Abdominal adiposity is not a mediator of the protective effect of Mediterranean diet on colorectal cancer

Adherence to the Mediterranean diet (MD) has a preventive effect on colorectal cancer (CRC). Several biological mechanisms have been hypothesized to explain this effect, but the involvement of clinical mediators has not been experimentally proven. We examined the role of abdominal adiposity (i.e., waist-to-hip ratio, WHR) as a potential mediator of the relationship between the MD and CRC in the Italian centres of the European Prospective Investigation into Cancer and Nutrition. We evaluated the effect of the Italian Mediterranean Index (IMI) on WHR and of WHR on CRC risk. We then estimated the natural indirect effect (NIE, mediated by WHR) and the pure direct effect (PDE, unmediated) of IMI on CRC risk using mediation analyses, considering age, sex, education, physical activity, smoking and EPIC centre as confounders. Increased IMI was associated with significantly decreased odds of high WHR (odds ratio [OR] for an IMI of 6–11 vs. 0–1: 0.88, 95% confidence interval [CI]: 0.81–0.97). There was a positive relationship between WHR and CRC (hazard ratio [HR] for high vs. low WHR: 1.34, 95%CI: 1.09–1.66). The total effect of IMI was protective on CRC risk and was mainly explained by the PDE (HR for an IMI of 6–11 vs. 0–1: 0.51, 95%CI: 0.31–0.83), whereas the NIE was 1.00 (95%CI: 0.94–1.10). In this Mediterranean cohort, the protective effect of the MD on the development of CRC was not mediated by abdominal adiposity. Since this is the first study to investigate the mediating effect of abdominal obesity, other studies are needed to replicate this result.