Fish oil lowers even normal blood pressure.

Fish oil lowers even normal blood pressure

Researchers showed last April that among mildly hypertensive men, diets high in fish oils can lower blood pressure -- a major risk factor for heart attack and stroke. Now, another team reports that such diets can similarly lower blood pressure in healthy, nonhypertensive men and women. The new findings also point to a possible mechanism behind the effect: increased excretion of sodium and body fluids by the kidneys.

Study director Constance Kies of the University of Nebraska at Lincoln described her group's results this week at the American Chemical Society's fall national meeting in Miami Beach. For 28 days, the researchers placed five men and five women on a diet supplemented with approximately 1 gram of polyunsaturated fat per day. Each participant received capsules of either safflower oil (rich in omega-6 fatty acids) or fish oil (rich in omega-3 fatty acids). Kies says the fish oil dose resembled the amount consumed in a single daily serving of salmon, lake trout or tuna. After two weeks, diners switched to the other supplement.

Though the two participants with the lowest starting blood pressure showed no change during the test, all others experienced a drop in both diastolic and systolic blood pressure -- but only when receiving the fish oil supplement. The reductions proved small but statistically significant, generally about 2 to 3 millimeters of mercury, Kies says. However, notes Garret A. Fitzgerald of Vanderbilt University in Nashville, who coauthored the earlier study involving hypertensive men, larger reductions would not be expected with the doses Kies used -- less than one-tenth those yielding discernible blood pressure changes in his study.

"Fish oils appear to operate much like a low-sodium diet," Kies says. By increasing urine output, they reduce the volume of fluids pressing against the inside of blood vessel walls, she explains. People on fish oil in her study increased their urine output by roughly 10 percent. And unlike most diuretics, fish oil did not increase the excretion of potassium, important in regulating blood pressure.

Kies suspects the changes may trace to the regulation of kidney function by eicosanoids -- a class of "biological activators" that can speed or slow many body activities. Eicosanoids derived from the omega-6 fatty acids in safflower oil are perhaps 1,000 times more active than those derived from fish oil's omega-3 fatty acids, Kies says. The les responsive eicosanoids from fish oil, she suggests, might "tie up reaction sites" in the sodium management system, slowing or tempering chemical processes that could otherwise lead to more fluid retention and higher blood pressure.