Long-chain n-3 polyunsaturated fatty acids (PUFAs), which are found only in fish oils, protect against cardiovascular disease by several mechanisms, including an anti-inflammatory effect that is thought to stabilize atherosclerotic plaques. In contrast, the n-6 PUFAs found in vegetable oils could produce inflammation and increase plaque instability. Thies and colleagues measured the effects of sunflower and fish oils on carotid plaque stability in patients scheduled for carotid endarterectomy at an English hospital.
All patients who agreed to participate were randomly assigned to one of three groups, and fasting blood samples were drawn for lipid studies. Sixty-one patients took fish oil, 59 took sunflower oil, and 68 took a control blend of 80:20 palm and soybean oils. Patients maintained their usual diet, monitored their eating with a food diary, and took two capsules of oil with each meal, for a total of six capsules (1.4 g of long-chain PUFAs or control oil) per day. Capsule counts and changes in blood lipid levels indicated 85 to 90 percent adherence to study protocols. Patients underwent carotid endarterectomy between seven and 190 days after randomization. A second blood sample was obtained on the morning of surgery, and the excised plaques were analyzed for fatty-acid composition.
The three groups of patients were comparable on entry to the study. The mean age was about 70 years, and the median carotid stenosis was 80 to 95 percent. Twenty-six patients withdrew from the study, mainly because surgery was performed within seven days or was not undertaken. Two patients withdrew because of symptoms they attributed to the capsules.
Plasma cholesterol levels were not affected by treatment with fish or sunflower oil. Plasma triglyceride levels were significantly lowered with fish-oil supplementation, and this benefit increased as treatment continued. The proportions of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the plasma low-density lipoprotein levels increased only in the fish-oil group. Patients in the fish-oil group also had higher proportions of EPAs in their excised carotid plaques. The proportion of EPAs in the plaques correlated with the duration of supplementation. The excised plaques showed significant differences in morphology. Plaques from patients in the fish-oil group were more likely to have thick, fibrous caps, no signs of inflammation, and fewer macrophages. Overall, patients taking fish oil had a greater proportion of type IV lesions (atheromas) and fewer type V lesions (fibrotic lesions and fibroatheromas).
The authors conclude that dietary supplementation with sunflower oil (3.6 g of linoleic acid per day) has little effect on plasma lipid levels and carotid plaque fatty-acid composition and stability. However, fish-oil supplementation lowers triglyceride concentrations and stabilizes carotid atherosclerotic plaques. Increased plaque stability could explain the beneficial effects, such as reduction in sudden death. The authors emphasize the potential benefit of fish-oil supplementation in reducing morbidity and mortality from cardiovascular and cerebrovascular disease, including myocardial infarction and stroke.
editor's note: This study adds to the impressive evidence showing benefit from fish oils in both primary and secondary prevention studies of cardiovascular disease. Should we be urging all patients to eat oily fish at least two or three times per week? Such advice must be provided with great care; frying the fish or serving it with rich sauces and French fries could make health problems worse. “Oily” fish (e.g., salmon and mackerel) are good, but “greasy” fish (i.e., battered and deep-fried) are bad.—a.d.w.