Hello, readers! This week, we're diving into one of the most polarizing topics in nutrition: seed oils. In recent years, the wellness industry has labelled them as toxic, even blaming them for the rise in chronic disease. Meanwhile, many healthcare professionals and major health organizations argue the opposite, recommending seed oils as part of a heart-healthy diet. So, why the stark divide? Today, we'll unpack both sides of the debate, and I’ll share my tips for navigating the grocery store in a world full of seed oils.
Here’s what you can expect in this post:
What are seed oils?
What the research says about seed oils and health
Do seed oils cause inflammation?
Does cooking with seed oils create harmful trans fats?
Do refining methods make seed oils harmful?
Tips for grocery shopping and assessing products that contain seed oils
In the free section of this post, I cover what seed oils are and why they’ve become such a hot topic.
To access the rest, including what the research actually says about seed oils and inflammation, trans fats, refining methods, and how to navigate seed oils in the grocery aisle, you’ll need to be a paid subscriber.
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What are seed oils?!
Let’s start with the basics. “Seed oils” is a term popularized by the wellness industry that refers to eight commonly used vegetable oils: canola, sunflower, soybean, corn, cottonseed, grapeseed, safflower, rice bran, and peanut oil.
Our consumption of these oils has increased substantially since the ‘60s, when large health authorities like the American Heart Association began recommending reduced intake of saturated fats for heart health. In response, food manufacturers started seeking alternatives to traditional sources of saturated fat, such as butter, lard, and palm oil, and turned to seed oils instead. Seed oils provided an inexpensive, shelf-stable, and versatile source of unsaturated fats that worked well in food production. For example, using seed oils in baking allowed manufacturers to lower saturated fat content while maintaining texture and flavour. As seed oils have become a staple in processed foods, our overall intake has increased significantly.
Today, polyunsaturated fats are still recommended as a replacement for saturated fats to support better health outcomes.
For example, the American Heart Association promotes seed oils as “better-for-you” cooking oils, and the Heart and Stroke Foundation identifies canola and soybean oils as healthy sources of polyunsaturated fats (PUFAs) that can help reduce cholesterol levels.
And they’re not wrong. There's a substantial body of scientific evidence showing that sources of PUFAs, including commonly used seed oils like canola and sunflower oil, have beneficial health effects. Large studies have linked seed oils to lower all-cause mortality, as well as reduced risk of death from heart disease and certain types of cancer.
For example, a study of over 400,000 adults found that a higher intake of plant-based fats, particularly vegetable oils, was associated with a lower risk of dying from heart disease.
Another study of over 220,000 adults found that replacing 10 g/d of butter with vegetable oil was associated with a 17% reduction in total mortality and cancer mortality.
In another study of over 520,000 adults, replacing butter or margarine with equal amounts of vegetable oil (corn oil, canola oil, or olive oil) was related to lower all-cause mortality and mortality from heart disease, diabetes, cancer, respiratory disease, and Alzheimer’s disease.
So why all the seed oil hate?
Before diving into the theories about why seed oils might be harmful, let’s look at where this all started. One of the most prominent voices in the anti–seed oil movement is Dr. Cate Shanahan, a licensed physician and author. She coined the term “The Hateful Eight” to refer to eight seed oils she claims are making us sick. As a medical professional, her credentials have given her message credibility, allowing her to amass a large following, influence a significant portion of the wellness industry, and sell a lot of books.
However, her arguments are flawed. One of her central claims, repeated across her books and online platforms, is based on a chart that shows seed oil consumption rising in parallel with the incidence of type 2 diabetes (shown below). She uses this correlation to suggest that seed oils cause type 2 diabetes.
But correlation does not equal causation, and this argument overlooks the many other related factors that occurred during the same period.
Here’s a classic example I can recall from my studies: the graph below shows that as ice cream sales increase, so do the number of drownings. By Dr. Cate’s logic, one could conclude that ice cream consumption causes drownings. From a scientific standpoint, these two events simply coincide with the summer months, when both ice cream consumption and swimming (and thus drownings) tend to increase.
Dr. Cate has also been quoted as attributing seed oils to “every other illness”, including inflammatory illnesses of the GI tract, brain cancer, dementia, and heart disease. She claims that eliminating seed oils will improve these conditions and lead to better overall health. Yet, she doesn’t cite any research specifically using seed oils. Instead, she relies on in vitro studies linking oxidative stress, inflammation, and various illnesses. While these studies are important to consider, they cannot be directly applied to the consumption of seed oils. Furthermore, they overlook the vast body of scientific literature that contradicts her claims.
There are plausible mechanisms by which seed oils may be harmful.
Over the past decade, the wellness industry has increasingly taken an anti-seed oil stance, and it’s not entirely surprising. Seed oils are widely used throughout the food system, especially in ultra-processed products. Given the extent of processing they undergo, it's easy to assume they might be harmful.
Several theories have been proposed to explain how seed oils could negatively impact health. Let’s take a closer look at each one to evaluate whether these concerns are supported by scientific evidence.
1. Do seed oils cause inflammation? The linoleic acid and inflammation hypothesis.
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