All statistical tests were two-sided. Results: After adjustment for age, a higher BMI was associated with higher PGE₂ levels (P = .001). A higher level of leisure- time physical activity was inversely associated with PGE₂ concentration (P<.03). An increase in BMI from 24.2 to 28.8 kg/m² was associated with a 27% increase in PGE₂. An increase in activity level from 5.2-to 27.7 MET- hours per week was associated with a 28% decrease in PGE₂. Conclusions: Physical activity and obesity may alter the risk of colon cancer through their effects on PGE₂ synthesis.