Browse evidence-based analysis of health-related claims and assertions
When perilla oil is heated, two chemicals—HHE and ethyl furan—are made together because they both come from the same fat molecule, linolenic acid.
Descriptive
The kind of fat in an oil—like how much linolenic, linoleic, or oleic acid it has—decides what chemicals are made when you heat it up, and how much of them.
When you heat olive oil at high temperatures for a long time, it makes more of two harmful chemicals—HNE and ONE—than the other oils, because olive oil is mostly made of oleic acid.
When you heat peanut oil really hot for a long time, it makes more of a chemical called pentyl furan than any of the other oils tested, because peanut oil has a lot of linoleic acid.
When you heat perilla oil (a type of plant oil) at high temperatures for a long time, it creates two specific chemicals—HHE and ethyl furan—that don’t show up when you heat other oils, because perilla oil has a special fat called linolenic acid.
When you heat raw or lightly cleaned camellia oil, new fat molecules form — but when you heat oil that’s been cleaned too much, almost nothing new forms, meaning the oil is too stripped down to react.
Raw camellia oil has the most natural antioxidants, but when you heat it, its fat molecules break down more than in oil that’s been lightly cleaned — so it’s a trade-off between protection and structure.
The natural antioxidants in camellia oil — like vitamin E and plant polyphenols — act like shields that slow down the oil’s breakdown when it’s heated. The more of these antioxidants, the less the oil goes rancid.
Correlational
When you heat camellia oil for a long time, it changes the structure of certain fat molecules in a very specific way — especially those involved in cell membrane function — and this change is more dramatic than any other chemical reaction happening in the oil.
Mechanistic
When camellia oil is cleaned too much, it loses its natural protective antioxidants, so it breaks down faster and turns rancid quicker when heated.
Camellia oil that’s been lightly cleaned but not over-processed stays fresher longer when heated, because it keeps some natural antioxidants but gets rid of gunk that makes it go bad.
When olive oil gets really hot, one of its healthy compounds — hydroxytyrosol acetate — goes up, which might be a clue that the oil has been overheated.
Quantitative
When you heat olive oil, the smell changes mostly because of how hot it gets — not because of what kind of olive it came from.
Different types of olive oil react differently to heat — one kind (Buža) keeps more of its healthy compounds when cooked, while others lose more, because of their natural chemical makeup.
Cooking olive oil at a moderate high heat doesn’t destroy all its healthy compounds, but it does break down some key ones — unless it’s a specific type of olive oil, which seems to hold up better.
When you cook olive oil too hot, it starts producing chemicals that smell rancid — like old nuts or grease — and these chemicals can be used to tell if the oil has been overheated.
Cooking olive oil at a very high temperature for an hour almost completely destroys the fresh, grassy smell that makes good olive oil taste good.
A super-sensitive machine called an 800 MHz NMR can spot tiny amounts of bad chemicals in oil that regular machines miss.
Extra virgin olive oil isn’t used for deep frying because it breaks down too easily at high heat — even experts avoid it for this reason.
Whether you leave oil in the sun or fry food in it, the same bad chemicals form — just at different speeds.
Oils with more unsaturated fats (like sunflower and sesame) break down faster and make more harmful chemicals when heated or left in the sun than oils with more saturated fats (like peanut or olive).
Frying food in oil at high heat for an hour creates toxic chemicals in the oil, especially in sesame oil, even after just 10 minutes.
Leaving cooking oils in the sun for a long time makes them break down and form harmful chemicals, especially if they’re made from seeds with lots of unsaturated fats.
Where the ‘kink’ is in the fat molecule decides whether it harms blood vessels — a kink at position 9 or 9+12 is bad, but at position 11 is harmless.