Fatty fish - are they really good for your heart?

An American study concludes that you can avoid atrial fibrillation by eating fatty fish. Danish researchers have come to the exact opposite conclusion.

In the summer of 2004, researchers at Harvard University in Boston published an article stating that if you eat lots of fatty fish, your risk of atrial fibrillation will be reduced.

Atrial fibrillation is the most common cause of irregular heartbeat that will occur in a large number of elderly people and will require lifelong anticoagulant treatment.

To be precise, the American researchers had found that the risk of atrial fibrillation was reduced by 35% in people who consumed fatty fish at least five times a week and by 24% in the ones who consumed fatty fish 1 - 3 times a month - compared to people who only ate fish very rarely.

However, the matter is more complex than that: In January 2005, two Danish doctors published an article stating that a large consumption of fatty fish increases the risk of atrial fibrillation. They found the risk to be increased by 44% in the 20% who consumed the most fatty fish compared to the 20% who consumed the least.

Were do we go from that, then? If we take a rough average of the two studies, it will indicate that it does not matter whether or not you eat fatty fish.
But why are the results so conflicting? If we move below the surface, it turns out that the two studies are very different.

The American study included 5000 trial subjects with an average age of 73 years.
The Danish study included 50,000 trial subjects with an average age that was 18 years less than the American study, i.e. 55 years.

Almost every fifth of the old Americans suffered atrial fibrillation during the 12-year duration of the American study. In comparison, the same was true for less than 2% of the Danes during the 6-year duration of the Danish study.
In total, the Americans observed more than twice as many trial subjects with atrial fibrillation compared to the Danes (980 and 456, respectively).

Frying distorts the fish oil
How were the person with atrial fibrillation indentified? The American trial subjects went through a cardiac examination every year in which all cases of atrial fibrillation were discovered. Contrary to this, information about the health state of the Danes was solely obtained from the hospitals - it was obviously assumed that all the trial subjects who had suffered atrial fibrillation had been hospitalized.

This discrepancy involves some uncertainty: Even though most trial subjects with atrial fibrillation were hospitalized, it is far from certain that this applied to everyone. Atrial fibrillation can very well be treated by a GP.

It was particularly important that the Americans observed that there is difference in the effect of fried fish and other kinds of fish. Fried fatty fish slightly increased the risk while non-fried (fatty) fish decreased the risk. Fried fatty fish neither increased the blood levels of fish oil (N-3 fatty acids) as non-fried fish does.
The Danish study does not explain how the fish were prepared. There was no distinguishing.

None of the studies explained how many and who took fish oil supplements: In the Danish study the trial subjects had not been given this question and in the American study, the information apparently did not alter the overall result.

To conclude: A detailed and precise American study has established that fatty fish reduces the risk of atrial fibrillation by approximately 33%.
A less detailed Danish study has established the opposite.

However, the size of the study is hardly decisive. It is probably more essential to pay some attention to how the fish was prepared. Fish oil is destroyed by high temperatures which, more or less, transforms the N-3 fatty acids into N-6 fatty acids, transfatty acids, and harmful oxidation products.

The American group has recently published some more news: Fatty fish - that has not been fried - reduces the risk of cerebral haemorrhage to approximately the same degree as is the case with atrial fibrillation. Fatty fish, on the other hand, increases this risk!


References:
Dariush Mozaffarian et al. Fish intake and risk of incident atrial fibrillation. Circulation 2004;116:368-73.
n-3 Fatty acids consumed from fish and risk of atrial fibrillation or flutter: The Danish Diet, Cancer and Health Study. Am J Clin Nutr 2005;81:50-4.
Mozaffarian D et al. Fish consumption and stroke risk in elderly individuals: the cardiovascular health study. Arch Intern Med. 2005 Jan 24;165(2):200-6.