Browse evidence-based analysis of health-related claims and assertions
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.
Descriptive
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.
Quantitative
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.
Mechanistic
These two trans fats interfere with how insulin tells blood vessels to relax by blocking key signals in the cell, but other fats don’t have this effect.
A type of trans fat found naturally in dairy and meat doesn’t harm blood vessel cells in the lab, even though it looks similar to the harmful ones in fried foods.
These two harmful trans fats cause blood vessel cells to produce excess harmful oxygen molecules, which then trigger inflammation and block the production of a key chemical that keeps blood vessels healthy.
Two types of artificial trans fats found in fried and baked foods make blood vessel cells more inflamed and less able to produce a chemical that helps blood vessels relax, but a different trans fat found in dairy doesn’t do this.
We don’t know if natural trans fats from meat and dairy affect cancer risk — no solid human studies have looked at this yet.
Small experiments on ruminant trans fats and heart health markers haven't given clear answers — maybe because they used too few people or too much fat that you can't actually eat in real life.
Some studies suggest that the natural trans fats found in milk and meat might not raise heart disease risk the same way that artificial trans fats in fried and baked foods do.
Correlational
When you block cancer cells from storing these fats and/or push them into ferroptosis, the cancer-killing effect of n-3 and n-6 fats gets even stronger.
Feeding mice a diet high in certain healthy fats (n-3 PUFAs) made their tumors grow slower than feeding them a diet high in other fats like olive oil.
When tumors are acidic, certain healthy fats (n-3 and n-6) can kill cancer cells by causing a specific type of cell death called ferroptosis, especially if you block a storage mechanism in the cells.
Eating a lot of soybean oil for a month lowered levels of DHA — a healthy omega-3 fat — in red blood cells, possibly because the two types of fats compete to get into cell membranes.
Causal
Even though soybean oil is full of linoleic acid, it didn’t make the body’s enzymes that turn it into other fatty acids work faster — so it doesn’t lead to more inflammation as some fear.