Polyunsaturated Fatty Acids and the Heart

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Polyunsaturated fatty acids (PUFA), are a family of fatty acids in which two or more double bonds join adjacent carbon atoms within the fatty acid carbon chain. PUFA are widely regarded as being good for cardiovascular health, and in fact the American Heart Association (AHA) unequivocally labels PUFA (along with monounsaturated fatty acids - MUFA) as being “good fats.” We are all urged to consume plenty of PUFA in our heart-healthy diets, in particular by consuming vegetable oil.

However, underlying these public assurances about PUFA is a lot of controversy among respected nutrition scientists. In this article I will review PUFA - the good they may do, and their potential downside.

What’s Good About PUFA?

What makes PUFA so attractive to public health experts is that they lower LDL cholesterol levels. Further, clinical studies have shown that when PUFA are substituted in the diet for saturated fats (that is, fatty acids that have only single bonds in their carbon chain), the risk of developing heart disease is reduced.

For this reason, the US government and the AHA both recommend that saturated fats be strictly limited in our diets (to 5 - 6% of total calories), and that we eat most of our fats instead in the form of PUFA (and MUFA).

  • Read why saturated fats may not be so bad after all.

Furthermore, unnoticed by many Americans, a few years ago the AHA quietly dropped its longstanding recommendation that we strictly limit the total amount of fats in our diets. This means that anyone following current dietary guidelines will be getting plenty of PUFA.

Are All PUFA the Same?

There are actually many different kinds of PUFA. The major dietary PUFA are the omega–3 and the omega–6 PUFA. (The term “omega” refers to the last carbon atom in the fatty acid’s carbon chain. In omega–3 PUFA there is a double bond three carbons away from the omega carbon, while in omega–6 PUFA one of the double bonds is six carbons from the omega position.) Both omega–3 and omega–6 PUFA are very important to the structure and function of cell membranes, and to vital physiologic processes.

Omega–3 PUFA. There is little or no disagreement among nutrition experts that omega–3 PUFA are good for your heart, and for your overall health. The most beneficial omega 3’s, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), come from fish. A third omega–3, alpha-linolenic acid (ALA), is found in chiefly in plant products. ALA is considered an essential fatty acid, because humans cannot synthesize it, and it must be consumed in the diet.

We can synthesize small amounts of EPA and DHA from ALA, so they are not strictly considered to be essential. However, because we need more EPA and DHA than we can easily make, dietary sources of these omega–3s remain important.

Omega–6 PUFA. The most important omega–6 PUFA are linoleic acid (LA) and arachadonic acid (AA). LA is an essential fatty acid, because we cannot synthesize it, and it must come from our diets.

Getting enough omega–6 PUFA (in contrast to the omega–3 PUFA) is generally not a problem in modern diets. In fact, it is nearly ubiquitous, and can be found in grains, nuts, all the vegetable oils used in food preparation, and even in modern, grain-fed poultry and beef. In the typical Western diet, the amount of omega–6 PUFA has massively increased over the past several decades.

In contrast to the omega–3 PUFAs, however, there is considerable controversy about the omega–6 PUFA.

The Omega–6 Controversy

The AHA and US government dietary guidelines pointedly decline to differentiate between omega–3 PUFA and omega–6 PUFA, and remain quite insistent in urging us to consume lots of all types of PUFA. Since omega–6 PUFA is ubiquitous in our diets, while we have to work to get omega–3 PUFA, the result is that most of us get lots of the first and little of the second.

For this reason it is disturbing that many respected experts believe that omega–6 PUFA, in the amounts that dietary guidelines recommend, may be risky for our hearts and our overall health. They urge limiting our omega–6 PUFA intake, or at the very least, substantially increasing our omega–3 PUFA intake along with omega–6 (since such “balance” may be protective). It is noteworthy that dietary guidelines in many countries (notably the UK and Europe), do not recommend unlimited consumption of omega–6 PUFA, and in fact suggest that the current average consumption of omega–6 might be too high.

I have looked pretty carefully at the evidence on both sides of the omega–6 argument, and have written a separate article about it:

My conclusion is that urging unlimited consumption of omega–6 PUFA is not justifiable. It’s possible that current US guidelines may turn out to be correct, but as I see it the evidence to the contrary is sufficient to justify caution. And, given the recent track record of US dietary experts (stigmatizing saturated fats, pushing a low-fat-high-carb diet, originally advocating for trans fats, and a few others), I have become far from reluctant to question their advice, even when it is presented with the force of dogma.

The Bottom Line

Studies have shown that people who substitute PUFA for saturated fats in their diet have fewer cardiac problems than people who don’t. So it does indeed make sense to eat PUFA.

However, there is evidence that eating a lot of omega–6 PUFA - or at least a lot more omega–6 than omega–3 - may have negative consequences. Studies suggest that taking pains to add plenty of omega–3 PUFA to our diet is not only good for us in itself, but also may help to mitigate the potential downside of consuming lots of omega–6 PUFA.

For anyone who is reluctant to adopt a diet contrary to recommendations from the US government and the AHA, be assured that a diet high in omega–3 PUFA keeps you legal, while still allowing you to hedge your bet.

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Article Sources
  • Harris WS, Mozaffarian D, Rimm E, 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 2009; 119:902.
  • Mozaffarian D, Micha R, Wallace S. Effects on coronary heart disease of increasing polyunsaturated fat in place of saturated fat: a systematic review and meta-analysis of randomized controlled trials. PLoS Med 2010; 7:e1000252.
  • 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.