Omega-3 Fatty Acids and Cancer RiskOmega-3 Fatty Acids and Cancer Risk

To the Editor: The systematic review of the effects of omega-3 fatty acids on cancer risk by Dr MacLean and colleagues¹ concluded that dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer. We would like to raise some of our concerns about this review.

Thirty-eight prospective studies were selected for review based on prespecified criteria. However, none of these studies measured fatty acid composition in patients. The food frequency questionnaire and dietary records correlate poorly with direct measurements of fatty acids in patient samples² and are prone to measurement error.³ Ultimately, the effect of omega-3 fatty acids depends on levels achieved in individuals. Fish cannot synthesize omega-3 fatty acids de novo but rather ingest eicosapentaenoic and docosahexaenoic acid present in phytoplankton and zooplankton.⁴ Thus, some fish (especially farm-raised fish) are poor sources of omega-3 fatty acids. We therefore think that a more accurate conclusion of this study is that a preventive effect of omega-3 fatty acids on cancers is still uncertain.

We also disagree with the authors on their assessment that animal studies “do not come close to replicating human exposures.” Animal models, especially those developed recently, can closely mimic the clinical course of cancer progression, and are invaluable tools for mechanistic studies that cannot be performed in humans.^{5 - 6} Animal diets are designed with varying ratios of omega-3 to omega-6 fatty acids, both of which are essential fatty acids required for mammals to survive. It is the ratio of omega-3 to omega-6 that is important for deducing their mechanistic functions, and animal diets have been formulated to reflect human consumption.