From Science Daily
People who eat fish a couple of times a week have fewer heart attacks than people who never eat fish. Researchers at the Japanese National Center of Neurology and Psychiatry in Kodaira may have discovered how fish protects the heart and blood vessels. They believe that eating fish protects the heart muscle from the effects of stress.
Stress raises the likelihood of contracting cardiovascular disease and eating fish reduces it. This is probably because of the fatty acids found in fish: EPA [structural formula shown here] and DHA. If you give people supplements containing fish fatty acids for longer than 12 weeks their heart rate goes down [Circulation. 2005 Sep 27; 112(13): 1945-52.] and the likelihood of them developing cardiovascular problems as well. That's why the Japanese wondered whether these two factors – stress and fish consumption – could cancel out each other's effect on the heart muscle.
The researchers did an experiment with 12 students who ate more than 70 g fish four times a week, and 13 students who ate less than 70 g fish not more than twice a week. The subjects had to keep subtracting 13 starting from 5000 [5000 – 4987 – 4974 etc.] and the researchers told them whether their answers were correct or not. At the same time the subjects' heart functioning was monitored.
The researchers discovered that a high-fish diet had no effect on how difficult or unpleasant the subjects found the test. So the amount of mental stress was the same in both groups.
Among the group that ate large amounts of fish, the systolic blood pressure [pressuring during the heart beat; SBP], the average blood pressure [MBP] and the diastolic blood pressure [pressure between two heart beats; DBP] rose by less than among the subjects that ate little fish. The heart rate increased less in the fish eaters than in the other group.
The pulse wave velocity [an indicator of the stiffness of the blood vessels; PWV] also rose less than in the low fish eating group.
The Japanese conclude that a diet rich in fish normalises the heart muscle under conditions of stress. They note, however, that the fish eaters' diet differed from that of the other group in a number of ways. "A group difference in fish-eating habit was a salient feature, but such a dietary pattern was accompanied by modulation in other food categories, including a higher intake amount of fruits, algae, and vegetables."
Nevertheless, the Japanese maintain that it's the fish that is responsible for the cardiovascular stress protective effect. "These findings suggest a possible physiological mechanism that may explain why frequent fish consumption reduces coronary heart disease risk", they write.
Circulation. 2005 Sep 27;112(13):1945-52.