Seed Oils: What the Evidence Shows

Seed oils are among the most argued about foods right now. Canola, soybean, sunflower, safflower, corn and cottonseed oil are described online as inflammatory and as a hidden cause of heart disease. Restaurants now advertise cooking without them. Here is what the research shows.

The claims

The main argument goes like this. Seed oils are rich in linoleic acid, an omega 6 fat. The body can turn omega 6 fats into compounds involved in inflammation, so more omega 6 should mean more inflammation, and more inflammation should mean more heart disease. A second is that these oils oxidize when heated and the damaged fats harm the arteries.

Both ideas sound reasonable on paper. The question is whether they hold up in people.

What the trials show

When people eat more linoleic acid in controlled studies, blood markers of inflammation generally do not rise. The body tightly controls how much linoleic acid becomes inflammatory compounds, so eating more does not turn up the dial.

For heart disease, the most consistent finding is about what the oil replaces. When polyunsaturated fats like linoleic acid replace saturated fat, LDL cholesterol falls. Most pooled analyses of the randomized trials that made that swap show fewer cardiovascular events, not more. Observational studies that measure linoleic acid in the blood tend to link higher levels with lower heart disease risk.

A couple of older trials, reanalyzed decades later, found no benefit, and in one of them the group eating more linoleic acid had more deaths. They are quoted often online and they deserve an honest look. They also had real limits: the margarine used in one may have contained trans fat, and in the other most people were followed for only about a year. They sit against a much larger body of evidence pointing the other way.

The oxidation concern has more to it. Oil heated again and again, as in a restaurant fryer, breaks down into compounds you would not choose to eat. That is a real issue.

Where the real problem is

Most seed oil in the American diet does not come from a bottle at home. It comes in fried food, chips, crackers, pastries, fast food and packaged snacks. Those foods are high in calories, low in fiber and protein, easy to overeat and often fried in reused oil. People who cut seed oils usually cut those foods along with them, and that alone can explain why many of them feel better.

That is a good result, reached for a slightly different reason than the one they were given.

At home:

• Extra virgin olive oil has the strongest evidence and works well as a default.
• Canola or avocado oil is fine for higher heat cooking.
• Cutting fried and packaged food matters far more than which oil is in your pan.

What helps most is a plan built around the foods you actually eat, measured against your own numbers. That's how we approach it in the nutrition program.

See how HYBRD Nutrition works

This article is educational and is not medical advice.