(20-09-11) Total and Lean Fish Consumption Associated with Lower Stroke Risk in Women

Several studies have explored the relationship between the risk of developing stroke and the consumption of fish or long-chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFAs) found in fish and shellfish. In Western countries, the predominant type of stroke is ischemic stroke (87% of all strokes in the U.S.), which arises when blood flow to the brain is blocked by a blood clot (thrombus). A clot may form in a cerebral blood vessel narrowed by the accumulation of plaque or be carried to the brain after it forms elsewhere in the body. Individuals with high blood pressure and those of African descent have the greatest risk of incurring a stroke. American men have a 61% greater chance of stroke compared with European men, while American women have twice the odds of a stroke as their European counterparts.

How strong is the evidence linking fish or n-3 LC-PUFAs with a lower chance of a stroke? Darius Mozaffarian and colleagues at the Harvard School of Public Health, Boston, USA, reported a prospective observational study in which there was a 27% lower risk of stroke among older adults aged 65 to 98 years who consumed tuna or other fish 1 to 4 times a week. Individuals who consumed fried fish or fish sandwiches once or more per week were at a 44% greater risk. On the other hand, 40- to 79-year-old participants in the European Prospective Investigation into Cancer reportedly had no significant reduction in stroke risk with fish or shellfish consumption. However, women in this study who consumed oily fish at least once or twice a week had a 30% lower chance of stroke compared with those eating fish less often than once a week. Several other prospective studies have reported conflicting results. Now, another examination of this relationship has been published based on data from 34,670 women who were part of the Swedish Mammography Cohort.

Participants included women aged 49 to 83 years who completed a detailed diet and lifestyle questionnaire and were free of stroke, ischemic heart disease or cancer at baseline. Over the 10-year observation period, there were 1,680 cases of stroke, of which 1,310 (78%) were cerebral infarctions. On average, the participants consumed 2 fish meals per week. Cod, saithe and fish fingers comprised about half the fish meals consumed. Women who consumed fish were more likely to be university educated, have a history of diabetes or hypertension and a family history of myocardial infarction compared with women with low fish consumption. Fish-eaters also consumed more alcohol, red meat, fruit and vegetables.

The most notable observation was a statistically significant 16% reduction in the multivariate-adjusted risk of stroke among women who consumed an average of 2 or more meals of fish per week (Figure). The trend for lower risk was dose-dependent. Furthermore, there was a 28% reduced risk of total stroke in those who ate 4 or more fish meals per week compared with women who ate fish infrequently. However, the multivariate relative risks specifically for cerebral infarction or hemorrhagic stroke did not reach statistical significance.

The investigators further examined the relative risks of different types of stroke by the amounts and types of fish consumed. Women who consumed 3 or more servings of lean fish were 32% less likely to experience any stroke or a cerebral infarction in multivariate analysis compared with women who did not eat fish. There was no association between the risk of any type of stroke and the frequency of consuming fried fish; salmon, whitefish and char; or herring and mackerel. The authors pointed out that herring and salmon are frequently consumed salted, a habit that might increase the risk of hypertension in this population. Hypertension is strongly associated with a greater risk of stroke.

Findings from this 10-year observational study confirm the conclusion of a meta-analysis of 9 cohorts that were assessed for the association between fish consumption and risk of stroke. The meta-analysis noted that fish consumption as infrequent as 1 to 3 times per month was associated with a significant 30% reduction in stroke risk and that lower risk was observed with eating fish more frequently. The findings reported here for lean fish consumption are at odds with other data linking lower stroke risk with greater fatty fish or n-3 LC-PUFA consumption. In the Mozaffarian study mentioned previously, eating tuna and presumably consuming the n-3 LC-PUFAs, was associated with a lower risk of stroke. Others have reported a significantly reduced risk of thrombotic stroke with fish consumption twice a week or more and in the highest quintile of n-3 LC-PUFA intakes.

In summary, this very large study among healthy Swedish women confirmed those studies reporting a lower risk of stroke among women who consume fish regularly. The 16% risk reduction was most pronounced for ischemic or thrombotic stroke, not the less common hemorrhagic stroke. Risk fell to about 30% less if fish, especially lean fish, was consumed 5 times a week. No significant increases in risk or adverse indications were observed.

Larsson SC, Virtamo J, Wolk A. Fish consumption and risk of stroke in Swedish women.

Source: Am J Clin Nutr 2011;93:487-493. [PubMed]

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