Long story short, plants that live where it is cold need to use less stable oils with a lower melting points. These oils lead to our cells being more fragile, and also lead to more free radicals.
Particularly if the oil is repeatedly heated. As happens in restaurants. And even more so when deep frying fast food.
For any kind of nutrient, saying that it is good or bad, without saying at which daily intake that is true, is totally meaningless.
Any essential nutrient, whose absence from your food can kill you, can also become a poison over a certain daily intake. For certain essential nutrients, the difference between the optimum daily intake and the lethal daily intake may be quite small, e.g. for selenium.
In the case of linoleic acid, it is likely that 10 g/day is very healthy, but 50 g/day is very unhealthy. It is not known for sure where the threshold between healthy and unhealthy lays, but it might be between 20 g/day and 30 g/day.
Most of the cheapest vegetable oils contain between 60% and 80% of linoleic acid.
With all kinds of industrially-made food containing large amounts of cheap oils, it has become very easy to eat too much linoleic acid per day, without being aware, like also too much sugar.
Wait a moment. The stereotypical unstable oil is alpha-linolenic acid, which seems to be generally regarded as quite healthy. Certainly oxidized ALA is disgusting (keep those flax seeds in the freezer!), but I’ve never heard of omega-3 fatty acids leading to fragile cells.
ALA is very unstable as oils go. It’s an omega-3 fatty acid. It’s generally regarded as healthy. The claim that unstable oils are somehow bad for you because they are unstable in your body seems entirely unsupported.
Certainly flaxseed oil is a terrible cooking oil unless you are trying to make furniture [0], but that’s a very different claim.
[0] You can also use flaxseed oil to produce a beautiful but mostly useless seasoning on cast iron. But the inside of your body is not made of hot iron, and the reactions that occur on a skillet don’t seem particularly relevant to what happens if you eat the oil.
Long story short, plants that live where it is cold need to use less stable oils with a lower melting points. These oils lead to our cells being more fragile, and also lead to more free radicals.
Particularly if the oil is repeatedly heated. As happens in restaurants. And even more so when deep frying fast food.