we have all heard about the three basic classes of food viz carbohydrate, protein and the alledged culprit in health conditions, FATS. All the aforementioned have roles to play in the human body and one cannot be essentially stated to be superior to the other. Our subject, fat, to many people is a villain in diets. Like humans, some fats are good while others may be bad depending on the level of intake and frequency. Permit me to include oils in this context to mean the same as fats because they have the same chemical structure but their physical appearance is different in that the former is liquid at room temperature while the latter is solid at room temperature. It is noteworthy that oils come from plant origin except some like fish oils, while fat is predominantly from animals. let us define some terms:
-- Saturated fats : these are fats that are solid ( not as hard as stone)at room temperature and they gradually become liquid when heated. These type of fats albeit appear to be semi-solid are also present as emulsion in liquid foods such as milk. about 3.5 g of fat is present per 100 g milk, of which 65-70% of the 3.5 g total fat is saturated fat. Milk is a healthy food, is it not? it is important in the diet of infants and even adult as a source of dietary protein, carbohydrate,fat as well as the most abundant mineral in the body CALCIUM.
-- Unsaturated fats: they are fats, can be called oils that occur as liquid at room temperature. When these type of fat predominantly from plant origin except oils from fish are forced to look and function like saturated fats, then another type of fat comes up known as Transfat. Transfat rarely occur in nature but industrial hydrogenation of oil has led to this, the major goal of hydrogenation in its formative years was to increase the shelf life of oils. Transfat is not a good fat to be consumed and should be reduced to the barest minimum in our diet, that is why you see on the containers of vegetable oils " NO TRANS FAT". Magarine is hydrogenated unsaturated fat and so it may contain Transfat if hydrogenation is not complete. Butter(dont want to mention brand name)as we generally call it is also hydrogenated vegetable oil fortified with minerals and vitamins. Originally butter is the fat got from animal milk like that of cow,goat,sheep and buffalo and not from plant origin. As such we should reduce the intake of hydrogenated vegetable oils like salad dressing and the earlier mentioned. These are the type of fat that can be implicated as a risk to health.
In contrast, naturally occuring unsaturated fats such as those found in groundnut,soya, avocado etc are good oils that contribute to good health if taken in adequate proportion. Do you know some type of unsaturated fat called polyunsaturated fats can help to lower blood cholesterol, these are the class of unsaturated oils found in fish and to a lesser extent in some nuts, they also have anti inflamatory response. You probably have heard about Omega-3, they are good. Omega-6 and 9 are also found in oil seeds like peanut, soya bean sessame seed etc and also contain varying amount of saturated fat too. Short and medium chain saturated fats are not as bad as we think , they are also good but unsaturated fats should be what we consume more in our diet. Overall,we should not consume more than 30-35% of fats in our diets and maximum 10% saturated fat out of the 30%
Most of us take vitamin supplements or eat some foods because of the minerals and vitamin they contain, do you know some of these vitamins can only be used by the body only when transported by fat. these vitamins include Vits. A(eye health,skin health, iron metabolism, enhance fertility etc),D(facilitate calcium absorption and mineralization into bones and teeth, also it regulates blood calcium level),E( an antioxidant to prevent body cells from attack by free radicals -implicated in cancer and diabetes) and K ( functions in blood clotting).
Fat serve as insulation during cold weather
Fat provide more energy that carbohydrate and protein. approximately 2.25 times more energy per gram
Fat protect vital internal organs like the heart and kidney .
Fat store up energy for later use especially during shortage of carbohydrate in the body, in addition fat dependent vitamins can be stored by
the body in fat cells for later use.

Prostaglandins are substances having hormone like funtions such as contraction of smooth muscles, good vasodilators (relaxation of blood vessels), avoid the formation of blood clot in blood vessels-when blood cloth in blood vessels, that can lead to blockage leading to cardiovascular problem, stroke can even occur if the artery supplying blood to the brain is blocked. Guess what? The precursor for prostaglandins are [b]Omega-6 fats[/b]present in soya oil, groundnut oil, sessame seed oil, cashew oil etc
You can now see that fat is not evil, only adequate amount need to be consume, too much of anything is not good, even carbohydrate must not be too much because it can lead to obesity,
As for cholesterol, avoid it . whether you take in cholesterol or not the body produces 300 mg everyday because without it some hormones cannot be synthesized such hormones are pregnancy hormone progesterone and estrogen, also vitamin D synthesis needs cholesterol intermediate to be complete. Our body cells maintain their fluidity and integrity due to the presence of sterols(cholesterol) in their membranes.
I am not saying taking cholesterol is good but this is just to let you know what you term as BAD. Nothing is useless!