Did you eat some fish today? In a case-control study (not good for proving causation but useful to demonstrate association & develop hypotheses), Swedish researchers compared 431 patients who had suffered a heart attack (myocardial infarction or MI) to 499 controls matched for body mass index, tobacco use, alcohol consumption, fruit & veggie consumption, and physical activity level.
Based upon self-report, fish consumption, ironically, was not associated with heart attack risk. However, plasma EPA+DHA (shorthand for specific omega 3 fatty acids in fish oil) did correlate inversely w/heart attack risk. In fact, plasma EPA+DHA was also associated with mercury (Hg) level, one of the heavy metal contaminants over which many are concerned, such that high Hg was associated w/lower heart attack risk, contrary to what was expected.
How do we interpret these results? Well, self-reported food consumption is fraught w/error while bio-markers, such as plasma EPA+DHA & erythrocyte Hg, more accurately reflect true consumption. The fact that plasma EPA+DHA correlates w/erythrocyte Hg makes sense, too, because the more fish you eat, the more Hg you ingest. But the good news is that the beneficial effect of fish consumption appears to override any negative effects from Hg.
One last bit of detail: cod & perch are considered lean fish while fatty fish (loaded w/EPA+DHA) are herring, lavaret & salmon. So again, just as with stroke risk, choose your fish carefully.