Wednesday, December 8th, 2010
Meta-analysis Reveals “Heart Healthy Omega-6 Fat” Increases Risk of Heart Disease
Bottom Line: The research upon which the American Heart Association based their “eat-your-omega-6-fat” advisory, is fatally flawed, according to the results of a meta-analysis study, which showed that a steady diet of omega-6 polyunsaturated fatty acids increases the risk of heart disease and death, especially for women [1]. British J Nutr. Dec 2010.
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Background: Omega-6 fats are the most commonly eaten polyunsaturated fatty acids (PUFA) in westernized countries. The top three sources are corn oil, soybean oil and cottonseed oil, which are the main ingredients in margarines, salad dressings, and mayonnaise. Many studies have shown that PUFA lower cholesterol.
Prior to industrialization, no population has been exposed to the current high levels of omega-6 polyunsaturated fats. We evolved on a diet with a balanced proportion of omega-6 to omega-3 fats of about 1:1. Today, that ratio in westernized countries is out of whack, near 20:1 of omega-6 to omega-3 fats. Different farming practices, new food processing and the urging by health authorities to use vegetable oils in place of animal fats for heart health, triggered an onslaught of omega-6 fats into the food we eat.
While it’s true that PUFA lowers cholesterol, cholesterol is not they key culprit in heart disease. Inflammation is the “new cholesterol” in matters of eating to protect the heart (and other chronic diseases for that matter). Notably many inflammation medications work by blocking the effects of excess omega-6 fat, including statins, aspirin, and asthma inhalers.
In 1999, there was enough scientific evidence to prompt scientists to recommend an upper limit for omega-6 fats, to no more than 7 grams per day. This ceiling is based on eating a maximum of 3% fat calories from omega-6 fat on a 2000 calorie diet.
Yet, a decade later, the 2009 the American Heart Association (AHA) published a health advisory touting the benefits of eating a high omega-6 polyunsaturated fatty acids (PUFA), and warned that eating less than 5% of calories from omega-6 PUFA would be hazardous to heart health. [2-3].
The conclusion of AHA’s advisory were seriously questioned in part, because of their failure to:
-Distinguish the types of PUFA used in studies, which also substantially increased omega-3 fats, which are also PUFAs.
-Include relevant trials with unfavorable results and excluded poorly designed studies.
Notably, these studies did not provide the specific fat content of the experimental diets used to lower cholesterol. (Yet the AHA recommended to specifically increase omega-6 PUFA). These flaws prompted a team of National Institutes of Health (NIH) scientists to re-evaluate the data, using techniques resembling a scholarly forensic investigation.
Design: Nine randomized controlled trials (the gold standard of study design), were identified, which met the criteria of having an intervention diet high in PUFA and reported clinical outcomes.
Next came the detective work, worthy of an Agatha Cristy who-done-it. Most of the studies did not provide omega-3 fat data–it was just lumped together in the PUFA category.
In order to track down the missing fatty acid data, (many of these studies were conducted over 40 years ago), they had to dig through newspaper archives, public records, scientific proceedings from national conferences and correspond with the study investigators (or their colleagues, if deceased).
Once the fatty acid data was collected, they were segregated into two categories:
-Mixed Diet (containing both omega-6 and omega-3 polyunsaturated fatty acids.)
-Omega-6 Specific Diet
Then the data was pooled to evaluate the effect of the Omega-6 Specific Diet, compared to the effect of the Mixed Diet, on clinical outcomes. They also evaluated the potential confounding role of trans fatty acids.
Results: Here’s what they found when evaluating the composition of the intervention diets.
Diet Composition:
-Only three of the nine so-called PUFA studies were “pure” omega-6 intervention trials, which upped omega-6, without a concurrent rise in dietary omega-3. Combined, these three studies had 9,500 participants.
-Four of the studies increased both the omega-3 PUFA (EPA and DHA) and omega-6 PUFA, which totaled over 1,700 participants. Notably, the researchers discovered that the Oslo Diet-Heart Study provided about 5 grams of EPA and DHA per day to the intervention group. (That’s equivalent to about 16 fish oil capsules).
-The control diets had a mean estimated trans fatty acid content of 3% (a significant confounding factor, which unquestionably increases risk of heart disease).
Heart Disease and Death Outcomes
When the effects of the Omega-6 Specific Diet were compared to the Mixed Omega 6/omega-3 PUFA; the following health outcomes were discovered:
Omega-6 Specific Diet:
-Increased risk of heart disease and death, compared to the Mixed Diet intervention studies.
-The relative risk of cardiac death increased by 28%.
-Increased the risks of all relevant cardiovascular outcomes.
-There was only one study with women, which showed significant harm.
The Mixed Omega-6/Omega-3 Diet:
-There was 8% risk reduction of death from all causes.
-There was 22% risk reduction from heart disease death.
Conclusion: The scientists concluded, that not only is there no indication of health benefit, from increasing dietary omega-6 PUFA, but a possibility of harm. Therefore, they recommended that the public health advice to maintain or increase dietary omega-6 PUFA, should be reconsidered.
Study Quote: “The increased cardiovascular heart disease risks from omega-6 specific PUFA diets in our meta-analysis may be underestimated as omega-6 PUFA also replaced substantial quantities of trans fatty acids.”
An accompanying editorial applauded the “extensive detective work” by the NIH research team, led by Christopher Ramsden [4]
Commentary : There is more to this story. In my next post, I’ll describe what went on behind the scenes, as three NIH scientists from this study, tried to get letters to the editor published in the AHA’s scientific journal, Circulation, in response to their omega-6 advisory. (I was actually a participant and witness).
[1] Christopher E. Ramsden,Joseph R. Hibbeln,Sharon F. Majchrzak and John M. Davis (2010).Omega-6 Fatty acid-specific and mixed polyunsaturate dietary interventions have different effects on CHD risk: a meta-analysis of randomised controlled trials. British Journal Nutrition. Dec 2010; 104(11): pp 1586-1600. [2] Harris WS et al. Omega-6 Fatty Acids and Risk for Cardiovascular Disease. A Science Advisory From the American Heart Association Nutrition Subcommittee of the Council on Nutrition, Physical Activity, and Metabolism; Council on Cardiovascular Nursing; and Council on Epidemiology and Prevention. Circulation published January 26, 2009. [Free Full Text.] [3] American Heart Asssociation News Release. Omega-6 fatty acids: Make them a part of heart-healthy eating DALLAS, Jan. 27, 2009. [Free full Text] [4] Philip C. Calder (2010).The American Heart Association advisory on n-6 fatty acids: evidence based or biased evidence? British Journal Nutrition. Dec 2010; 104(11): pp 1575-1576.[Abstact]
Copyright © 2010 by Evelyn Tribole, MS, RD Published at www.EvelynTribole.com
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DISCLAIMER: The information is intended to inform readers and is not intended to replace specific advice from a health care professional. Copyright 2010 Evelyn Tribole, MS, RD
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