To suggest otherwise, he added, “just undermines the science”.

We asked scientists to help clear up the confusion about how these oils affect our bodies.

What are seed oils?

Seed oils have become shorthand for refined plant-based vegetable oils. Technically, not all of the “hateful eight” oils, which also include cottonseed, soybean, safflower, grapeseed and rice bran oils, are made from seeds. Soybean oil, for example, is made from a bean. And there are other seed oils, such as sesame and flaxseed oils, that aren’t on that list. All of these oils are mainly composed of unsaturated fats. Most of them are high in one type, omega-6 fatty acids, and low in another, omega-3 fatty acids.

Seed oils are made by pressing the seeds to draw out the oils, said Eric Decker, a professor of food science at the University of Massachusetts Amherst. If you stop there, you have a cold-pressed oil, such as extra-virgin olive oil, which is rich in beneficial plant-based compounds, but prone to smoking and degradation when cooked at high heat.

Manufacturers usually process the oils further with heat and solvents to help pull more oil from the seeds, Decker said. They also often remove certain components that can contribute to rancidity, splattering, unpleasant flavours or a dark colour.

These processes result in a neutral-tasting oil that is relatively shelf stable and can be used at high heat without easily smoking, Decker said.

Why is there controversy about seed oils and health?

Several concerns about seed oils have been simmering online, but none are borne out in the research, experts say.

Some detractors claim their high levels of omega-6 fatty acids relative to omega-3 fatty acids may drive up chronic conditions by increasing inflammation in the body. This is based on an overly simplistic idea, said William Harris, a professor of basic biomedical sciences at the University of South Dakota.

It’s true, for example, that our bodies convert some omega-6 fatty acids into inflammatory compounds, but they change them into some anti-inflammatory ones as well. And researchers have found people who consume more omega-6 fatty acids generally do not have higher markers of inflammation in their blood. Instead, they are healthier.

In fact, a large body of research has shown people who consume more omega-6 fatty acids or who have higher levels of those fats in their blood have lower risks for cardiovascular disease, type 2 diabetes, cancer and earlier death.

Clinical trials have also consistently shown that when people replace some saturated fats (from animal fats such as butter or lard) with seed or other plant-based oils, their LDL, or “bad”, cholesterol levels, and risk for cardiovascular disease decrease, said Alice H. Lichtenstein, a professor of nutrition science and policy at Tufts University.

Opponents also say the fatty acids in seed oils can easily degrade during refining, storage or cooking – breaking into smaller molecules that can damage cells.

Decker said omega-6 and omega-3-rich oils do break down more quickly than solid sources of fat, but the refining steps help to prevent that process, as does their high level of vitamin E, an antioxidant. As with many food products, seed oils can go bad when they’re stored for several months at room temperature, Decker said. (He stores his in the refrigerator.) In that case, they should be thrown away.

A final claim is that we’re eating more of these oils than in the past and, as a result, increasing certain chronic health conditions. One study, for example, found that levels of linoleic acid – the main omega-6 in seed oils – in US adults have more than doubled during the past 50 years.

But correlation does not equal causation. We’re eating more of these oils because they’re used in ultra-processed and fast foods, which make up a larger share of our diets today than in past decades, Gardner said. Those foods aren’t good for us, he said, but there’s no evidence to suggest seed oils are what makes them unhealthy.

“That’s just bizarre to blame them and not the foods that they’re in,” Gardner said.

What’s the bottom line?

If you want to reduce your consumption of seed oils, do so by eating fewer ultra-processed foods, Gardner said. That would likely be a health win.

But it would be a mistake, Lichtenstein said, to replace seed oils with ingredients such as butter, lard or tallow, which are high in saturated fats.

Seed oils work especially well in recipes that require oils with a neutral flavour, like salad dressings, or for high-heat cooking, like with a vegetable stir-fry.

That, Gardner said, would be a healthful meal – not a harmful one.

This article originally appeared in the New York Times.

Written by: Alice Callahan

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