FISH CONSUMPTION AND RISK OF STROKE:
A REAL BENEFIT OR A FISH STORY?

(November 2003)

According to a study from Harvard Medical School and the Harvard School of Public Health, all you have to do to reduce your risk of the most common form of stroke by about 40 percent is eat one serving of fish a month. They followed about 44,000 men ages 40 to 75 years for twelve years, during which time 608 strokes occurred; 377 of these were due to reduced blood supply to the brain (called ischemic). This is, by far, the most frequent form of stroke.

Fish consumption was divided into five groups (quintiles) ranging from less than once a month to at least five times a week. Those eating fish as infrequently as one to three times a month showed a 44 percent reduction in risk of ischemic stroke. Importantly, there was no further reduction in risk from eating fish once a week, two to four times a week, or even daily. The investigators then estimated the intake of omega-3 fatty acids, the component of fish thought to be responsible for the benefit, and found the same pattern - a 44 percent reduction in ischemic stroke risk in the second lowest omega-3 intake and no further reduction with higher intakes. Surprisingly, those men who took fish oil supplements (which provide very large amounts of omega-3 fatty acids) showed no reduction in ischemic stroke occurrence.

The authors concluded "in this large cohort of men, we observed a significantly lower risk of ischemic stroke in men who consumed fish once per month or more compared with men who ate fish less often".

Commentary: The authors argue that, once you pass a threshold - one serving of fish a month - that is all you need to reduce the risk of ischemic stroke and increasing fish intake beyond that does not increase the benefit. They quote other "supportive" studies, but the other studies are not really supportive. In one of the studies, the fish intake was about once per week, not once per month. In three other of the quoted studies, the results were not statistically significant, and in one case, the strongest (but still not statistically significant) hint of reduced risk was at five times or more per week. In a separate study of fish oil supplements, no benefit was found for stroke, even though the risk of heart attack was significantly reduced among fish oil supplement users.

There is then really no support for this specific threshold of once a month fish intake. Furthermore, the lack of greater benefit with greater fish intake makes the whole study suspect. Additionally, if the mechanisms of action relate to the omega-3 fatty acids, why was there no reduced risk for those consuming large amounts of omega-3 fatty acids in supplements? The authors say this may be related to the relatively small number of fish oil supplement users and their small number of ischemic strokes; but, even with the small numbers, there should have been some hint of reduced risk and there was not.

Omega-3 fatty acids are found most in oily fish such as mackerel and salmon. They have multiple actions, including reducing the risk of severe, potentially fatal abnormal heart rhythms and reducing clotting of the blood. It is the reduced clotting that would presumably help prevent ischemic strokes.

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The evidence is reasonably clear that eating oily fish once, or preferably twice, a week reduces the risk of death from heart attack or death from abnormal heart rhythms not associated with heart attack. So, oily fish should be a part of your diet.

CLICK HERE for table of oily fish and for table of plant alternatives that supply good amounts of omega-3 fatty acids. Also note that pregnant women should probably not eat king mackerel, tilefish, shark, and swordfish, and should eat tuna in moderation because of concerns about their mercury content (mercury content of tuna is ordinarily quite low, so three-quarters of a pound or even a pound a week should be safe).

Whether eating fish reduces risk of stroke is unclear. Because omega-3 fatty acids reduce clotting, those taking fish oil supplements should watch the dosage carefully because they could be getting huge amounts of omega-3 fatty acids and that potentially could increase the risk of brain hemorrhage (but no such increased risk has been documented thus far).

Healthful Life continues to believe the best way to get your omega-3 fatty acids is by eating oily fish once or twice a week (note, mackerel has about twice the omega-3 content as other fish on the list). Fish oil supplements are probably quite safe if you do not go overboard on dosage. But, their benefit has not been fully documented for heart attack reduction and fish could have some benefits not found in the supplements.

He, K., et al. Fish consumption and risk of stroke in men. Journal of the American Medical Association. Vol 288 (December 25) Pgs 3130-3136. 2002.