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Editor,—In a recent article on diet and colorectal cancer, Stoneham et al 1reported that 76% of the variation in incidence rates between 28 countries could be explained by three significant dietary factors (meat, fish, and olive oil). The authors concluded that olive oil may have a protective effect on the development of colorectal cancer. While a plausible mechanism is proposed, it is worth noting that 21 of the 28 populations included in the ecological study were essentially non-consumers of olive oil (<0.01 kg/year per capita) and the inverse association between consumption and colorectal cancer risk is therefore based on a relatively small number of influential data points (fig 1). Regional colorectal cancer rates in Italy and Spain (for example, Trieste and Mallorca) are among the highest in Europe2despite high levels of olive oil consumption, while lower rates in Greece are consistent with a diet characterised by intermediate meat and high fibre consumption.1 The authors have demonstrated a strong positive association between meat supply and colorectal cancer risk, which persists after controlling for a range of potential dietary confounders. The association is supported by a plausible causal agent in the form of heterocyclic amines produced in the cooking of meat and fish products and shown to be both mutagenic in the standard Ames' test and carcinogenic in animal studies.3 Further studies on olive oil are clearly warranted, including research on the possible inhibition of heterocyclic amine formation by constituents in olive oil such as oleic acid4 and vitamin E.5 Nevertheless, the data presented suggest that meat consumption may be a more important determinant of colorectal cancer risk at the population level and also warrants further investigation.
Editor,—We agree with Maric and Cheng that meat consumption is likely to be the most important dietary factor to be associated with rates of colorectal cancer at a population level. This association is well recognised. We did not intend our study1-1 to distract attention from it but rather add information about olive oil. Nevertheless, there were some anomalies in the association between colorectal cancer and meat. For instance, we found Japan and Norway had much lower meat consumption than countries such as France, Ireland and Denmark, yet their colorectal rates were similar. Our analysis suggested that this discrepancy may be attributable to the high fish consumption in the former countries. We also found that the southern Mediterranean countries, particularly Greece, had much lower colorectal cancer rates than expected from their meat and fish consumption. Our analysis indicated these lower rates may be related to their high consumption of olive oil, more so than to their relatively high intake of fruit and vegetables. However, we recognise the limitations of this, as with all ecological studies, and consider it a hypothesis generating study, with the possible preventive effect of olive oil worthy of further research.
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