Cooking with oil
Light, temperature and air all denature oil making it bad to consume…
What you are trying to avoid is heating any polyunsaturated fats so if you are ever wondering about an oil you can look it up and see if its got lots of polyunsaturated fats in it. Only saturated fats should be used for cooking with at high temperatures (e.g. the best would be coconut oil, or less good, but still a saturated fat, butter).
Extra virgin olive oil is thought to be ok as it is a monounsaturated (single unsaturated bond) + so is less sensitive to heat - i.e. can be heated but to a lesser amount than coconut oil.
Vegetable oils do have polyunsaturated (many unsaturated fat) which means they should not be heated! They can be put onto warm food after its been cooked, but better if you wait for the food to cool, so as little heat touches the oil as possible.
You should keep all oils away from light in the house wherever you store it, and store it in a dark bottle - thats why you usually see olive oil in a dark bottle, but this applies to all oils. You should also close the lid as soon as you have used it, allowing as little air to enter the bottle as possible.
Things like linseed or flaxseed oil should also be kept in dark bottles and in the fridge, they should only be used cold for say dressing a salad never to be cooked with.
You hear about 'trans fats' being bad for you. As a fat turns into a trans fat it becomes more solid. Hence, margarine is having turned oil into a trans fat, making it that buttery composition (and making it bad for you). Equally, when you reheat oils many times they get more sticky, this is whats happening - the oils turn into trans-fats.
In general, it would be best NOT to cook with oil at all, and instead to add some organic extra virgin olive oil to your food after it’s cooked!