Common Seed Oil Misconceptions: a Q&A with Dr Cate Shanahan
Dr. Cate Shanahan has been instrumental in bringing awareness of the dangers of seed oils from the fringes to mainstream health discourse in just a few years. She has spent decades researching and advocating for real, traditional food and seed oil avoidance since before these topics were fashionable — and she knows more about them than almost anyone else in the space.
People are increasingly aware that seed oils are unhealthy, but few actually know why. There is still a great deal of confusion and ambiguity around the topic. In this Q&A, Dr. Shanahan helps clear up some of the common misconceptions and sheds light on the real issues surrounding seed oils.
Q: On page 17 of Dark Calories, you talk about the chain reaction of PUFA oxidation. Why do seed oils still look like oil even after oxidation?
Dr. Cate:
The oxidation process happens slowly due to limited oxygen and light exposure while the oil is in the bottle. You may not see changes, but you can smell them. Refined oils shouldn't have any smell, so if there’s an odor, it’s a sign of rancidity, meaning some level of oxidation has already started.
Q: Even if you store seed oils properly, does oxidation still occur once inside the body?
Dr. Cate:
Yes. Even if stored in a vacuum or frozen, once the oil enters your body, an oxygen-rich and warm environment, the oxidation process accelerates. The small amount of toxins in the oil multiplies rapidly, which is why deep fryer oils, for example, develop solids over time—those are essentially polymers forming from oxidized oil, that basically turned into plastic.
Q: Do monounsaturated fats require more stabilizers, like Vitamin E, than saturated fats?
Dr. Cate:
Yes. Monounsaturated fats oxidize more quickly without stabilizers like Vitamin E. Saturated fats, on the other hand, are much more stable. But once you get into polyunsaturated fats (PUFAs), oxidation happens much faster, especially in omega-3s and omega-6s.
Q: What about modern monounsaturated cooking oils, like those from algae?
Dr. Cate:
Those oils are often synthetic and don’t have natural antioxidants like olive oil, so they are less stable. They fall into a middle category—not as toxic as seed oils, but lacking in nutrition.
Q: You mentioned seed oil processing removes antioxidants like Vitamin E. Why can’t they leave Vitamin E in?
Dr. Cate:
Vitamin E is removed during the deodorizing process, which eliminates free fatty acids that cause rancid smells. Unfortunately, deodorizing also strips away Vitamin E and other nutrients. While they could add it back, studies show that adding isolated Vitamin E without its natural companions can promote oxidation.
Q: Why do some oils, like flax or fish oil, have a fishy smell?
Dr. Cate:
The fishy smell comes from rancidity, which occurs when free fatty acids are released. Free fatty acids vaporize, allowing you to smell them, but this doesn’t mean the oil is oxidized yet. Rancidity and oxidation are different processes—rancidity releases free fatty acids, while oxidation produces toxic byproducts.
Q: You mentioned 30-40% of our cell membranes are made of PUFA. Is this normal, or is it due to overconsumption of PUFA-rich oils?
Dr. Cate:
Membrane PUFA is regulated and supposed to be there, but the high levels of PUFA in body fat reflect our diet and shouldn’t be as elevated as they are today.
Q: Does having a low-PUFA diet reduce inflammation from things like mosquito bites?
Dr. Cate:
Yes, absolutely. A healthier diet allows your body to better control oxidation. This reduces discomfort from things like bee stings, sprains, and even exercise-induced inflammation. The inflammatory response is necessary, but with less PUFA in your diet, you can control it more effectively.
Q: Alzheimer's has increased with PUFA consumption, but what caused it back in 1906?
Dr. Cate:
The root cause of Alzheimer’s is oxidative stress, not just PUFA consumption. In 1906, malnutrition was likely the main contributor to oxidative stress. Today, seed oils exacerbate it due to their PUFA content. The tau proteins seen in Alzheimer’s patients are the result of oxidized fats and proteins.
Q: Is the issue with PUFAs really about how many antioxidants we have to control their oxidation?
Dr. Cate:
Yes. The problem is not just too much PUFA, but a lack of antioxidants to control PUFA oxidation. Our bodies need a certain amount of PUFA, but without enough antioxidants, the oxidative stress becomes overwhelming. Seed oils deplete your antioxidants, and the natural antioxidants (like Vitamin E/mixed tocopherols) in the plants they are derived from get eliminated when the oils are processed.