What are seed oils?

Seed oils refer to the fats squeezed from the seeds of vegetables like corn and soybeans, which are then refined with heat and chemical solvents. They were first produced in the late-1800s and have significantly risen in popularity since.

In 1961, as concerns over heart disease rose, the American Heart Association recommended people use polyunsaturated oils, like vegetable oils, as a replacement for saturated fats, and by the 1980s, many restaurants had started cooking with vegetable oils.

In recent years, many people associated with the MAHA movement have raised concerns about seed oils, with Kennedy himself saying Americans are being "unknowingly poisoned" by the oils.

"Seed oils are one of the most unhealthy ingredients that we have in foods," Kennedy said, adding that they're "associated with all kinds of serious illnesses including body-wide inflammation which affects all our health."

A number of health experts and influencers involved with the MAHA movement have raised concerns about seed oils known as the "hateful eight," referring to corn, canola, cottonseed, grapeseed, soy, sunflower, safflower, and rice bran oils.

What the research says about seed oils

Experts say that, so far, there's no clear evidence that seed oils are harmful to your health, and switching to alternatives like butter or lard could actually be worse for you.

Most fats are converted to energy within the body, Gardner said. "A very small number of our fats — and it's these omega-6s and omega-3s — actually get converted into hormone-like substances," he said, adding that these fatty acids play a role in regulating blood pressure, vasodilation, clotting, and triglyceride levels in the blood, which are all processes related to the inflammatory response.

"The omega-3s are a little less inflammatory than the omega-6s," Gardner said. "There are some byproducts of omega-6s that could contribute in some way to inflammation, but the net impact is not pro-inflammatory."

Research has linked an increased ratio of omega-6s to omega-3s to a variety of conditions, including mood disorders, knee pain, back pain, menstrual pain, and even preterm birth. However, the ratios of omega-6s to omega-3s in seed oils varies significantly. For example, safflower oil and sunflower oil have ratios of 125:1 and 91:1 while corn oil's ratio is 50:1, soybean oil's ratio is 8:1, and canola oil's ratio is 2:1.

However, there are also health benefits that have been associated with omega-6s.

"Omega-6s in dozens and dozens of randomized controlled trials in people, improve blood cholesterol levels — multiple aspects of blood cholesterol levels, from increasing the good cholesterol, like HDL, reducing LDL cholesterol, reducing triglycerides to improving glucose and insulin levels," said Mozaffarian. "And it's ironic, because many of the influencers talk about diabetes — and there's well-established randomized trials showing that omega-6 fatty acids actually improve glucose, improve insulin resistance, improve insulin secretion by the pancreas."

Other research on seed oils has been mixed. One meta-analysis that synthesized the effects of seed oils on 11 inflammatory markers generally found no effects with the exception of one inflammatory signal that was elevated in people with the highest intake of omega-6s.

Meanwhile, a review of seven randomized controlled trials found the effect of seed oils on the risk of heart attacks varied depending on the type of seed oil, and more than 15 randomized controlled trials found that refined seed oils had no effect on inflammation. These trials even found that linoleic acid, which is found in sunflower and grapeseed oils, typically lowers inflammation.

According to Alice Lichtenstein, a professor of nutrition at Tufts, it's tempting to zero in on one specific nutrient and blame it for America's health problems, but it's more helpful to consider overall dietary patterns.

"Certainly dietary patterns that are higher in fish … are associated with lower risk of cardiovascular disease," Lichtenstein said. But, she added, diets that are higher in fish also tend to be lower in saturated fat and higher in monounsaturated fat. "People [who eat fish also] tend to be more active and they tend to smoke less and have other factors that are associated with better health outcomes," she added.

Experts noted that choosing to avoid seed oils doesn't pose an inherent danger to your health, though there are downsides. For example, avoiding seed oils means cooking all of your food at home, including things like salad dressing, sauces, and even bread, according to Julia Zumpano, a registered dietitian at Cleveland Clinic.

There's also concern about what people are substituting seed oils for. Butter, lard, and beef tallow are all high in saturated fat, and coconut oil contains even more saturated fat than butter.

"Animal fats are healthier than white bread, but they're not healthier than seed oils," Mozaffarian said.

Instead of avoiding seed oils, experts said it'd be better to avoid fast and highly processed foods.

"For years, we have told you to eat less sugar and salt and chips and junk food," Gardner said. "If suddenly telling you that the seed oil in them is killing you [means] you stop eating all the crap for the wrong reason — I would be thrilled."