Colorectal cancer risk based on extended family history and body mass index

Family history and body mass index (BMI) are well‐known risk factors for colorectal cancer (CRC), however, their joint effects are not well described. Using linked data for genealogy, self‐reported height and weight from driver's licenses, and the Utah Surveillance, Epidemiology, and End‐Results cancer registry, we found that an increasing number of first‐degree relatives (FDR) with CRC is associated with higher standardized incidence ratio (SIR) for overweight/obese probands but not for under/normal weight probands. For probands with two CRC‐affected FDRs, the SIR = 1.91 (95% CI [0.52, 4.89]) for under/normal weight probands and SIR = 4.31 (95% CI [2.46, 7.00]) for overweight/obese probands. In the absence of CRC‐affected FDRs, any number of CRC‐affected SDRs did not significantly increase CRC risk for under/normal weight probands, but for overweight/obese probands with at least three CRC‐affected SDRs the SIR = 2.68 (95% CI [1.29, 4.93]). In the absence of CRC‐affected FDRs and SDRs, any number of CRC‐affected third‐degree relatives (TDRs) did not increase risk in under/normal weight probands, but significantly elevated risk for overweight/obese probands with at least two CRC‐affected TDRs was observed; SIR = 1.32 (95% CI [1.04, 1.65]). For nonsyndromic CRC, maximum midlife BMI affects risk based on family history and should be taken into account for CRC risk communication when possible.