Cochrane review on omega-3 fats – Expert Reaction

A Cochrane review, examining results from 79 randomised trials, has found strong evidence that taking long-chain omega-3 supplements such as fish oil does not benefit heart health or reduce our risk of stroke or death from any cause.

The review found some evidence omega-3 from plant oils and nuts may slightly protect the heart and circulatory system.

The Science Media Centre gathered expert commentary on the review. Please feel free to use these comments in your reporting. The review and press material is available on Scimex.

Professor Barbara Meyer, School of Medicine, University of Wollongong, NSW, Australia, comments:
Professor Meyer recently authored a study on the effects of long-chain omega-3 on heart health, available here

“I do not think that this is a pivotal review because the meta-analysis included studies that had low dose of omega-3 that resulted in only a 1% increase in omega-3 levels in the blood and therefore had little or no chance of showing any beneficial effect of omega-3s. Given today’s health care of people with cardiovascular disease (e.g. medications for high cholesterol, high blood pressure etc), it is of no surprise that a 1% increase in omega-3 levels in the blood does not result in reduced death from cardiovascular disease.

“This Cochrane review concluded that ‘long-chain omega-3 fats do not have important positive or negative effects on mortality or CVD events’, but let’s put this conclusion into perspective.

“Historically the Greenland Eskimos [in two of the studies] had low rates of cardiovascular mortality (7% of all deaths) compared to Europe and the United States with much higher rates of CV mortality (45% of all deaths). The Greenland Eskimos and other countries consuming a lot of fish and seafood (e.g. Korea and Japan) had high levels of omega-3 long chain polyunsaturated fatty acids in their blood, compared to countries consuming Western diets.

“The later intervention studies involved increased cardiovascular care, conducted in people who were on cholesterol lowering medication.

“A great deal of the studies used in the meta-analysis had lower than expected cardiovascular event rates, which could be attributed to the increased CV care being provided. However, it could also be due to the low doses used in these studies. Furthermore, the dose of DHA may be important.

“Finally, not all of these studies measured the levels of long chain polyunsaturated fatty acids (n-3 LCPUFA) in the blood. Of these studies that did measure n-3 LCPUFA in the blood, their levels only increased by approximately 1% – which is not enough of a change to see a difference.”

No conflict of interest.

The UK Science Media Centre gathered the following comments, feel free to use them in your reporting:

Victoria Taylor, Senior Dietitian, British Heart Foundation, said:

“This is a systematic review of 79 randomised trials, which included over 100,000 people and included the most recent research in this area.  But, as the majority of these trials looked at supplements, it doesn’t give us reliable information about the effect of omega-3 from eating fish.  Advice on omega-3 supplements changed a few years ago so that increased levels and supplementation were no longer recommended for people who had had a heart attack.

“Supplements are no replacement for a healthy diet.  Guidance which was updated a few years ago is consistent with this study in that omega-3 supplements are not recommended to prevent heart and circulatory diseases.  However, this doesn’t mean that oily fish or vegetarian sources of omega-3 such as walnuts or rapeseed oil should be taken off the menu.

“Our message is clear – rather than taking supplements to reduce your risk of having another heart attack or stroke, you should focus on eating a healthy, balanced, Mediterranean style diet.  This includes white and oily fish along with plenty of fruit, vegetables and pulses, lentils, nuts, seeds, unsaturated oils and wholegrains.  We need to focus on our whole diet rather than the use of supplements of individual nutrients to ward off heart disease.”

No conflict of interest declared.

Prof Tim Chico, Professor of Cardiovascular Medicine and Honorary Consultant Cardiologist, University of Sheffield, said:

“Although diet plays an important role in preventing heart disease, this is complex and unlikely to relate much to any single element of the diet.  In addition, it is hard to know whether the effects of diet are due directly to what someone actually eats, or other influences such as income.

“Previous experience has shown that although some types of diet are linked to lower risk of heart disease, when we try to identify the beneficial element of the diet and give it as a supplement it generally has little or no benefit.  This was the case for vitamins; we know a diet rich in vitamins is associated with lower risk of heart disease, but studies giving people vitamin pills showed that these gave no benefit and indeed may have caused harm.

“This analysis of many studies shows clearly that omega-3 supplements do not reduce heart disease.  This is in keeping with medical practice; although there was a period where people who had suffered a heart attack were prescribed these on the NHS, this stopped some years ago.  Such supplements come with a significant cost, so my advice to anyone buying them in the hope that they reduce the risk of heart disease, I’d advise them to spend their money on vegetables instead.”

No conflict of interest.

Dr Ian Johnson, Nutrition researcher and Emeritus Fellow, Quadram Institute Bioscience, said:

“This is a high-quality, comprehensive analysis of all the best available evidence from human intervention studies on the supposedly protective effects of omega-3 fatty acids from fish and some plant foods against cardiovascular disease.  The results show little or no evidence for important beneficial effects.  Given the strong evidence from previous epidemiological studies this conclusion is somewhat surprising, but it needs to be taken seriously.  Either the protective effects of oily fish consumption that are observed in populations are due to mechanisms that cannot be reproduced by relatively short-term interventions with purified omega-3 supplements, or perhaps they are caused by other unidentified environmental factors somehow linked to oily fish consumption.”

Conflict of interest statement: Ian Johnson has previously held honorary academic appointments in the medical school at the University of East Anglia.

Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:

“The major limitation of this review of randomised controlled trials is that it has been unable to allow for the increased intakes of omega-3 fatty acids over the past 20 years.  This has occurred because the food industry recognised the need to redress the balance between omega-6 and omega-3 fatty acids in oils used in food processing as well as ceasing to partially hydrogenate vegetable oils such as rapeseed and soybean oil, which destroyed alpha linolenic acid.  The increased use of unhydrogenated canola (rapeseed oil) and soybean oil in processed food has resulted in higher intakes of linolenic acid than in the past.

“Most of the trials in this review were in patients with pre-existing cardiovascular disease (CVD), which is a further limitation when extrapolating to the prevention of heart attacks in the general population.  This is important as a substantial proportion of first heart attacks are fatal (it is worth noting that this proportion has fallen greatly in the past twenty years).  Previous observational cohort studies, not looked at here, suggest that omega-3 fatty acids may reduce the risk of sudden cardiac death.  This outcome can only be studied in a primary prevention trial (that is among individuals who have not had a cardiovascular event).

“The data from previous observational cohort studies, which this review doesn’t take into account, suggest a threshold intake where intakes below 1g/d of alpha linolenic acid are associated with increased risk of fatal heart disease.  It follows, therefore, that intakes above this level are unlikely to have any further benefit.  Because a small amount of an essential nutrient is needed, it does not follow that more is always better.

“Fish consumption is more consistently associated with reduced risk of cardiovascular disease, again in previous observational studies.  Oily fish (sardines, mackerel, salmon) are the main source of long-chain omega-3 fatty acids (eicosapentaenoic, docosapentaenoic and docosahexaenoic acid).  These fatty acids are made by marine algae and accumulate in the tissues of fish, particularly oil fish.  These long-chain omega-3 fatty acids have different physiological effects from alpha-linolenic acid (lowering blood pressure, decreasing serum triglycerides and reducing the inflammatory response) but these are only seen clearly at high intakes usually in excess of 3g/d, which is generally much higher than the amounts used in the trials (which is typically 1g/d).  The recent trials show no benefit of omega-3 supplements in patients with cardiovascular disease who are treated with drugs such as statins, aspirin, and blood pressure lowering medication.  However, there may still be a role for omega-3 fatty acids in patients with heart failure, which is still under investigation.  Current dietary guidelines to prevent cardiovascular disease encourage fish consumption (two portions of fish a week of which one should be oily) – that would supply in the range of 0.2-0.4 g/d long-chain omega-3 fatty acids – rather than taking supplements.  This study provides no evidence to suggest that this dietary advice should change.”

Conflict of interest statement: Scientific governor of British Nutrition Foundation, Honorary Director of Nutrition HEART UK.