Scientific expert reaction to Cochrane Review on omega-3 fatty acids

Scientific expert reaction to  Cochrane Review on omega-3 fatty acids

New evidence published today in the Cochrane Library shows that there is little or no effect of omega 3 supplements on our risk of experiencing heart disease, stroke or death. Below is a Science Media Centre roundup of third-party expert reaction to this review.

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.”

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.”

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.”

Declared interests

Prof Tim Chico: “No conflicts.”

Dr Ian Johnson: “Ian Johnson has previously held honorary academic appointments in the medical school at the University of East Anglia.”

Prof Tom Sanders: “Scientific governor of British Nutrition Foundation, Honorary Director of Nutrition HEART UK.”

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Wednesday, July 18, 2018
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