Abstract:
Context: Many population studies, as well as clinical and experimental findings, have suggested beneficial effects of increased fish or fish oil (FO) consumption in relation to cardiovascular health. Although the presence and/or extent of cardio-protective effects of fish oil still remains controversial in some areas, accumulated evidence, particularly in population health studies, favours the benefits of dietary FO fatty acids. Among many putative benefits and their mechanisms of diets rich in FO, mechano-energetic protection of the myocardium was also suggested in the studies by Pepe & McLennan (2002 and 2007). They observed a two-fold increase in the efficiency of O2 use in the FO-fed rat hearts compared to the control-diet group. They also observed a near-halving of the efficiency in the SFA (saturated fatty acid)-fed group compared to the control group in the same study. These findings imply that a simple augmentation of the diet with FO fatty acids would reduce the severity of adverse cardiovascular events such as ischaemic heart disease and myocardial infarction, since the hearts enriched with these fatty acids require less O2 for survival and functioning. However, clinical intervention results are still in disagreement and are elusive, not confirming the mechanistic proposition of the experimental findings by Pepe & McLennan. In addition, other previous studies which similarly investigated the effects of dietary FO on parameters of interest, such as the total rate of O2 consumption (VO2), coronary flow rates and mechanical work/power, were not suggestive of such dramatic changes as observed by this group. It is also worth mentioning the existence of a gap of knowledge concerning the effect of dietary FO on cardiac efficiency at a more fundamental level of cardiac contraction – namely, myocardial tissue per se. Findings at this level would help elucidate the presence and/or extent of the mechano-energetic benefits of dietary FO on the whole myocardium. Aims: The current study aimed to answer the question, “Can increased dietary supplementation of FO fatty acids enhance the myocardial contractile efficiency?” In the process of answering the question, this study also aimed to investigate possible effects of the dietary fatty acids on other mechanical and energetic indices of the heart and of its contractile tissue. Methods: To examine the putative mechano-energetic benefits of FO supplementation, rats were fed foods prepared with different levels of FO fatty acids. Three Diet groups were examined: REF group (control diet), FO group (n-3 PUFA rich), and SAT group (n-6 PUFA and SFA rich). Two experimental methods were adopted: isolated working-hearts (at 37°C and 32°C) and isolated, intact LV trabeculae (only at 32°C). For each heart, various left ventricular mechanical and energetic measurements were made at the whole-organ level, and a subsequent, comparable tissue experiment was carried out, using an excised trabecula. Thus experiments at two different physiological levels gave independent measures of the mechano-energetic status of the hearts with different FO content. Results: the main findings of this study are: 1. Myocardial contents of fatty acids in the rat hearts were altered with the 6-to-8 week feeding with the specifically formulated diets. The intended changes in the myocardial n-3 PUFA and n-6 PUFA among the diet groups were achieved with no significant difference in the SFA content. The ratio of n-3 PUFA to n-6 PUFA in the hearts from the FO group was 3- fold higher than in the SAT group. 2. No mechanical or energetic indices were altered by the differential myocardial FO content (or different dietary FO level) in the isolated working-heart experiments. There was no difference in the peak mechanical efficiency among the Diet groups: 14.8±1.1%, 13.9±0.6% and 13.6±0.7% in the REF, FO and SAT groups, respectively. 3. There was no difference in the peak mechanical efficiency of the LV trabeculae from different Diet groups: 13.3 ± 1.4%, 11.2 ± 2.2% and 12.5 ± 1.5% in the REF, FO and SAT groups, respectively. There was a 7% increase in the duration of contraction at the level of 50% peak stress (tc50) in isometric contractions of trabeculae at their optimal lengths in the FO group compared to the other two groups. Conclusions: Overall, the current study presents no evidence of any effect of dietary FO on the efficiency of the heart, at either independently assessed levels: whole-heart and cardiac tissue. Since neither cardiac muscle mechanics nor energetics was altered by the increased level of myocardial FO fatty acids, the enhancements observed by Pepe & McLennan (2002) must have arisen due to other factors, but not from a change in the fundamental mechano-energetic status of the myocardium. The experimental findings of the current study are in alignment with the rather inconclusive population studies and clinical findings; there is no O2 saving effect of dietary fish oil that makes the heart more efficient which, otherwise, is deemed to produce better prognosis and survival in major cardiovascular events such as ischaemic heart disease and myocardial infarction.