Eating More Fish And n3 Fatty Acids Associated With Reduced Risk Of Stroke For Women

Hiroyasu Iso, M.D., Ph.D., of the Channing Laboratory, Brigham and Women’s Hospital and Harvard Medical School, Boston, and colleagues followed 79,839 women over a period of 14 years, from 1980-1994. The women ranged in age from 34 to 59 at the start of the prospective study period. All were enrolled in the Nurses’ Health Study, which collected information on lifestyles and medical history from female nurses in 11 states.

According to background information cited in the study, an inverse relationship between fish intake and risk of stroke has been reported in several, but not all, prospective studies. But no prospective study has previously examined the relationship between intake of fish and n3 fatty acids and stroke risk, by stroke subtypes.

According to The American Medical Association Encyclopedia of Medicine, stroke may be caused by any of three mechanisms. Thrombosis is blockage by a thrombus, or clot, that has built up on the wall of a brain artery. It accounts for 40 to 50 percent of strokes. Embolism is blockage by an embolus (usually a clot) that is swept into an artery in the brain. It accounts for 30 to 35 percent of stroke cases. Thrombosis and embolism both lead to cessation or marked decrease of the blood supply to part of the brain, and thus to infarction, or tissue death. The third mechanism is rupture of a blood vessel in or near the brain, which may cause an intracerebral hemorrhage (bleeding within the brain), or a subarachnoid hemorrhage (bleeding over the surface of the brain). Hemorrhages account for 20 to 25 percent of stroke cases.

The authors report that during their 14-year follow-up study, 574 strokes were documented - including 181 hemorrhages, 303 ischemic strokes (264 thrombotic and 39 embolic infarctions), and 90 strokes of undetermined type.

"We observed a significant inverse association between fish intake and risk of stroke, primarily thrombotic stroke, after adjustment for cardiovascular risk factors and selected dietary variables," the authors write.

"Compared with women who ate fish less than once per month, those with higher intakes of fish had a lower risk of total stroke," they continue. After adjusting for age, smoking, and other cardiovascular risk factors, women who ate fish one to three times per month had a 7 percent lower risk than those who ate fish less than once per month. Women who ate fish once a week had a 22 percent risk reduction; those who ate fish two to four times a week had a 27 percent lower risk; and for those who ate fish five or more times per week, their risk of total stroke was reduced 52 percent.

The authors examined the effect of fish and n3 fatty acid consumption on the risk of different types of stroke, and on women who regularly took aspirin. Aspirin itself can potentially lower stroke risk by reducing platelet aggregation.

"Among stroke subtypes, a significantly reduced risk of thrombotic infarction was found among women who ate fish two or more times per week [51 percent lower risk]," the authors write. "Women in the highest quintile of intake of long-chain n3 polyunsaturated fatty acids had reduced risk of total stroke and thrombotic infarction [28 percent risk reduction for total stroke and 33 percent lower risk for thrombotic infarction]."

"When stratified by aspirin use, fish and n3 polyunsaturated fatty acid intakes were inversely associated with risk of thrombotic infarction, primarily among women who did not regularly take aspirin," they continue.

The study found no association between consumption of fish or n3 fatty acids and the risk of hemorrhagic stroke.

The authors suggest several mechanisms may be involved in the lower stroke risk associated with n3 fatty acids. A high-dose supplementation of these fatty acids can reduce platelet aggregation. "High-dose supplementation of n3 fatty acids (e.g., 15 g/d) also lowers blood pressure levels in hypertensive persons, and reduces plasma fibrinogen concentrations in healthy volunteers," they write. "These effects may contribute to the prevention of atherosclerotic development and the thrombotic process."

"In conclusion, consumption of fish and n3 fatty acids was associated with a reduced risk of total stroke and thrombotic infarction primarily among women who did not take aspirin regularly. Consumption of fish and n3 fatty acids was not related to risk of hemorrhagic stroke," the authors summarize.

"These results suggest that regular intake of fish may be beneficial for the prevention of thrombotic infarction in middle-aged U.S. women," they conclude.

(JAMA. 2001; 285:304-312)

Editor’s Note: This study was funded by research grants from the National Institutes of Health. Dr. Iso is the recipient of an overseas research fellowship from the Japan Society for the Promotion of Science.