Inconsistent and sometimes contradictory results from nutrition studies can be puzzling. However, the challenge of reproducibility is the foundation of science. Such is the case of clinical trials that show opposing endorsements regarding the cardioprotective benefits of eating fish. While some studies have confirmed the efficacy of fish oil supplementation in reducing cardiovascular disease risk, other studies describe an overall lack of effect. An explanation for these inconsistencies could be attributed to the intakes of fish oil used in animal-model studies being far too high to be replicated in the human diet, although this has not been verified experimentally.

To provide further clarity on fish oil supplementation, a study published in The Journal of Nutrition compared the efficacy of fish oil doses, equivalent to regular consumption of fish and fish oil supplements to a diet equivalent to that of non-fish eaters. This study tested the hypothesis that the low omega-3 polyunsaturated fatty acid dietary threshold for myocardial incorporation in rats, equivalent in dose to what humans derive from eating fish, can reduce heart rate and irregular heart rhythms.

Authors Michael Macartney, Gregory Peoples, and Peter McLennan (University of Wollongong) fed rats diets that differed only in the fatty acid composition. The control diet provided a non-fish eater western diet, without fish oil, whereas the other two diets provided the western diet with either a small or a moderate dose of fish oil. All three diets were of equal caloric value. The anesthetized rats were then subjected to procedures that blocked coronary artery blood flow to simulate a heart attack, with acute myocardial ischemia and cardiac arrhythmia, followed by restoration of blood flow to the heart.

The results showed that ventricular arrhythmias were prevented, and heart rate was slowed by lower omega-3 polyunsaturated fatty acid intake in rats than previously reported, equivalent to a human diet with regular fish consumption. To the author’s knowledge, this study is the first to demonstrate that a nutritionally relevant dietary intake of fish oil slowed resting heart rate and prevented irregular heart rhythms and heart attack deaths. This aligns experimental data with human observational studies indicating that the risk of cardiovascular death is reduced by modest dietary intakes of omega-3 fatty acids, with higher intakes providing little further risk reduction. According to the study authors, the efficacy of low-dose fish oil demonstrates biological plausibility for nutrition omega-3 fatty acid-mediated cardioprotection and suggests that effectiveness in human clinical trials may be obscured by failure to exclude fish eaters. A corresponding editorial by Erik Schmidt (Aalborg University Hospital) and Philip Calder (University of Southampton) provides further insights regarding fish consumption as an integral part of a healthy dietary pattern.

References Macartney MJ, Peoples GE, McLennan. Cardiac Arrhythmia Prevention in Ischemia and Reperfusion by Low-Dose Dietary Fish Oil Supplementation in Rats. The Journal of Nutrition, Volume 150, Issue 12, December 2020, Pages 3086–3093, https://doi.org/10.1093/jn/nxaa256.

Schmidt EB, Calder PC. Marine n-3 Fatty Acids, Sudden Cardiac Death, and Ischemic Heart Disease: Fish or Supplements? The Journal of Nutrition, Volume 150, Issue 12, December 2020, Pages 3055–3057, https://doi.org/10.1093/jn/nxaa319.

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