This is a good question and commonly misunderstood, and and area with common misdirection from the industry wishing to sell more vegetable oils.

It is an issue of simple chemistry. When a fat is solid at room temperature it is stable, and if good quality and free from chemicals and preservatives it can be recognised by the body and used for helping the body function and heal itself. The same can be true of vegetable oils, which when at room temperature, are liquid and less stable, but in their original form can be used by the body.

The problem comes when you HEAT these oils, the more solid a fat the more it resists heating. Think of what burns more quickly on your fire, a lump of coal or a bag of wood chippings. The solid fat such as butter, ghee or coconut oil will remain in a useable state by the body until it gets to about 150-160 degress Celsius, whereas sunflower oil, canola oil and other vegetable oils become oxidized and rancid at approximately 60 degrees Celsius.

I can think of many cooking processes that remain under 60 degrees. Note though it is not good to over heat or heat for a long time any oil as this this will oxidize all fats in the end.

So all vegetable oil spread that are made are heated to over 120 degrees celsius. Those vegetable oils will have been turned to oxidised, rancid fat. That is why the heated solid oil has to be heated and de-odorised then coloured yellow. This tricks the unknowing and accepting public into eating it. Then they add plant sterols in the name of heart health!! How ironic.

So solids fats for cooking, vegetable oils are ok when not cooked, and olive oil and nut oil are somewhere in between.

I’ll make this into a full article soon, thanks for the question.