(02-06-06) Conflicting Evidence for Fish Oil's Effects on the Heart

By Kate Johnson

BOSTON (EGMN) -New studies showing conflicting results on the effect of fish oil on the heart add weight to the notion that perhaps age and health status influence whether omega-3 fatty acids prevent or promote cardiac arrhythmias, according to Dr. Anthony Aizer.
His study, which he presented at the annual meeting of the Heart Rhythm Society, linked increased fish consumption with a higher risk of developing atrial fibrillation (AF) in healthy male physicians (aged 40-84 years) who were enroled in the previously reported Physicians' Health Study (N. Engl. J. Med. 1989; 321:129-35).
The findings corroborate recently published results from the Danish Diet, Cancer, and Health study (Am. J. Clin. Nutr. 2005;81:50-4), but contrast with data from the Cardiovascular Health study (Circulation 2004;110:368-73), said Dr. Aizer, who is an electrophysiologist at New York University Medical Center. He said one explanation for the conflicting findings could be that fish oil may have profibrillatory effects in healthy individuals but antifibrillatory effects in subjects with preexisting cardiovascular disease (CVD).
"One hypothesis is that omega-3 fatty acids have some effects on the autonomic nervous system-in particular, by increasing parasympathetic tone. In generally healthy individuals without CVD, it is sometimes thought that an increase in autonomic tone may play a role in the development of AF," he said in an interview. "In contrast, in older, less healthy individuals it's possible that other effects are more significant. In certain individuals, the effect of omega-3 fatty acids that enhance cardiac tissue refractoriness may have a more significant impact, thereby preventing AF."
Dr. Aizer's analysis of the Physicians' Health Study included 17,679 men who had completed a fish consumption questionnaire in 1983 and of whom 7% reported AF 15 years later. He found that men who reported eating five or more fish meals per week had a 55% higher rate of AF, compared with men who ate fish only once a month.
But two other smaller studies that were presented as posters at the meeting reported the cardiac benefits of omega-3 fatty acids.
A prospective study of six patients with paroxysmal AF showed that an infusion of 100 mL of omega-3 fatty acids resulted in an increase in atrial refractoriness, a reduction in AF inducibility, and a prolongation of fibrillatory cycle length, reported Dr. Hercules E. Mavrakis, from Heraklion University Hospital in Heraklion, Crete, Greece.
And another study of 26 patients with inducible ventricular tachycardia (VT) at 3 or more months postmyocardial infarction showed strong benefits of oral omega-3 fatty acid capsules (180 mg eicosapentaenoic acid and 120 mg docosahexaenoic acid) daily, compared with placebo, over a 40-day treatment period, reported Dr. Glenn D. Young from the Cardiovascular Research Centre in Adelaide, Australia.
At the end of the study, VT was no longer inducible in 5 of the 12 treated patients, and 5 of the remaining 7 patients required more aggressive stimulation to induce arrhythmia. By contrast, in the 14 control patients, VT was no longer inducible in only 1 patient, and 3 of the remaining 13 patients required more aggressive inducement.
"Statistically, it was a very significant result. There seems to be a direct effect of fish oil in preventing ventricular tachycardia," said Dr. Young in an interview. "People can achieve that level with only [two or three] oily fish meals a week."
Dr. Aizer said his finding of a higher incidence of AF with increased fish consumption cannot be interpreted as causal. "This wasn't a randomised, controlled trial [of fish consumption]-there could always be an association of fish with some other factor that's causing the atrial fibrillation."
And he noted that another important association within this same cohort was a reduced rate of sudden cardiac death in those who ate more fish.
"The message of this study is not to stop eating fish," he stressed. "It adds more information to the picture suggesting that perhaps population groups play a significant role in what's going on, and that needs to be considered in terms of further analysis."

Source: 2006 Elsevier Global Medical News.

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