Body mass index and risk of colon and rectal cancer stratified by tumor location and sex

The association between body mass index (BMI = weight in kg/height in m2) and colorectal cancer (CRC) risk is complex. It is somewhat established that high BMI is associated with increased risk of cancer in the colon among men, but risk estimates among women vary according to age and menopausal status. There is some evidence suggesting that risk also differs according to tumor subsite (proximal colon, distal colon, rectum) for both men and women, but further studies are needed to confirm the pattern of the association. Here, we examine the association between BMI and CRC risk, stratified according to tumor location in the colon or rectum, and sex. Data for this case–control study were collected from the Ontario Familial Colorectal Cancer Registry. Cases were men and women diagnosed with incident colon or rectal cancer identified through the Ontario Cancer Registry since July 1997. Controls were identified from provincial property assessment files in 2000. Family history was determined through review of a family history questionnaire and reviewed by a genetic counselor for classification. Height and weight 2 years prior were assessed with a self-administered questionnaire. BMI was calculated and categorized according to World Health Organization criteria: low BMI, <18.5 kg/m2; normal, 18.5 to <25 kg/m2; overweight, 25 to <30 kg/m2; and obese, BMI ≥30 kg/m2. Among men, an overweight or obese BMI was associated with about a 50% increased risk of colorectal cancer relative to men with a normal BMI. The BMI risk for colorectal cancer was attenuated among premenopausal women, and null for postmenopausal women. Increased risk from overweight and obesity were also found among men, but not women, for proximal and distal subsites of colon cancer. Overweight was associated with increased risk of rectal cancer in women only. These results suggest that BMI among men and women is differentially associated with colon and rectal cancer risk. Endogenous or exogenous estrogens may, in part, influence this discordance.