Good Fish and Bad Fish

Summarized by Robert W. Griffith, MD
September 12, 2005

Introduction

We've urged readers in the past to eat more fish - preferably 'deep-sea' fish that contain plenty of omega-3 fatty acids. The general statement "eating fish stops strokes" is broadly correct, but it does depend somewhat on what is meant: which sort of fish, and which sort of stroke. These points have been clarified by a recent study reported in the Archives of Internal Medicine, which we summarize here.

Stroke is often a devastating cardiovascular event that occurs chiefly in the elderly. It can be classified broadly into ischemic stroke (due to a clot in an artery supplying a part of the brain) and hemorrhagic stroke (where a brain vessel bursts and bleeds, denying the brain tissue its blood supply). The study reported focuses on ischemic stroke, which is roughly ten times more common than hemorrhagic stroke.

What was done

The Cardiovascular Health Study enrolled volunteers over 65 from four US communities in 1989-1990. People with a history of stroke, transient ischemic attacks (TIAs) or who had undergone a carotid endarterectomy (a 'clean-out' of the carotid artery) were excluded.

At baseline, all participants completed health questionnaires and had a full health exam, with emphasis on the cardiovascular aspects. Food-frequency questionnaires at baseline were analyzed to assess the participants' usual dietary intake; the questionnaires were aimed at determining the type of fish consumed - tuna fish, other broiled or baked fish, and fried fish or fish sandwiches (fish-burgers).

During the next 12 years participants were followed to determine if they had a stroke, and if so, what sort it was. Diagnoses were carefully confirmed with appropriate testing.

Statistical calculations were used to estimate the risk of participants having a stroke during follow-up until their death, or 2001, whichever came first. Then the risk was calculated for those that ate tuna or other fish on the one hand, and fried fish or fish burgers on the other.

What was found

There were over 4750 participants in the final analysis; their average age at baseline was 73 years. Overall, the average consumption of tuna/other fish and fried fish/fish burgers was 2.2 and 0.7 servings a week. In general, fried fish/fish burger consumption was more common in men, those of a nonwhite race and lower educational level, and who took in more calories and saturated fat.

During the follow-up period, 626 strokes were recorded - 529 ischemic strokes, 65 hemorrhagic strokes, and 32 which couldn't be classified due to inadequate information.

The risk of having a stroke were compared for various groups of participants, after adjusting for differences in age, gender, education diabetes, smoking, coronary heart disease, and aspirin use:

• There was a 27% lower risk of stroke in those eating tuna/other fish 1 to 4 times a week, compared with those eating it less than once a month.
• There was a 30% lower risk in those eating tuna/other fish 5 or more times a week.
• There was a 44% higher risk of stroke in those eating fried fish/fish burgers more than once a week, compared with those eating these foods less than once a month.

These different risk levels were for ischemic stroke. There was no increased or lessened risk for hemorrhagic stroke.

What these results mean

The findings confirm what one might expect, based on our knowledge of results from earlier studies. Eating tuna or other broiled or baked fish was linked to a lower chance of having a stroke. It seems highly likely that this effect is due to the presence of omega-3 fatty acids in these fish.

Some studies have suggested there is a higher risk of hemorrhagic stroke with high omega-3 fatty acid intake, because of its effect on platelet function. It's therefore important that this study showed no relationship between fish eating and any change in hemorrhagic stroke frequency. Of course it doesn't follow that high doses of omega-3 supplements are without risk.

Why were fried fish and fish burgers linked to an increased risk of stroke? This may be because of the preparation method used. Frying can greatly alter the nutrient composition of a fish meal, increasing the omega-6 and decreasing the omega-3 acids, and raising trans-fatty acid levels; such changes are known to increase cardiovascular risks.

The 'takeaway message' is therefore: tuna and other fish that are broiled or baked are good for you, while fried fish or fish sandwiches (fish burgers) are definitely bad for you.

Source

• Fish consumption and stroke risk in elderly individuals. The Cardiovascular Health Study. D. Mozaffarian, WT. Longstreth, RN. Lemaitre,  et al., Arch Intern Med, 2005, vol. 165, pp. 200--206

Related Links
Fish Appears to Reduce Stroke Risk in Women
Stroke Can be Prevented
A Study Shows That Some Fish is Better Than None

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