Wednesday August 5 2009
Oily fish is known to help people with existing heart problems
“Eating oily fish can help ensure a long life as it slashes the risk of heart failure by a third,” reported the Daily Express. It said that scientists claim that omega-3 should now be taken daily by everyone in Britain. The newspaper said that oily fish is known to help people with existing heart problems, but “compelling new evidence” suggests it could also cut the risk in healthy people, potentially reducing heart attack deaths in the UK by up to 30%.
The news story is based on a review of the evidence for the ability of omega-3 fatty acids to prevent and treat heart conditions. The reduction of deaths by a third as referred to in the newspaper only happened in people with known heart disease. There it still little evidence to promote the idea that everyone should take fish oils to prevent heart disease and for a longer life.
The review contributes to the discussion about the role of omega-3, but it is not a systematic review and on its own is unlikely to change the current advice that moderate amounts of fish can be part of a healthy diet, and that supplements are currently only recommended for people with heart disease.
Where did the story come from?
This research was carried out by Dr Carl J Lavie from the Ochsner Medical Center, New Orleans, and colleagues. Sources of funding for the study are not reported, competing interests are declared. The study was published in the peer-reviewed medical journal The Journal of the American College of Cardiology.
What kind of scientific study was this?
This review looked at the evidence for the prevention and treatment of several heart conditions with omega-3 polyunsaturated fatty acids (PUFAs). The reviewers looked at observational and randomised trials of fish oil supplements and oily fish consumption for coronary heart disease (CHD), irregular heart rhythms (arrhythmia), congestive heart failure (enlarged hearts) and in people with high cholesterol or other fats in the blood. They also discuss some of the safety concerns about the mercury content of oily fish.
The reviewers explain that omega-3 fatty acids are found in oily fish, such as herring, mackerel, salmon, albacore tuna and sardines. A high intake can also be obtained from fish oil supplements or cod liver oil. The oils come from the marine micro-organisms that are eaten by fish and not made by fish themselves.
The evidence relates to two main types of omega-3 fatty acids, both of which are polyunsaturated oils: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These are the long-chain fatty acids in the family of omega-3 fatty acids.
The authors say that “numerous” trials have shown that moderate fish oil consumption decreases the risk of various heart diseases in people who already have heart disease or heart failure, such as heart attack, sudden cardiac death, angina, atrial fibrillation (an irregular heart beat).
They report that the American Heart Association recommends that patients with documented CHD consume about a gram a day of combined DHA and EPA, either in the form of oily fish or fish oil supplements. However, research is ongoing as to the role of these supplements in primary prevention (where people do not have a history of heart problems).
What were the results of the study?
The researchers refer to the results of both observational studies and randomised trials of omega-3 fatty acids in people with heart disease (treatment) and without known heart disease (prevention). They also discuss trials investigating intermediate outcomes, these outcomes are less important to patients as they do not involve illness or death. For example, there are several trials that look at the effect of fish oils on blood tests or lipid levels.
The researchers discuss in detail the three randomised trials on major outcomes in prevention and treatment. These trials include the DART (Diet and Reinfarction Trial), the GISSI Prevenzione study and the JELIS (Japan EPA Lipid Intervention Study).
- The DART study was published in 1989 and was conducted in 2,033 men who had had a recent heart attack. It found that two years after the attack there was a 29% reduced number of deaths from any cause in men who ate oily fish or took omega-3 supplements compared to men who did not. The researchers say this was largely due to reductions in death from coronary heart disease.
- In the GISSI prevenzione study, published in 2001, 5,666 patients who had survived a recent heart attack were randomised to either 850mg of EPA/DHA alone or in combination with vitamin E. These people had an overall 21% reduction in deaths and a 30% reduction in deaths from heart disease compared to those who did not take the omega-3 supplements. Later analyses showed that this reduction was driven by a highly significant 45% reduction in sudden cardiac death, which was evident after only four months.
- In the JELIS trial published in 2007, 18,645 patients with high cholesterol were randomised to a cholesterol-lowering drug (statin) or a statin combined with 1,800mg of EPA a day. Most of the patients were women and about 15,000 had not had previous heart disease. After five years, those randomised to EPA and statin had a 19% reduction in all the major cardiovascular events recorded compared to the statin only group. In the EPA and statin group, 2.8% had a major event within five years compared to 3.5% in the statin only group, an absolute difference of 0.7%. This means that 1,000 people would need to take EPA and statins for five years for seven of them to benefit from the additional fish oil.
Common side effects from fish oils are nausea, gastrointestinal upset and a fishy burp. The researchers also comment on mercury content, saying that salmon, sardines, trout, oysters and herring are quite low in mercury, and claim that, as mercury is water-soluble and bound to protein, there is less present in the oil than in the muscle of fish. They maintain that this supports the theory that fish oil supplements should contain negligible amounts of mercury.
What interpretations did the researchers draw from these results?
The researchers say that “convincing evidence from extensive research over the past three decades points out the potential beneficial effects of omega-3 PUFA”. They agree with comments made over 20 years ago that "fish oil is a whale of a story, that not surprisingly gets bigger with every telling".
What does the NHS Knowledge Service make of this study?
The findings of this non-systematic review correspond with the findings of previous studies that the evidence indicates that fish oils benefit people with heart disease. However, despite the newspaper headlines, there is little evidence to promote the idea that everyone should take fish oils to prevent heart disease and prolong life.
The researchers found only a few randomised trials investigating whether omega-3 can prevent death and coronary disease in people without known heart disease. The most recent study was the JELIS study in Japan, which was mostly in women who were already at higher risk of heart disease because of their high cholesterol. As such, the results may not apply to populations that do not follow a Japanese diet, have normal cholesterol levels or do not take statins.
As the researchers admit, there are still several unanswered questions about fish oils, such as ideal doses, the ideal ratio of DHA to EHA and whether oily fish is as good as supplements. There is also an issue with the mercury content of oily fish. Pregnant women in particular are cautioned against high intake of oily fish due to the mercury content.
Overall, this review has collected data that contribute to the discussion about the benefits of oily fish. However, nothing new or convincing on the effects of omega-3 fatty acids in prevention of heart disease can be concluded, because of the small number of relevant randomised trials. In addition, the review does not describe how the published literature was searched or how the researchers selected the studies to report. As such, it is unlikely to change current recommendations on its own.