Are Refined Oils Unhealthy? What Science Says

Over the past century, refined oils like soy, canola, and peanut oil have become a larger and larger part of the Western diet. 

These oils are inherently high-calorie and often used in processed, packaged foods, and this rise in refined oil consumption has also coincided with a large increase in preventable diseases like cancer, diabetes, and obesity. Thus, it’s logical to wonder if there’s a connection.

On the other hand, many highly processed oils such as olive oil, avocado oil, and safflower oil are high in monounsaturated fats, which are associated with a number of health benefits.

So, are refined oils harmful, benign, or helpful? 

Or does it depend on which refined oils you consume, and if so, which ones are healthiest and how much should you eat?

What Is Refined Oil?

When food manufacturers extract oils from plants and plant materials, such as palm oil, peanut oil, coconut oil, avocado oil, soybean oil, canola oil, sunflower oil, and sesame oil, they generally use various chemicals and processing techniques to purify it. 

They do this because “crude oils” (oils in their natural state) contain substances that consumers find undesirable with respect to taste, stability, appearance, or odor.

Oils that go through these processing steps are referred to as “refined” or “processed” oils.

Are Refined Oils Unhealthy?

Blogs, magazines, and the mainstream media make many claims about how refined oils affect our health.

Here’s what science says about the most common ones.

Refined Oils and Omega-6 to Omega-3 Ratio

When refined oils became more readily available at the beginning of the 20th century, food manufacturers began using them to prepare pre-packaged convenience foods, such as crackers, biscuits, cookies, pastries, pies, mayonnaise, and margarine.

This meant that people consumed far more refined oil than ever before. And since vegetable oils typically contain more omega-6 fatty acids than omega-3 fatty acids, the amount of omega-6 fatty acids in the standard Western diet skyrocketed.

For instance, research conducted by scientists at The Center for Genetics suggests that for most of history, humans consumed roughly equal amounts of omega-6 and omega-3. However, since vegetable oil intake increased around 120 years ago, the ratio is now as high as 20:1 in favor of omega-6.

Some researchers believe this imbalance contributes to chronic inflammation, which underlies many common Western maladies, including cardiovascular disease, type 2 diabetes, irritable bowel syndrome, macular degeneration, rheumatoid arthritis, asthma, some cancers, and psychiatric disorders.

The current weight of evidence doesn’t support this theory, though, with several systematic reviews finding no link between omega-6 and omega-3 consumption and increased systemic inflammation.

In fact, in one study conducted by scientists at Maastricht University Medical Center, researchers found that when participants ate meals high in omega-6 fatty acids, they had lower levels of inflammatory markers in their blood than when they ate meals high in saturated fat.

Thus, there’s no scientific basis to curtail your omega-6 fatty consumption. That said, consuming too much of anything—especially at the expense of other health-promoting nutrients such as omega-3—is suboptimal from a health perspective.

A better approach is to consume a variety of healthy foods rich in omega-3, omega-6, and omega-9 fatty acids, such as salmon, mackerel, and anchovies; walnuts, almonds, and cashews; and olive, avocado, and peanut oil. 

Refined Oils and Oxidation

Refined oils oxidize during cooking, which means that the polyunsaturated fatty acids in the oil react with chemicals and moisture in the air, making it potentially harmful to eat. Refined oils that are heated to very high temperatures for long periods or repeatedly reused are particularly susceptible to oxidation.

While some animal research shows that consuming repeatedly heated vegetable oils may adversely impact health, other studies show that there are no obvious negative consequences despite the increased oxidation. 

Given the conflicting evidence on refined oils and oxidation, it’s perhaps safest to follow the guidance of a 2015 review published in The British Journal of Nutrition. In this study, the researchers concluded that cooking with vegetable oils is probably safe, though it still makes sense to minimize oxidation.

To do this, they recommend that you avoid heating refined oil to very high temperatures for long periods while cooking and only use oil once. 

This is easy to monitor with your own cooking, but becomes impossible to police when you dine out. Thus, when deciding what to eat at a noshery, it’s probably sensible to limit foods that may have been cooked in oxidized oil, notably shallow- and deep-fried foods.

Refined Oils and Heart Health

Aside from being cheap and readily available, one of the main reasons vegetable oils have become popular is their association with heart health. 

For decades, public health officials recommended substituting saturated fats for polyunsaturated fats because there’s evidence that those who do suffer fewer cardiovascular-related problems. 

However, the studies that support this claim tend to be observational, which means they can show two things are associated but not that one causes the other. They also don’t take into account outside factors that may contribute to poor heart health, such as lifestyle, physical activity levels, stress, sleep, and other dietary habits. 

As such, they don’t provide strong evidence that polyunsaturated fat is better for maintaining heart health than saturated fat.

Importantly, several meta-analyses of randomized controlled trials (the gold standard of scientific evidence) show that switching to vegetable oil does little to improve heart health or your risk of dying from cardiovascular complications.

Thus, there’s no reason to swap saturated fat for polyunsaturate fat entirely. Instead, look to get a mix of both in your diet. That said, you also want to limit your intake of saturated fat to less than about 10% of your daily calorie intake. 

Refined Oils and Trans Fats

Some vegetable oils naturally contain trans fat, while in others, trans fat forms when the oil goes through hydrogenation—the process of making liquid vegetable oils solid at room temperature. 

This is important because consuming large amounts of trans fat contributes to multiple chronic diseases, including heart disease, obesity, cancer, and diabetes.

As such, it’s sensible to avoid regularly consuming trans fat.

Thankfully, this is relatively simple in the US since the FDA recently banned food manufacturers from adding trans fat to food.

Outside of the US, countries like Denmark, Switzerland, and Austria have also imposed bans, whereas countries like the UK have only urged food producers to cut down on the amount of hydrogenated oil they use. 

In countries where there isn’t yet a ban, the best way to limit your intake of trans fat is to avoid foods containing hydrogenated and partially hydrogenated oils. Be aware, though, that trans fat may appear in the ingredients lists under a name like “mono and diglycerides of fatty acids.”

Conclusion

While many health gurus like to demonize refined oils, there’s substantial scientific evidence that they’re safe to consume so long as you don’t heat them to extreme temperatures for too long, reuse them several times, or consume them in large amounts (primarily because of their calorie content).

The only refined oils you should be wary of are hydrogenated and partially hydrogenated vegetable oils as they contain trans fat, which can be detrimental to your health if you consume it regularly.