Dr. Rajiv Chowdhury at Cambridge University in the U.K. and Prof. Oscar Franco at Erasmus MC in Rotterdam analyzed data from 38 studies involving 794,000 participants in Europe, North America and the Asia-Pacific region.

There were more than 34,000 stroke events (such as ischemic strokes, hemorrhagic strokes and mini-strokes known as transient ischemic attacks) that are collectively called cerebrovascular disease.

"This review found that higher fish consumption is moderately but significantly associated with a reduced risk of incident cerebrovascular disease," the study’s authors concluded in a recent issue of the British Medical Journal.

They assessed fish and long chain omega-3 fatty acid consumption using dietary questionnaires, identified markers of omega-3 fats in the blood and recorded use of fish oil supplements.

Those who ate two to four servings a week showed a 6% lower risk of cerebrovascular disease compared with those eating one or fewer servings of fish a week. People who dined on five or more servings a week had a 12% lower risk, the researchers said.

However, the biomarkers and fish oil supplements were not associated with a reduced risk of stroke.

"These findings therefore suggest that single nutrients may have limited effects on chronic disease outside of their original food sources," the researchers said.

They offered several possible explanations for the difference:

• Fish has a wider range of nutrients besides omega-3 fatty acids, such as vitamin D and B vitamins, that are also linked to lower stroke risk.
• People who eat more fish may eat less red meat.
• Higher fish consumption may indicate a healthier diet pattern overall or higher socioeconomic status that are also associated with vascular health.
• How the fish is prepared, such as deep frying, could also make a difference by potentially adding harmful trans fats during the cooking process, the study's authors said.

Fish consumption is low in most European countries, Canada and the U.S., Jannette de Goede, of the human nutrition division at Wageningen University, said in journal commentary.

The studies included in the review were also not designed to detect an effect on stroke and many of the participants were being treated for heart disease, de Goede noted.

"On the basis of available evidence, it is reasonable to advise people that eating one or two portions of fish a week could reduce the risk of CHD and stroke," the editorial concluded.

In June, a smaller review published in the Cochrane Database of Systematic Reviews concluded that omega-3 supplements taken in capsules or margarines resulted in no improvement in cognitive function in healthy older people.

The researchers received scholarships, studentships or grants from Gates Cambridge, U.K.’s Medical Research Council and Pfizer Nutrition.