Is fat bad for you?
Almost every time I go to the supermarket, the loudspeakers advertise some foodstuff with a message that finishes with "and it's low in fat" or "it's fat free". Food packaging carries messages such as "97% fat free". There are many diet and cookery books whose title is centred on the "low fat" or "no fat" idea. From such sources people pick up the idea that "fat is bad for you".
And yet it is only a half-truth. To understand why, consider an analogy.
There are people who damage their health by drinking too much alcohol, or even kill themselves that way.
Do we conclude that all liquids are bad for you and that therefore we should cut back on them? Of course not, because we all see the fallacy of that argument.
We know that there are many types of liquid.
We know that there are liquids that will harm you even in tiny quantities, e.g. methylated spirits.
We know that there are liquids that are harmful if taken in excess, e.g. red wine.
We know that there are liquids that are good for you.
We know that there is one liquid (water) that is essential - a complete lack of water will kill you.
Exactly the same considerations apply to fat.
There are many types of fat.
There are fats that will harm you even in tiny quantities (e.g. the trans-fatty acids found in hydrogenated vegetable oil, which is used in many manufactured foods such as some brands of margarine).
There are fats that are harmful if taken in excess e.g. some types of saturated fat.
There are fats and oils that are good for you. E.g. they can help with the absorption of fat-soluble vitamins. They can slow down the digestion of carbohydrates which is of benefit. They can be a useful source of calories, so that you do not need to get them all from carbohydrate and protein - a balance between the three sources is better and more pleasant.
There are fats/oils that are essential - a complete lack of them would kill you. They are the ones based on omega 3 and omega 6 essential fatty acids. In our typical western diet, the omega 3 fats/oils are in short supply and most people would be healthier if they had more.