Fish Consumption and the Risk of Coronary Artery Disease
March 1998; Volume 1: 32-34
By Matthew Sorrentino, MD
Coronary artery disease (cad) is the number one killer of men and women in the United States. Can we change this dismal statistic by changing our diet? A diet rich in cold water fish may be the answer. Over the past decade there has been a steady decline in the death rate due to coronary disease. A reduced intake of saturated fat and cholesterol and a substitution of fish in the diet may be a simple lifestyle intervention that we all can do to decrease the risk of developing heart disease.Historical and Clinical Studies
Interest in eating fish rich in omega-3 fatty acids as a way to protect against heart disease was first suggested by observations that Greenland Eskimos had a low coronary mortality despite a diet as high in fat and cholesterol as that of other North Americans. The different composition of the fats in the diet was thought to be an explanation for the low incidence of cardiovascular disease among the Eskimos.1 The Eskimo diet is rich in long-chain n-3 polyunsaturated fatty acids (omega-3 fatty acids) found in fish and in the flesh of the seal, walrus, and whale meat that are the principal components of the Eskimo diet.An association between fat intake and coronary disease was noted in the Seven Countries Study2 in which a low coronary mortality was noted in Japan and Mediterranean Southern Europe that was explained in part by the higher fish consumption in those cultures compared with that of the inhabitants of North America or Northern Europe. Two prospective studies, the Chicago Western Electric study3 and the Zutphen trial,4 showed that the regular consumption of fish in the diet was associated with lower rates of coronary artery disease.
Not all studies, however, have shown a positive association between fish in the diet and a reduction in coronary risk. The Health Professionals Follow-up Study found that increasing fish intake from one to two servings per week to five to six servings per week did not reduce the risk of heart disease.5 Carefully done studies evaluating fish oil supplements on coronary atherosclerosis regression6 and prevention of restenosis after angioplasty7 failed to show any benefit. Studies such as these suggest that fish oil capsules are not a substitute for changing to a healthy diet.
Mechanism of Action
Fish are a rich source of omega-3 polyunsaturated fatty acids. Omega-3 and omega-6 fatty acids found in plant sources are essential to the human diet. Omega-3 fatty acids may have multiple mechanisms of benefit on the cardiovascular system.Omega-3 fatty acids in the diet lower triglyceride levels and may inhibit the synthesis of VLDL particles. There does not appear to be any effect on low density lipoprotein (LDL) levels.
A diet rich in omega-3 fatty acids may lower blood pressure. A study of two sets of people of the same ethnic background compared cardiovascular risk factors in a group consuming a fish-based diet with a second group consuming a vegetarian diet. The frequency of definite and borderline hypertension was lower in the fish consuming group.8 Additional studies documented that the intake of cold water fish at 280 g/d can bring about a decrease in blood pressure by approximately 12%.9 The mechanism of this blood pressure decrease is unknown.
The omega-3 fatty acids also exert significant effects on coagulation parameters. Greenland Eskimos, for example, were found to have reduced platelet aggregation and prolonged bleeding times. Omega-3 fatty acids have been found to replace arachidonic acid in the membrane of platelets altering their ability to aggregate.10 Omega-3 fatty acids may also increase levels of tissue plasminogen activator and lower concentrations of plasminogen activator inhibitor and fibrinogen;11 all actions that further decrease the potential for thrombus formation.
Omega-3 fatty acids may also have effects on inflammation and the immune system.10 Cellular immunity may be decreased when fish is added to the diet. The clinical significance of this finding is unknown.
Finally, omega-3 fatty acids may have an effect on cardiac arrhythmias. Dietary supplementation with fish or fish oils has been shown to reduce PVCs12 and improve heart rate variability in myocardial infarction survivors.13 This may explain the finding of reduced sudden death in patients with a previous myocardial infarction advised to eat fish twice a week for two years compared with similar patients on a normal diet.14
Adverse effects
Adverse effects may occur with a diet high in fish. The increase in bleeding time due to inhibition of platelet aggregation may lead to an increased risk of bleeding especially if combined with aspirin or anticoagulant therapy. A high intake of fish has also been associated with an increased risk of myocardial infarction in one report.15 One explanation is that fish may also be a source of methyl mercury, which exerts a harmful toxic effect. Finally, the inhibitory effects on the immune system raise some questions about potential adverse consequences.Diet/Dosage
Observational studies and prospective evaluations have shown a cardiovascular benefit in individuals who consume fish in the diet compared with no fish diets. The Health Professionals Follow-up Study, however, did not show any incremental benefit when increasing the fish servings from one to two per week to five to six servings per week.5Conclusion
The observed decrease in the mortality due to cardiovascular disease may be due in part to an improvement in the American diet. The inclusion of 4-oz. portions of broiled, baked, or grilled cold-water fish at least twice a week may protect against heart disease, because of the effects of the omega-3 fatty acids on platelet function, lipid metabolism, blood pressure, and cardiac arrhythmias. Over 95% of heart attacks occur because of the development of a platelet thrombus associated with a pre-existing atherosclerotic lesion. A diet high in cold-water fish, therefore, may serve as an anticoagulant preventing thrombus formation similar to the protective effect that aspirin has in preventing coronary events.The protective effect of fish may be a direct effect of the omega-3 fatty acids or may occur because of a reduction in animal fat and other saturated fats in the diet when fish is substituted. The protective effects of cold-water fish in the diet may be why some cultures have a significantly lower incidence of cardiac disease compared to western countries.
Recommendations
Include at least two cold water fish servings per week in the diet for a beneficial effect on preventing heart disease. A larger amount of fish (> 280 g, or 9 oz/d) may be required to realize a significant (>10%) blood-pressure lowering effect. The type of fish is important. Cold-water fish such as mackerel, salmon, albacore tuna, rainbow trout, and sardines are rich in omega-3 fatty acids. The fish should be prepared with little or no fat—either grilled, broiled, baked, sauteed, or poached; the effect on coronary disease of frying or deep frying will offset any potential protective gains from the ingestion of the fish and its omega-3 oils. It has not been well-demonstrated that fish oil capsules give the same benefit as a diet rich in cold-water fish and currently are not recommended as a substitution for the inclusion of fish in the diet.References
1. Leaf A, Weber PC. Cardiovascular effects of n-3 fatty acids. N Engl J Med 1988:318:549.2. Keys A, ed. Coronary heart disease in seven countries. Circulation 1970;41(suppl I):I1.
3. Shekelle RB, et al. Diet, serum cholesterol, and death from coronary heart disease: The Western Electric Study. N Engl J Med 1981;304:65.
4. Kromhout D, Arntzenius AC, Kempen-Voogd N, et al. The inverse relation between fish consumption and twenty year mortality from coronary heart disease. N Engl J Med 1985;312:1205.
5. Ascherio A, et al. Dietary intake of marine n-3 fatty acids, fish intake, and the risk of coronary disease among men. N Engl J Med 1995;332:977.
6. Sacks FM, Stone PH, Gibson CM, et al. Controlled trial of fish oil for regression of human coronary atherosclerosis. J Am Coll Cardiol 1995;25:1492.
7. Leaf A, et al. Do fish oils prevent restenosis after coronary angioplasty? Circulation 1994;90:2248.
8. Pauletto P, et al. Blood pressure and atherogenic lipoprotein profiles of fish-diet and vegetarian villagers in Tanzania: The Lugalawa study. Lancet 1996;348:784.
9. Singer P, et al. Lipid and blood-pressure lowering effect of mackerel diet in man. Atherosclerosis 1983;49:99.
10. Yetiv JZ. Clinical applications of fish oils. JAMA 1988;260:665.
11. Stone NJ. Fish consumption, fish oil, lipids, and coronary heart disease. Circulation 1996;94:2337.
12. Sellmayer A, et al. Effects of dietary fish oil on ventricular premature complexes. Am J Cardiol 1995;76:974.
13. Christensen JH, et al. Fish consumption, omega-3 fatty acids in cell membranes, and heart rate variability in survivors of myocardial infarction with left ventricular dysfunction. Am J Cardiol 1997;79:1670.
14. Burr ML, et al. Effects of changes in fat, fish, and fibre intakes on death and myocardial infarction: Diet and reinfarction trial (DART). Lancet 1989;2:757.
15. Salonen JT, Nyyssonen K, Salonen R. Fish intake and the risk of coronary disease (letter). N Engl J Med 1995;333:937.
CME Questions
a. True
b. False
16. A patient comes to your office with arthritis and requests information on the efficacy of bee venom therapy (BVT)? What advice do you provide?
a. BVT is a potentially dangerous treatment with little reliable information about its effectiveness in humans.
b. Many anecdotal success stories in the treatment of arthritis have been reported.
c. Studies in animal models suggest that bee venom has an anti-inflammatory affect and can suppress induced arthritis in rats.
d. All of the above
17. Co-Q10 works by improving oxygen uptake and utilization in cardiac myocytes.
a. True
b. False
18. Which of the following is not a clinically proven outcome of using Co-Q10 in patients with CHF?
a. A decrease in the number of hospitalizations
b. A decrease in mortality rates
c. An increase in ejection fraction
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