There is also a lack of evidence of its benefits on haemorrhagic stroke, said the team from the Northwestern University in Chicago.
The researchers carried out a meta-analysis of eight independent studies published since 1966, involving more than 200,000 people, who were followed for up to 30 years in some trials.
The risk of stroke was 13 per cent lower among those who ate fish at least once weekly compared with those who did so less than once per month. But those eating fish only one to three times per month also saw a risk reduction of 9 per cent, while the protective effect went up to 30 per cent for those eating fish five or more times per week, finds the report in this month's issue of Stroke (35:1538).
However, the benefit was limited largely to reduced risk of ischaemic stroke as most studies were carried out in western countries where this is the most frequent type of stroke. The three large studies that included data regarding different types of stroke showed no less risk for haemorrhagic stroke (caused by bleeding in the brain rather than a blockage of blood supply to the brain in ischaemic stroke) with increasing fish intake.
The researchers also noted that "one should be cautious when advising people to use fish oil supplements instead of eating whole fish".
Although numerous experimental studies have indicated protective effects from the use of fish oil supplements on cardiovascular diseases, the dose of long-chain omega-3 PUFAs used in this research is much higher than the amount typically found in the diet, they write.
"It is not clear how much intake of long-chain omega-3 PUFAs may be required to significantly reduce risk of stroke. In addition, whether fish consumption provides other beneficial nutrients not present in pure fish oil remains uncertain," they write.
This is because of the findings that a small amount of fish intake may lower risk of ischemic stroke, suggesting that constituents other than omega-3s may have synergistic effects.
"Moreover, considering the different amounts of long-chain omega-3 PUFAs in different types of fish, one would expect to observe more benefit by eating fatty fish rich in long-chain omega-3 PUFAs if any beneficial effect of fish intake on risk of stroke is largely attributable to its content of long-chain omega-3 PUFAs. However, data are very limited regarding the effect of intake of different types of fish on stroke risk," they write.