Robert F. Kennedy Jr. and others claim they're harming our health, but the evidence suggests otherwise.
To their many vocal detractors, they're referred to as "the hateful eight." Canola oil, corn oil, sunflower oil and other refined oils made from the seeds of certain plants have become lightning rods for wellness influencers -- and some politicians.
Robert F. Kennedy Jr. says Americans are being "unknowingly poisoned" by them. Online forums, blogs and influencers say they're "toxic," "slowly killing you" and driving up rates of diabetes, obesity and other chronic diseases.
The claim that seed oils are ruining our health is especially rankling to nutrition scientists, who see them as a big step forward from butter and lard.
Decades of research have shown that consuming seed oils is associated with better health, said Christopher Gardner, a professor of medicine at Stanford University.
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, like sesame and flax seed 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 -- like 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, Dr. Decker said. They also often remove certain components that can contribute to rancidity, splattering, unpleasant flavors or a dark color.
These processes result in a neutral-tasting oil that is relatively shelf stable and can be used at high heat without easily smoking, Dr. 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 that 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 that 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 that 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 like 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 claim that the fatty acids in seed oils can easily degrade during refining, storage or cooking -- breaking into smaller molecules that can damage cells.
Dr. Decker said that 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, Dr. 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 that is also increasing certain chronic health conditions. One study, for example, found that levels of linoleic acid -- the main omega-6 in seed oils -- in U.S. adults have more than doubled during the last 50 years.
But correlation does not equal causation. We're eating more of these oils because they're used in ultraprocessed and fast foods, which make up a larger share of our diets today than in past decades, Dr. Gardner said. Those foods aren't good for us, he said, but there's no evidence to suggest that seed oils are what makes them unhealthy.
"That's just bizarre to blame them and not the foods that they're in," Dr. Gardner said.
What's the bottom line?
If you want to reduce your consumption of seed oils, do so by eating fewer ultraprocessed foods, Dr. Gardner said. That would likely be a health win.
But it would be a mistake, Dr. Lichtenstein said, to replace seed oils with ingredients like butter, lard or tallow, which are high in saturated fats.
Seed oils work especially well in recipes that require oils with a neutral flavor, like salad dressings, or for high-heat cooking, like with a vegetable stir fry.
That, Dr. Gardner said, would be a healthful meal -- not a harmful one.