Mounting evidence shows that omega-3 polyunsaturated fatty acids (PUFAs) prevent cardiac death and nonfatal myocardial infarction. The types of PUFAs that have been most often studied include eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and α-linolenic acid (ALA). Dietary sources of PUFAs include meat, poultry, fish, vegetable oils, salad dressings, and grain products. Fatty cold-water fish such as halibut, mackerel, herring, and salmon are good sources of EPA and DHA. Soy and pinto beans, walnuts, and flaxseed are good sources of PUFAs, as are vegetables such as leeks and purslane. Oils high in ALA include canola, soybean, and flaxseed. Harper and Jacobson review evidence supporting the value of PUFAs in preventing coronary heart disease (CHD).
Epidemiologic studies that confirmed the relationship between PUFAs and decreased CHD include comparisons of the rates of heart disease among Eskimos and Green-landers who ate higher amounts of PUFAs versus Danes whose diet contained a much lower PUFA intake. In the U.S. Physicians Health Study, U.S. male physicians aged 40 to 84 years who ate more fish had a decreased risk of sudden cardiac death. A higher intake of ALA among participants in the Nurses Health Study correlated with a lower relative risk of fatal CHD.
The omega-3 PUFAs are probably cardioprotective through several mechanisms. They have antiarrhythmic and antithrombotic effects, and improve endothelial function. Atherosclerotic plaque formation has been shown to be inhibited by ingestion of EPA and DHA. Total cholesterol and triglyceride concentrations are lowered with consumption of fish oil without a drop in high-density lipoprotein levels. Working mostly through anti-atherogenic effects, it is mainly the marine-derived PUFAs, EPA, and DHA that have the clearest value. The benefit of ALA needs further clarification.
|Fish (mostly EPA and DHA)||Plants (mostly ALA)|
|Atlantic herring||Butternuts (dried)|
|Albacore tuna||English walnuts|
|Chinook salmon||Soybeans (raw)|
|Coho salmon||Wheat germ|
|Atlantic cod||Pinto beans|
Prospective trials have confirmed the benefit of PUFAs on CHD. In a large prospective study, men who had recovered from a myocardial infarction and were assigned to eat fish or take fish oil capsules had a significant decrease in all-cause mortality. A prospective study of a Cretan Mediterranean diet, high in fruits and vegetables, rich in monounsaturated fatty acids (olive oil), and high in ALA, among a population who had survived a first myocardial infarction, demonstrated a significant reduction of risk for cardiovascular death and nonfatal myocardial infarction. Other prospective diet-based studies have demonstrated similar beneficial results.
The authors conclude that PUFAs are useful in secondary prevention of CHD. In the U.S. diet, the principal sources of PUFAs are vegetable oils and fish. Guidelines recommend increased consumption of ALA, EPA, and DHA. For persons who cannot tolerate an increase in fish to one to two fish meals per week, supplements are available, including a vegetarian source derived from algae. One or two fish-oil capsules containing 750 to 1,000 mg EPA can be used as an alternative. Cod liver oil is a good source of PUFAs but also contains high amounts of vitamins A and D. Although more evidence is needed to confirm the value of PUFAs in the primary prevention of CHD, it would be prudent to increase ingestion of PUFA-containing foods (see accompanying table).
editor's note: The cardioprotective value of omega-3 fatty acids is becoming more widely accepted. Data tell us that the type of fat is more important than the total amount of fat in the diet. Replacing saturated fat with unsaturated fat is more effective in lowering CHD risk than simply reducing total fat intake. Clear evidence of reduced cardiac mortality has been demonstrated in studies using omega-3 fatty acids for secondary prevention of cardiac morbidity and mortality. Other potential benefits of consumption of fatty fish may include decreased risk of prostate cancer (Terry P, et al. Fatty fish consumption and risk of prostate cancer. Lancet 2001;357:1764–6) and decreased rates of depression (Mischoulon D, Fava M. Docosahexanoic acid and ω-3 fatty acids in depression. Psych Clin North Am 2000;23:785–94). When recommending fatty acid supplements, physicians must monitor patients closely, keeping in mind the lack of data about long-term safety.—r.s.