Sound Science: Polyunsaturated Fats and Heart Health

We’re back with another installment of Sound Science, the forum where we spotlight quality research studies and scientific conclusions that may have flown under your radar. This time, we take a peek at the peer-reviewed literature on polyunsaturated fatty acids and heart health.

The big, fat debate about the fats we eat is nothing new. In the middle of the last century, the lipid hypothesis saturated scientific discussion, but by the 1980s and ’90s, the low-fat craze was in full swing.

In this century, butter apparently is back. Yet as the pendulum of public opinion swings, science advances. While questions about the healthfulness of certain types of dietary fatty acids remain, the understanding of the role others play in health has become clearer.

Not All Types of Fat Are the Same

Although many people may associate certain foods with only one type of fat (like olive oil and monounsaturated fat, or butter and saturated fat), the truth is that the foods we eat are made up of a blend of fatty acids. We’re never eating just one type of fat, and this is a good thing because dietary fats play diverse and important roles throughout the body.

But fats can be a complicated topic for many, so let's start at the beginning. The fats we eat can be either saturated or unsaturated. Polyunsaturated (PUFA), monounsaturated (MUFA) and trans fatty acids are all of the unsaturated variety, and a tremendous amount of research has been conducted on their health impacts.

Trans fatty acids are known for their association with coronary heart disease, while PUFA and MUFA (particularly from plant sources) have been shown to be health-promoting. The strongest evidence for heart-health benefit, however, supports the substitution of PUFA in place of saturated fats.

Polyunsaturated Fatty Acid Varieties

Within the class of PUFA there are different types of fatty acids, with the two most common varieties in our diet being omega-3 and omega-6.

Omega-3 fatty acids include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). ALA is the plant form of omega-3 and is found in foods like flaxseed and walnuts, as well as canola and soybean oils. EPA and DHA are marine forms of omega-3. They are produced by microalgae and end up in the tissues of fish like anchovies, mackerel, salmon and sardines.

Our bodies can convert ALA into EPA and DHA, but it happens inefficiently, yielding only small quantities; therefore it’s recommended that we consume EPA and DHA directly.

Omega-6 fatty acids include linoleic acid and arachidonic acid. Linoleic acid is found in canola, corn, cottonseed, sunflower and soybean oils. Arachidonic acid is found in animal sources like fish, meat and eggs. Our bodies can convert linoleic acid to arachidonic acid, but this process is also inefficient. Because ALA and linoleic acid can be obtained only in our diet, they are considered essential fatty acids.

Now that we’ve covered some basics, let’s take a closer look at the evidence supporting PUFA’s role in heart health. Research has shown both omega-3 and omega-6 fatty acids are beneficial to health, but in slightly different ways.

Omega-3 Fatty Acids

Both randomized control trials (RCT) and observational studies have shown that omega-3 fatty acids positively impact cardiovascular health. RCT evidence demonstrates that omega-3s can lower triglyceride levels, blood pressure, inflammation and heart rate, while also increasing HDL cholesterol (the “good” kind of cholesterol that we want more of) and enhancing endothelial function.

Early observational data on people in Japan and Greenland in the 1970s hinted that eating higher amounts of fish may lead to better heart health. Since that time, a vast amount of observational research has shown omega-3 fatty acids to be protective against cardiovascular disease, which may not come as a surprise, given the improved cardiovascular biomarkers seen in RCT data.

To help get an adequate amount of omega-3 in our diet, U.S. Dietary Guidelines recommend that we eat at least two servings of seafood (≥ 8 ounces total) per week. You can also get omega-3 in supplement form, with the most common forms being fish and krill oils.

Much has been written in the popular press about omega-3 supplementation and its contribution to health, which comes with the territory when scientific research reaches mixed conclusions. According to the National Institutes of Health (NIH), omega-3 supplements don’t reduce the risk of heart disease, but whether omega-3 supplements are beneficial for other health outcomes remains unclear.

Research continues, but it’s worth mentioning an interesting footnote here: Previous research studies indicate that omega-3 supplements could protect against heart disease, and the NIH cites potential increases in seafood consumption and cholesterol-lowering medications as possible explanations for more recent research conclusions.

Omega-6 Fatty Acids

Although omega-3s grab more headlines, studies have repeatedly demonstrated that omega-6 fatty acids are beneficial to heart health as well. The main omega-6 fatty acid in our diet is linoleic acid, which when compared with carbohydrates has been shown to lower LDL cholesterol (the “bad” kind of cholesterol that we want less of), the ratio of total cholesterol to HDL cholesterol, and blood triglyceride levels. These biomarkers are important for improving heart health and may explain, in part, the large observational studies and clinical trials that have found that higher omega-6 consumption reduces risk for heart disease.

The Bottom Line

Don’t fear fats: They play important roles in our health. Decades of research has revealed that the type of fat we eat matters. As a result, dietary advice has evolved to place more emphasis on type of fat (unsaturated > saturated) rather than amounts. While polyunsaturated and monounsaturated fatty acids contribute to good health, the strongest evidence for heart health is found when polyunsaturated fats replace saturated fat in the diet.

This blog post includes contributions from Alyssa Ardolino, RD.

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