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expert reaction to research into dietary fats and heart disease

A clinical trial in the BMJ suggested that replacing saturated animal fats with omega-6 polyunsaturated vegetable fats is linked to an increased risk of death among patients with heart disease. A before the headlines analysis accompanied this roundup.

 

Prof Iain Broom, Research Professor, Centre for Obesity Research and Epidemiology (CORE), Robert Gordon University, said:

“My comments are wider than the current paper’s focus, which is solely in n-6 PUFA intake, but I would say that the outcomes are not in the least surprising and the paper does carry out an exhaustive analyses of the data now available.  In addition the authors do come up with plausible explanations as to the reason for the increased CVD mortality in the shape of more easily oxidisable low density lipoprotein particles (LDL).  I find it incredible that the CVD data were not published in the original paper.  This paper could have repercussions amongst both Governments and the food industry where recommendations may not have been appropriate, and may also have implications for the American Heart Association’s recent suggestion of increasing n-6 PUFA dietary content.”

 

Prof David Spiegelhalter, Winton Professor of The Public Understanding of Risk, University of Cambridge, said:

“These are typically 50-year-old men, mainly smokers, who have had a heart attack already, and there were only 63 deaths altogether (35 in the control vs 28 in the intervention group) with barely statistically significant results.  So I would not want to get very excited about this study on its own, but it does add some evidence to a complex issue.”

 

Catherine Collins, Principal Dietitian, St George’s Hospital London, said:

“This article re-examines dietary information given 40years ago to young men who had had a major heart event at a young age (30-59y) despite being normal body weight. This re-analysis shows that a high intake of omega-6 polyunsaturated fat (derived from safflower oil – an oil rarely used in the UK but available in healthfood shops) – actually increased the risk of death from a second cardiac event, even though changing dietary fat choice reduced saturated fat intake and blood cholesterol levels.

“Our knowledge of diet and heart disease has become much more sophisticated over the intervening 40 years. We now know that reducing artery inflammation – by boosting monounsaturated fat intake (from rapeseed oil and olive oils) helps stabilise artery walls and make them more resistant to damage. More mono-unsaturates, with a modest amount of polyunsaturated fats, also helps to lower blood cholesterol levels. This together with other aspects of the Mediterranean diet – more dietary fibres, less salt, and more fruit and vegetable intake along with modest alcohol, are all part of the robust heart-healthy ‘portfolio’ of foods relatively unknown 40 years ago.

“Should we be concerned about our current intake of omega-6 polyunsaturates- linoleic acid in particular? As a Dietitian I think not. We already have a healthier diet than 40 years ago, and for those with resistant cholesterol levels medication can help regulate.

“Our diet is now naturally higher in mono-unsaturates which is protective against omega-6 fats, but for the older generation who still choose polyunsaturated margarines, and fry foods regularly in corn or sunflower oils, a change to ‘vegetable oil’ (rapeseed oil ) is all that is necessary to limit risk from linoleic acid. After all, it’s all about proportion of each fat in the diet, not absolute amounts.

“Almost 70% of subjects in both groups were smokers, which was common at the time but less so now with only 20% of the population smoking (see here). Why is this important? Smoking damages your arteries, and so in these subjects having a high polyunsaturated fat intake – naturally less stable than other fats – the risk of damage and health effects from the combination of smoking and linoleic acid intake would have been enhanced.”

 

Prof Tom Sanders, Head of Diabetes & Nutritional Sciences Division, King’s College London, said:

 “This is data dredging of a study conducted many years ago in Australia. It has little relevance to diets today. It reports the results of a study conducted in Sydney where subjects were asked to consume large amounts of safflower oil. The study was stopped because of adverse effects – I heard about this from a colleague whose brother was involved in the trial more than 30 years ago! Hopefully science has moved on. Firstly, the study was enormously underpowered to detect any meaningful outcome with only 458 men. Assuming a high mortality of 12% per year, the chances of detecting meaningful differences in mortality are small, particularly when follow-up is short.

“A meta-analysis of other randomised controlled trials including n–6 PUFA which has been published in the last few years refutes this (including a Cochrane review by Lee Hooper). It shows that dietary advice to replace saturated fatty acids with polyunsaturated fatty acids reduces incident coronary heart disease (CHD; Mozaffarian et al. Plos Med ). The prospective cohort studies also show benefit of replacing saturated fatty acids with polyunsaturated fatty acids (mainly linoleic acid).

“Safflower oil is rather unusual in that it contains mainly linoleic acid whereas commonly used vegetable oils such as soybean oil and rapeseed oil contain both linoleic and linolenic acid. The evidence for the benefits of long-chain n-3 PUFA have also received a drubbing by the systematic reviewers in JAMA in September. Furthermore, in support of the benefits of n-6 PUFA is a current report from the Oxford EPIC group which shows a 32% reduction in incident CHD in vegetarians (published in the current AJCN). Vegetarians have high intakes of n-6 PUFA, much lower blood cholesterol concentrations and slightly lower blood pressure.

“You need to look carefully at the paper because the dietary intervention was as a supplement of margarine and oil rather than the type of dietary modification currently advocated for cardiovascular disease (CVD) prevention such as the DASH diet or the NICE guidelines. We do not know the level of trans fatty acids in the margarine, which are now known to increase risk of heart disease. Current advice should be based on present knowledge and not veered off course by this new study. Taking extra fat on board in patients with CHD is not a good idea. Current dietary guidelines focus on decreasing sources of saturated fat and partially replacing saturated with unsaturated fatty acids i.e. vegetable oils rather than animal fats.”

 

Prof Brian Ratcliffe, Professor of Nutrition, Robert Gordon University, Aberdeen, said:

“This paper does not provide evidence for changes to the current recommendations for a healthy diet.  It provides interesting additional information regarding the role of dietary fats in the development of cardiovascular diseases.  The study was specifically a secondary prevention and it may have limited relevance to primary prevention. 

“Nevertheless, it is worth pointing out that current advice in the UK (dating back to the COMA report of 1994) does not recommend replacing saturated fatty acids with n-6 PUFA.  It was recommended that there should be no increase in the average intake of n-6 PUFA and that the proportion of the population consuming this type of fat in excess of 10% of energy intake should not increase, and the target is about 6% of energy intake.  Subjects in this study achieved PUFA levels of 15% of food energy.  Also, it is recommended not to exceed a PUFA:SFA ratio of 1 whereas in this study, in the intervention group at follow up, the ratio was 1.7. 

“It has been known for some time that there is potentially a problem with high intakes of linoleic acid leading to oxidised linoleic acid metabolites that would be potentially atherogenic.  The authors of this latest paper suggest that this may indeed be the mechanism that has contributed to the increased mortality from cardiovascular diseases seen in the study. 

“It is generally assumed that eating a healthy diet involves achieving a balance of n-6 and n-3 PUFA and many authorities have suggested that diets in developed countries are too imbalanced in favour of n-6 PUFA.  It is also worth pointing out that this study used a safflower oil with a particularly high content of linoleic acid (n-6 PUFA) and the level of this fatty acid varies considerably in commercially available safflower oils.”

 

‘Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis’ by Ramsden et al. published in the BMJ on Tuesday 5 February.  

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