TLDR; — A study heated the ten most common cooking oils and found that Extra Virgin Olive Oil (EVOO) contained the fewest toxic compounds at the end, even though it has one of the lowest smoke points.

The Prevailing Theory of Cooking Oils

For years, my oil selections in the kitchen have followed the standard guidelines passed around in health-focused circles. These consist of:

  1. Use extra virgin olive oil (EVOO) or butter for dressings or low-heat applications.
  2. Use regular/refined olive oil for medium heat.
  3. Use coconut oil, avocado oil, or ghee for high heat.

The core principle behind these guidelines was to constantly balance the applied heat with the oil's smoke point. Once oil hits its smoke point, it starts oxidizing, breaking down, and releasing toxic compounds and free radicals into your food.

The theory was that EVOO, being largely unfiltered, contained a more significant number of compounds that the high heat would damage, and thus, its health value would drop off rapidly when used for cooking.

Conversely, it was assumed that heavily processed oils such as canola oil, grapeseed oil, and sunflower oil, while being bad for you for many other reasons, at least were better at high temperatures due to their high smoke points (the better option for this category being avocado oil). Most of the top articles about healthy oils for cooking still list canola as one of the top choices.

A 2018 study from Down Under has turned this entire theory on its head.¹

Study Design

Rather than just measuring the smoke point of an oil and assuming that this correlated with toxic compounds released, researchers measured the toxic compounds themselves. The results were surprising. — Click To Tweet

Two tests were performed:

  1. Oils were heated gradually (over 20 minutes) from 25 to 240°C / 77–464°F (higher than domestic cooking temperatures) and sampled every 30˚.
  2. Oils were heated at 180°C / 356°F for 6 hours (longer than most slow cooking methods), collecting samples at 30, 60, 180, and 360 minutes.

All heated samples were cooled at room temperature (25 ± 1°C, 77 ± 1ºF) and stored until chemical analysis.

For reference, deep frying is generally carried out between 170–180°C and shallow pan frying tends to be uncontrolled with temperatures between 180–230°C, whereas eggs will cook at ~70˚C/160˚F.

Ten Oils were tested, listed here along with their starting smoke points (in ˚C):

  1. Extra virgin olive oil (EVOO) — 206.7 +/-2.5
  2. Virgin olive oil (VOO) — 175.3 +/- 0.6
  3. Olive oil (OO) — 208.3 +/- 1.5
  4. Canola oil (CO) — 255.7 +/- 0.6
  5. Rice bran oil (RO) — 237.0 +/- 1.7
  6. Grapeseed oil (GO) — 268.0 +/- 1
  7. Coconut oil (CoO) — 255.7 +/- 0.6
  8. High oleic peanut oil (PO) — 226.3 +/- 2.1
  9. Sunflower oil (SO) — 254.7 +/- 1.5
  10. Avocado oil (AO) — 196.7 +/- 0.6

NOTE: These smoke point values vary considerably from some more commonly reported numbers.

What They Found

In each sample, they measured the degree of oxidation, the current oxidative stability, the smoke point, the fatty acid profile (including trans fats), and other toxic byproducts called polar compounds.

Many of these polar compounds have been associated with various forms of cancer and neurodegenerative diseases such as Alzheimer's and Parkinson's Disease.⁴

At this point, trans fats shouldn't need much explanation. They are bad. They increase your risk of developing diabetes, heart disease, cancer, etc. The studies showing this are too numerous to count. While this portion of the study was not particularly surprising, it enforces that olive, coconut, and avocado oils are a step above the seed oils.

Next up, we get some more surprising results. (Apologies for the low-quality images. I couldn't procure the raw data, and converting this type of graph is almost impossible.)

Unsurprisingly, coconut oil remained at the bottom of the charts due to its extremely high saturated fat content (~90%).

Why are saturated fats stable at high temperatures?For an oil to be oxidized, chemical bonds must be broken. This process requires energy-the precise amount of which is determined by the specific combination of fatty acids that the oil is made up of. The bonds in saturated fats typically require more energy to break than those in polyunsaturated fats, with monounsaturated falling in between. The higher the temperature, the more energy is present for the breaking of these bonds and subsequent formation of free radicals.³

What was more surprising from these results is how EVOO, full of monounsaturated fats, stayed neck-in-neck with the coconut oil. And yet, upon further reflection, this does indeed make sense. The activation energy of lipid oxidation is higher in the presence of antioxidants. This means that for oils rich in antioxidants, more energy is required to break the bonds, and thus, the rate at which toxic byproducts are formed is lower. This also explains why virgin and refined olive oil underperformed the EVOO across the board: increased processing and filtration means fewer antioxidants.