Fish oil for the reduction of heart rate and oxygen consumption

Dietary fish oil (FO) has been shown to have several beneficial health effects for humans. For instance, FO prevents fatal arrhythmia (1, 2), reduce resting heart rate (3, 4) and have been shown to improve insulin sensitivity (5-7). The reduction of heart rate due to fish oil supplementation have also been shown in a meta-analysis which is more reliable than separate studies (8).
Skeletal muscles and heart have the ability to incorporate higher levels of docosahexaenoic acid (DHA) (an omega-3 fatty acid in fish oil) compared to many other organs (9, 10).

Increased DHA levels in the membranes of the skeletal muscles modifies the function which include increased insulin sensitivity (5), and possibly substrate preference toward carbohydrate oxidation (11). Previously, it has been shown that FO administration increase the efficiency of O2 use by the heart and skeletal muscle in rat (12, 13). A recent dubbel blind study on humans showed a significant reduction of whole body oxygen consumption and heart rate during exercise when administered 8g fish oil/day in 8 weeks (14), indicating an increased efficiency of O2 use.
The reduced oxygen consumption may have several explanations. The increased insulin sensitivity may alter shift substrate preference to glucose in skeletal muscle (11), which is a more oxygen efficient energy substrate than fat. However, the respiratory exchange ratio (RER) was unchanged in the human study (which should increase if increased glucose oxidation was the main explanation) in the study which indicated that another mechanism may be involved. One such explanation may be an improved Ca2+ cycling (12), which has been calculated to use up to 37% of energy in the skeletal muscle (15).
Interrestingly, a decreased heart rate has been observed also in cardiac transplant patient abscent of a vagal tone which excludes a neurogenic mechanism (16).

Of course, this is of interest for the breath hold diver which may decrease the oxygen consumption with the administration of fish oil. It is important to understand that not every product labelled with "contains omega-3" means that it is efficient. For instance, alpha linolenic acid is an essential vegetable omega-3 fatty acid which is useless when it comes to most of the beneficial effects seen in studies on fish oil. This is because the conversion of alpha linolenic acid to DHA and eicosapentaenoic acid (EPA) (which is associated with the positive effects) is highly inefficient in humans. At the moment, I know of only one alternative for the vegan available on the market where the EPA and DHA is derived from algae. This product is also interesting for the "fish eater" since the levels of toxins are substantially lower than in fish oil which contains high levels of toxins like PCB. However, I can not guarantee the quality of the product and that it contains the promised amounts of EPA and DHA.

References

1. Burr ML, Fehily AM, Gilbert JF, et al. Effects of changes in fat, fish, and fibre intakes on death and myocardial reinfarction: diet and reinfarction trial (DART). Lancet 1989;2:757-61.

2. Dietary supplementation with n-3 polyunsaturated fatty acids and vitamin E after myocardial infarction: results of the GISSI-Prevenzione trial. Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto miocardico. Lancet 1999;354:447-55.

3. Dallongeville J, Yarnell J, Ducimetiere P, et al. Fish consumption is associated with lower heart rates. Circulation 2003;108:820-5.

4. Shah AP, Ichiuji AM, Han JK, et al. Cardiovascular and endothelial effects of fish oil supplementation in healthy volunteers. J Cardiovasc Pharmacol Ther 2007;12:213-9.

5. Pan DA, Lillioja S, Milner MR, et al. Skeletal muscle membrane lipid composition is related to adiposity and insulin action. J Clin Invest 1995;96:2802-8.

6. Helge JW, Wu BJ, Willer M, Daugaard JR, Storlien LH, Kiens B. Training affects muscle phospholipid fatty acid composition in humans. J Appl Physiol 2001;90:670-7.

7. Andersson A, Sjodin A, Hedman A, Olsson R, Vessby B. Fatty acid profile of skeletal muscle phospholipids in trained and untrained young men. Am J Physiol Endocrinol Metab 2000;279:E744-51.

8. Mozaffarian D, Geelen A, Brouwer IA, Geleijnse JM, Zock PL, Katan MB. Effect of fish oil on heart rate in humans: a meta-analysis of randomized controlled trials. Circulation 2005;112:1945-52.

9. Charnock JS, Abeywardena MY, Poletti VM, McLennan PL. Differences in fatty acid composition of various tissues of the marmoset monkey (Callithrix jacchus) after different lipid supplemented diets. Comp Biochem Physiol Comp Physiol 1992;101:387-93.

10. Owen AJ, Peter-Przyborowska BA, Hoy AJ, McLennan PL. Dietary fish oil dose- and time-response effects on cardiac phospholipid fatty acid composition. Lipids 2004;39:955-61.

11. Rustan AC, Hustvedt BE, Drevon CA. Dietary supplementation of very long-chain n-3 fatty acids decreases whole body lipid utilization in the rat. J Lipid Res 1993;34:1299-309.

12. Pepe S, McLennan PL. Cardiac membrane fatty acid composition modulates myocardial oxygen consumption and postischemic recovery of contractile function. Circulation 2002;105:2303-8.

13. Pepe S, McLennan PL. (n-3) Long chain PUFA dose-dependently increase oxygen utilization efficiency and inhibit arrhythmias after saturated fat feeding in rats. J Nutr 2007;137:2377-83.

14. Peoples GE, McLennan PL, Howe PR, Groeller H. Fish oil reduces heart rate and oxygen consumption during exercise. J Cardiovasc Pharmacol 2008;52:540-7. 15. Bergstrom M, Hultman E. Energy cost and fatigue during intermittent electrical stimulation of human skeletal muscle. J Appl Physiol 1988;65:1500-5.

16. Harris WS, Gonzales M, Laney N, Sastre A, Borkon AM. Effects of omega-3 fatty acids on heart rate in cardiac transplant recipients. Am J Cardiol 2006;98:1393-5.

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