Many people consider the nutritional information they get to be a maze of contradictions. What is good for you one day is bad for you the next. Much of this problem is caused by the propensity of the media to report only on the latest scientific study without regard to how well the study was done or whether it conflicts with all other studies done on the same subject. There is not enough room here for a comprehensive overview of the subject, but hopefully this will help fill in the gaps in the generally available information. There are many types of fats and oils.
Saturated fats are the most chemically stable.
Some saturated fats have been strongly linked to elevated cholesterol and to cardiovascular disease. These saturated fats cause the body to produce excess LDL cholesterol, which then oxidizes and builds up on the inner surfaces of the human circulatory system. Polyunsaturated fats are the least stable and can easily become rancid at room temperature, especially if they do not contain antioxidant preservatives. The chemical instability of polyunsaturated fats is believed to be the reason why they have been linked to cancer in laboratory animals. Monounsaturated fats are widely thought to be a good compromise between saturated and polyunsaturated fats.