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(Circulation. 2003;108:820.)
© 2003 American Heart Association, Inc.
Clinical Investigation and Reports |
From INSERM U 508 (J.D., M.M., P.A.), Institut Pasteur de Lille, Lille, France; Department of Epidemiology and Public Health (J.Y., A.E.), Queens University Belfast, Belfast, Ireland; INSERM U 258 (P.D., A.B., G.L.), Hôpital Paul Brousse, Villejuif, France; Laboratoire dEpidémiologie et de Santé Publique (D.A., B.H.), Strasbourg, France; and INSERM U 558 (J.F., J.-B.R.), Faculté de Médecine Purpan, Toulouse, France.
Correspondence to Dr Jean Dallongeville, INSERM U 508, Institut Pasteur de Lille, 1 rue du Pr Calmette, 59019 Lille Cedex, France. E-mail jean.dallongeville{at}pasteur-lille.fr
Received December 31, 2002; de novo received April 23, 2003; accepted May 1, 2003.
| Abstract |
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Methods and Results A cross-sectional analysis was conducted of 9758 men, age 50 to 59 years, without coronary heart disease (CHD) who were recruited in France and Belfast, Ireland, from 1991 to 1993. Heart rate and CHD risk factors were compared among 4 categories of fish consumption, as follows: (1) less than once per week (n=2662), (2) once per week (n=4576), (3) twice per week (n=1964), and (4) more than twice per week (n=556). Fatty acid profiles of erythrocyte phospholipids were determined in a random subsample of 407 subjects. In erythrocyte phospholipids, eicosapentaenoic acid (P<0.0005), docosahexaenoic acid (P<0.0001), and total n-3 fatty acid (P<0.0008) increased across the categories of fish intake. Triglycerides (P<0.0001), systolic blood pressure (P<0.006), and diastolic blood pressure (P<0.0001) were lower and HDL cholesterol levels (P<0.004) were higher in fish consumers than in nonconsumers. Similarly, heart rate decreased across the categories of fish intake (P<0.0001). After adjustment for age, center, education level, physical activity, smoking habit, alcohol consumption, body mass index, and antiarrhythmic medications, heart rate remained statistically lower among fish consumers than among nonconsumers (P for trend <0.0001). Docosahexaenoic acid content of erythrocyte phospholipids was inversely correlated with heart rate (P<0.03).
Conclusions Fish consumption is associated with decreased heart rate in men. Because heart rate is positively associated with risk of sudden death, this association may explain, at least in part, the lower risk of sudden death among fish consumers.
Key Words: nutrition heart rate fatty acids death, sudden
| Introduction |
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Increased heart rate is a risk factor for sudden death.1013 Therefore, one possible mechanism by which n-3 fatty acid may prevent sudden death and fatal cardiovascular events is by regulating cardiac rhythm14 and preventing arrhythmia.15 In support of this hypothesis, clinical evidence has accumulated showing a significant impact of n-3 fatty acids on ECG parameters. For example, the n-3 fatty acid content of blood cells and serum cholesterol esters is associated with increased heart rate variability.1618 In short-term pharmacological trials, highly purified n-3 fatty acids reduced heart rate, diminished ventricular extrasystoles, and normalized cardiac rhythm in healthy and high-CHD-risk subjects.1923
Most of the evidence linking n-3 consumption to ECG parameters has been obtained in clinical studies using high doses of purified fatty acids, and to date, it is not known whether regular fish consumption has any effect on cardiac rhythm. This is particularly important because fish is the most important source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the staple diet. Therefore, we investigated the association between fish consumption and heart rate in a cross-sectional study of European men at contrasting levels of CHD risk.
| Methods |
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Self-administered questionnaires related to demographic factors, socioeconomic factors, and diet were completed at home by the participants and checked with the subject by survey staff at the clinic. Data on educational level, occupational activity, personal and family history, tobacco and alcohol consumption, drug intake, and physical activity were also collected. Educational level was classified into 3 categories: primary school, secondary school, and technical and university. Physical activity was assessed during work and leisure, on working days and weekends. Subjects were classified into the following 4 categories: intense exercise, if they took intense physical activity of more than 20 minutes 3 times or more per week; regular exercise, corresponding to intense physical activity 1 to 2 times per week; light physical activity, every week; and no leisure-time physical activity. Intense physical activity was defined by a physical activity that provoked sudation, increased heartbeats, and shortness of breath. Smoking habits were determined from questions on present and past habits and number and type of cigarettes, cigars, or pipes smoked per day. Smokers were defined in 3 categories: currently smoking at least 1 cigarette per day, ex-smokers, and never-smokers. Alcohol consumption was assessed by a questionnaire in which the subject reported his mean consumption (in units) of wine, beer, cider, and spirits for each day of the week. Intake of alcohol was expressed in drinks per day (1 drink equals 10 mL of pure ethanol). CHD was assessed with the London School of Hygiene Cardiovascular (Rose) Questionnaire for Chest Pain on Effort and Possible Infarction,25 and a standard 12-lead ECG was recorded.
Fish consumption was assessed by means of 1 question from a semiquantitative food-frequency questionnaire that asked for the frequency of a standard portion of fish consumption. No data were available to differentiate the type of fish or, particularly, fatty from nonfatty fishes. Individuals were divided into 4 categories: (1) consumption of a standard portion of fish less than once per week (n=2662), (2) once per week (n=4576), (3) twice per week (n=1964), and (4) more than twice per week (n=556).
Anthropometric measurements included height (to the nearest centimeter) and body weight (to the nearest 200 g) of subjects in light clothing without shoes. Body mass index (BMI) was computed as weight (in kilograms) divided by height squared (m2). Blood pressure was measured once at the end of the examination, after a 5-minute rest in the sitting position. Hypertension was defined by systolic blood pressure >140 mm Hg, diastolic blood pressure >90 mm Hg, or current blood pressurelowering treatment. Heart rate was measured after 5 minutes of rest in the sitting position with an automatic device (the Spengler SP9) for blood pressure measurement that also recorded heart rate.
Blood Sampling and Assay Procedures
Plasma total cholesterol and triglyceride levels were measured by enzymatic methods with reagents from Boehringer Mannheim. HDL cholesterol was measured after precipitation of apolipoprotein Bcontaining lipoproteins with phosphotungstate magnesium chloride (Boehringer). The erythrocyte phospholipid fatty acid composition was determined in a random subsample from each center (for a total of 407 men) by gas chromatography (Palo Alto, Calif) equipped with a flame ionization detector and capillary column as described previously.26
Statistical Analysis
Statistical analysis was conducted with SAS software (release 6.12; SAS Institute). The general linear model procedure was used to compare mean values of fatty acids, coronary risk factors, and heart rate across categories of fish consumption. Adjustment variables were age, recruitment center, education level, physical activity, smoking habit, alcohol consumption, BMI, and use of antiarrhythmic drugs (ß-blockers, amiodarone, digitalis, and calcium channel blockers).
| Results |
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Table 2 shows the fatty acid content of erythrocyte phospholipid according to fish consumption. There was no statistically significant difference among categories of fish consumption for palmitic, stearic, oleic, total saturated, total monounsaturated, total polyunsaturated, and n-6 fatty acids. In contrast, EPA (P<0.0005), DHA (P<0.0001), and total n-3 fatty acid (P<0.0008) content of erythrocyte phospholipid increased across fish consumption categories.
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Triglycerides (P<0.0001) and systolic (P<0.006) and diastolic (P<0.0001) blood pressure were lower and HDL cholesterol levels (P<0.004) were higher among fish consumers than among nonconsumers (Table 3). In contrast, there was no evidence for any association between fish consumption and total cholesterol or LDL cholesterol levels.
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Heart rate decreased across categories of fish consumption (P<0.0001; Table 4). After adjustment for age, center, education level, physical activity, smoking habit, alcohol consumption, BMI, and antiarrhythmic drugs, heart rate remained statistically lower in fish consumers (P for trend <0.0001). Adjusted heart rate ranged from 67.5 bpm in men consuming fish less than once per week to 65.6 bpm in those consuming fish more than twice per week. There was no evidence for heterogeneity among recruitment centers, which suggests a consistent association between fish consumption and heart rate in populations at contrasting levels of CHD risk.
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Finally, in the subsample of 407 men, there was a statistically significant association between age- and center-adjusted erythrocyte DHA content (but not with EPA or other fatty acids) and heart rate (ß=-0.73; P<0.03; Table 5). After further adjustment for education, physical activity, smoking habits, alcohol consumption, BMI, and antiarrhythmic drugs, this association was attenuated (ß=-0.55; P<0.11). In addition, the DHA and total n-3 fatty acid content of erythrocyte was associated with diastolic blood pressure (ß=-1.85, P<0.01 and ß=-0.48, P<0.03, respectively), and EPA levels of erythrocyte were associated with plasma triglycerides (ß=-0.16; P<0.05). There were no other statistically significant associations between erythrocyte free fatty acid content and cardiovascular risk factors.
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| Discussion |
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In the present cross-sectional population study, we investigated whether fish consumption affects heart rate in healthy men without a personal history of CHD. This is particularly important because sudden death most often occurs in men without a known history of CHD and also because dietary fish intake is the principal source of EPA and DHA in the staple diet. The results showed that regular consumption of fish is associated with decreased heart rate in healthy men. These findings suggest that fish consumption affects regulation of heart rate in middle-aged men. The present study adds to evidence from pharmacological trials that consumption of fish, rather than supplements, may have a beneficial effect on heart rate. This observation has important implications for primary prevention of sudden death.
The most likely active component of the effect of fish on heart rate is n-3 fatty acids and possibly DHA. This interpretation is supported by the finding of an inverse relationship between heart rate and DHA content of erythrocyte phospholipid. It is also consistent with short-term dietary trials with n-3 fatty acids.19,20 In those studies, pharmacological supplementation with highly purified DHA but not with EPA reduced heart rate in healthy and overweight subjects. In contrast, studies in dog models of sudden death showed that purified EPA, DHA, and
-linolenic acid can prevent ischemia-induced ventricular fibrillation.31 Therefore, additional studies are necessary to assess the contribution of individual n-3 fatty acids to cardiac electrophysiological regulation.
In the present study, the relationship between fish intake and heart rate was linear between men who consumed fish less than once per week and those declaring their consumption to be more than twice per week. For each increase in fish consumption of 1 portion per week, there was an average reduction of 0.5 bpm. Although this effect appears somewhat limited in absolute terms, it might still be important at the population level for the prevention of fatal cardiac events. In the Paris Prospective Study, which included more than 7700 men followed up for 23 years, the mean difference in heart rate between controls and patients who died of sudden death was 4.1 bpm,33 in the range of the average 2-bpm difference between frequent consumers and nonconsumers of fish. Furthermore, the relationship between heart rate and risk of sudden death was linear,13 which suggests that even a small reduction in heart rate might have a significant public health impact because of the high incidence of fatal events in the population.
Heart rate may be affected by n-3 fatty acids in different ways. There is now strong in vitro evidence that n-3 fatty acids stabilize the electrical activity of isolated cardiac myocytes by elevating the action potential threshold and prolonging the relative refractory time. These striking electrophysiological effects result from the modulation by free n-3 fatty acids of sodium and calcium channels through the cell membrane (for review, see Kang and Leaf15,32). These potent actions might also affect electrical stimuli, resulting in a lowering of heart rate. Alternatively, n-3 fatty acids could affect the sympathetic and parasympathetic control of heart rate by virtue of their interaction with the adrenergic system. Finally, the changes in heart rate may be due to some underlying benefit of fish components on cardiac physiology, such as improved ventricular or endothelial function and lower blood pressure, which may result in reduced heart rate.20
The present study has several limitations. First, the cross-sectional design does not enable us to examine the temporality of the observed associations. Second, because fish consumers have different nutritional habits than nonconsumers, it is also possible that our findings are due to some unknown confounding nutritional variables that we were unable to control for in our analyses. Fish consumers may also consume special foods or nutrients that have heart ratemodifying properties, such as alcohol. To minimize the possibility of a residual effect, adjustments were performed with alcohol as a continuous variable or different categories of consumption. Third, fish consumers also differ from nonconsumers in a number of life habits and CHD risk factors that may play the role of a confounding factor associated both with an increased heart rate and cardiac fatal events (ie, physical activity and smoking). Although statistical adjustments for known confounders were made, a residual effect or an influence of unmeasured factors is still possible. However, the finding of consistent associations between fish intake and heart rate in Belfast and France, despite different life styles and contrasting cardiovascular risk factors between countries, suggests that this association is partly independent of lifestyle factors. Among possible confounders, physical activity is particularly important because of its strong association with reduced heart rates and fish intake. Therefore, to account for its possible residual effect, several analyses were performed that included activity at work and during leisure time, which did not affect the conclusions. Finally, fish consumption was assessed by means of a food-frequency questionnaire, the accuracy of which is somewhat limited. However, the finding of a significant agreement between reported fish intake and n-3 fatty acid content of erythrocyte fatty acids suggests that reported fish consumption was reasonably accurate. In any case, a misclassification of fish consumers would tend to underestimate the strength of the association.
In conclusion, the results of the present study provide evidence that fish consumption is associated with a decreased heart rate in men age 50 to 59 years without a known history of CHD. Given the existing evidence implicating increased heart rate as a risk factor of sudden death, this finding provides a possible explanation for the lower risk of sudden death among fish consumers. It is now well established that EPA and DHA have pleiotropic effects on the cardiovascular system. Therefore, the combined effect of these fatty acids on cardiac electrophysiology, vascular function, and atheroma plaque structure very likely contributes to prevention of fatal cardiac events.
| Appendix |
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| Acknowledgments |
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| Footnotes |
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| References |
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