Doctor’s Note: Seed Oils — How Bad Are They? – The Saturday Evening Post

You may have heard people talking about removing seed oils from their diet. What exactly are seed oils, and is it true that they’re poison? Let’s go over the facts.

The most common seed oils are canola oil, corn oil, cottonseed oil, soybean oil, sunflower oil, safflower oil, grapeseed oil, and rice bran oil. These are often referred to as the “hateful eight” by anti-seed-oil advocates. Of course, there are more than eight seed oils in the world. People have consumed oils made from sesame seed, pumpkin seed, chia seed, and other less common seeds.

So why is there so much controversy over seed oils?

Fundamentally, it’s because seed oils are an invention of modernity. Animal fats like butter, tallow, and lard were the primary sources of dietary fat for most of human history, with fruit oils such as olive oil, avocado oil, palm oil, and coconut oil as secondary dietary fats. These so-called ancestral oils require no special tooling to produce and have been used by human civilizations since the beginning of recorded history.

In comparison, seed oils are pressed in high-pressure mills that weren’t invented until the last few centuries. Cottonseed oil was considered a waste product until the late-19^th century, when it was first used as an industrial oil and then turned into edible shortening. Since the mid-20^th century, soybean oil has become the predominant seed oil in the U.S., with canola (rapeseed) oil as the second-most-popular.

Therefore, the first argument against seed oils is the argument against modernity. Human metabolic health is much worse today than in the distant past, with people across the world developing obesity, diabetes, hypertension (high blood pressure), and hyperlipidemia (high cholesterol) at an exponentially increasing rate over the past century. If the change in diet over the past century is partially to blame for our declining metabolic health, the rise of seed oils is one of the largest such changes.

Of course, correlation is not causation. Just because seed oil consumption increased at the same time that metabolic disorders did, doesn’t imply that the seed oils were to blame. But there are number of theoretical rationales for why seed oils could be harmful.

The second argument is that many seed oils are produced by hexane extraction. Hexane is a petroleum product that is similar to the gasoline you put in your car, and it is used as a solvent to separate oil from solids. Most of the hexane is removed from food oils before it makes its way to your table, and as a highly volatile fuel, hexane should be completely destroyed by cooking. The U.S. Food and Drug Administration does not monitor or regulate hexane residue in most foods, but the European Union does, and international tests have generally shown extremely low levels of hexane in cooking oils. It’s likely that average Americans are exposed to far more hexane from motor vehicles than from food. That said, if you want to completely avoid dietary hexane, USDA Organic foods are certified hexane-free.

Third, Seed oils contain an unusually high ratio of omega-6 to omega-3 fatty acids. This refers to the two classes of essential fatty acids present in nature. Both omega-6 and omega-3s are essential nutrients, but it’s very rare to become deficient in either fatty acid as our diets contain plenty of both. The fatty acid ratio hypothesis states that the pre-modern human diet contained a relatively similar amount of omega-6 and omega-3 fatty acids. As seed oils have increased in prevalence, we are now eating many times more omega-6 fatty acids than omega-3s. Omega-6 fatty acids are used in the biosynthesis of inflammatory compounds, and some laboratory animals can suffer cardiovascular or cerebrovascular disease when fed high doses of omega-6 fatty acids. However, it’s not certain whether this happens in normal humans consuming normal amounts of seed oils.

Finally, polyunsaturated fatty acids found in seed oils are more susceptible to lipid oxidation than monounsaturated or saturated fats. Lipid oxidation is one of the ways that fats naturally degrade over time, leading to rancidity. Rancid oils can contain toxic degradation byproducts such as peroxides and aldehydes, which can cause acute nausea and gastrointestinal distress, and may have more subtle effects with gradual low-level exposure. You can typically smell and taste a high dose of rancid oils, but vegetable oils are more susceptible to oxidation than animal oils, so we could be eating slightly-rancid oil without knowing it.

It’s also possible that omega-6 fatty acids could oxidize after they’ve been incorporated into lipoproteins in our body. Low-density lipoprotein (LDL), also known as “bad cholesterol,” is a well-known cause of heart disease and strokes. Oxidized LDL appears to be even more harmful than regular LDL. If omega-6-containing LDLs are actually more susceptible to oxidation than omega-3 or saturated-fat LDLs, then it could explain a linkage between seed oils and poor health.

That said, all of these theoretical harms of seed oil are currently unproven. No one has run any high-quality controlled trials of a diet high in animal oils, versus fruit oils, versus seed oils. In the past century we’ve seen plenty of promising dietary theories fail in the real world.

So, what’s my personal practice? Well, we know that many ultraprocessed foods are made with seed oils, and there are many different reasons to limit ultraprocessed foods. By focusing on reducing the most obviously unhealthy sources of seed oil, you can improve your diet quality whether or not the seed oil hypotheses turn out to be true!