Omega-6 Fatty Acids and Our Health

Heart Healthy, or Dangerous?

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Dietary guidelines from both the US government the American Heart Association (AHA) urge us to severely restrict “bad fats” (saturated fats and trans fats) from our diets, and replace them with “good fats.” The guidelines specifically define good fats as the unsaturated fatty acids, both polyunsaturated and monounsaturated fatty acids (PUFA and MUFA). Furthermore, the experts who wrote the guidelines make no differentiation between omega–3 PUFA and omega–6 PUFA.

Both of these types of PUFA are strongly recommended, without any qualifications or limitations. And no hint of a controversy is alluded to.

But in fact there is a controversy. Many nutrition experts are alarmed by the recommendation to consume omega–6 PUFA in virtually unlimited quantities. They tell us that omega–6 PUFA, while essential to our diets in small quantities, can lead to heart disease, cancer, arthritis, and other medical conditions, when they are consumed in large amounts. And indeed, in many countries, notably in the UK and Europe, recommendations regarding omega–6 PUFA are much more circumspect than they are in the US.

So - are omega–6 PUFA really healthy for us, or not?

This question is particularly important, not only because of current dietary guidelines, but also because omega–6 PUFA are nearly ubiquitous today in the Western diet.

They are plentiful not only in their natural sources (nuts and grains) but also in virtually any of the processed foods we eat, in vegetable oils, and even in poultry and beef (which today are usually grain-fed, and thus loaded with omega–6). Our intake of omega–6 PUFA is many times higher today than it has ever been in human history - much to the delight of the guideline writers, we must suppose.

Why Large Amounts of Omega–6 Might Be Bad

Omega–6 PUFA are an important component of cell membranes, and are required for the normal functioning of many of the body’s systems. Linoleic acid is the omega–6 we most commonly consume in our food, and it is considered an essential fatty acid because we cannot synthesize it ourselves. The fact that first made omega–6 PUFA attractive to nutritional experts, however, is that it decreases cholesterol levels, and the level of LDL cholesterol in particular.

But many scientific studies have suggested that too much omega–6 PUFA might also increase the risk of some serious problems. There is evidence that large amounts of omega–6 PUFA can increase the risk of cancer (especially prostate and breast cancers), reduce the efficiency of the immune system, increase inflammation, and increase the risk of heart disease. Some of these effects, apparently, are related to the fact that omega–6 PUFA are highly subject to oxidation, which can lead to an excess of free radicals, and which can make LDL cholesterol particles more likely to cause atherosclerosis.

There is also evidence that omega–6 PUFA might be especially damaging when combined with habits that lead to high oxidative stress - especially smoking.

Experts writing on behalf of the AHA and in defense of current dietary recommendations have acknowledged these theoretical concerns. But in response, they point to clinical studies showing that when saturated fats are replaced in the diet with PUFA, cardiac outcomes are significantly improved. Notably, however, in most of the studies they reference, the increased intake of omega–6 PUFA was supplemented by added omega–3 PUFA. And in a meta-analysis of studies using only omega–6 PUFA (without also adding omega–3), this beneficial result was not seen, and instead the risk of death was increased.

In addition, a new analysis of the Sydney Diet Heart Study raises particular alarms about omega–6 PUFA. This rigorous study randomized men who had survived heart attacks to receive either a diet in which all fats were replaced by omega–6 PUFA (which can be accomplished by using safflower oil), or a more typical “heart-healthy” diet. The Sydney study originally was reported as favoring the omega–6 patients, since cholesterol levels became significantly lower in the treatment group. But in 2013, data from the Sydney study was rediscovered, and it was learned that the actual cardiovascular outcomes of the enrolled subjects had been recorded (though never reported). A re-analysis of this new data revealed a significantly increased mortality rate among the men eating the omega–6 PUFA diet - despite the improvement in their cholesterol levels.

So What Does The Evidence Say About Omega–6?

The available evidence, therefore, is consistent with at least two things. First, replacing saturated fats with a diet balanced in omega–6 and omega–3 PUFA appears to improve cardiovascular outcomes. But second, adding lots of omega–6 PUFA to the diet without “balancing” it by adding omega–3 may possibly lead to worse outcomes. Virtually all experts accept the first point. And many experts (though not the ones who write the US guidelines) also find strong evidence that the second point is valid.

In late February, 2015, the Final Scientific Report of the Dietary Guidelines Advisory Committee (DGAC) was publicly released. This report will serve as the basis for the next 5-year revision of the US Dietary Guidelines, due in late 2015. Not surprisingly, this report continues to emphasize the importance of eating plenty of PUFA, without specifying which kind. In studying this report, I can find no discussion of the evidence that substantially increasing omega–6 PUFA, without also adding omega–3 PUFA, might be detrimental to cardiac health. Further, the re-analyzed Sydney study is not included in the references of the DGAC document, and apparently was not even considered by the committee.

The Bottom Line

The US government and the AHA encourage everyone to eat lots of PUFA in place of saturated fats, and if you do that, they assert, it does not matter whether you are eating omega–3 or omega–6 PUFA. However, there is sufficient evidence to question this advice. Experts in other countries have indeed questioned it, and have even set upper limits on recommended dietary omega–6 PUFA.

Whether or not you choose to give credence to the evidence that high dietary omega–6 can be detrimental to your health, it makes good sense to go out of your way to add omega–3 PUFA to your diet. (And you do indeed need to go out of your way, because omega–3 doesn’t just show up in your diet like omega–6 does.)

Adding omega–3 PUFA is a good idea because: a) you are almost certainly eating a lot of omega–6 PUFA, whether you’re trying to or not; b) if unbalanced omega–6 indeed turns out to be bad for you, it appears you can counteract some of the negative effects by adding omega–3 to your diet; and c) eating plenty of omega–3 PUFA keeps you compliant with current dietary guidelines, whatever the truth turns out to be regarding omega–6. So you can hedge your bet without alarming the diet police.

Oh, and be careful with that vegetable oil.

Sources:

U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans 2010. 7th Edition, Washington, DC: U.S. Government Printing Office, December 2010. http://www.dietaryguidelines.gov.

Ramsden CE, Hibbeln JR, Majchrzak-Hong SF. All PUFAs are not created equal: absence of CHD benefit specific to linoleum acid in randomized controlled trials and prospective obsrvational cohorts. World Rev Nutr Diet 2011;102:30–43.

Ramsden CE, Zamora D, Leelarthaepin B, et al. 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. BMJ 2013; 346:e8707.

Farvid MS, Ding M, Pan A, et al. Dietary linoleic acid and risk of coronary heart disease: a systematic review and meta-analysis of prospective cohort studies. Circulation 2014; 130:1568.

Chowdhury R, Warnakula S, Kunutsor S, et al. Association of dietary, circulating, and supplement fatty acids with coronary risk: a systematic review and meta-analysis. Ann Intern Med 2014; 160:398.

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