Doctors recommend that we should cook our food in oil rather than in vanaspati ghee. Why?

Cooking oil is a plant, animal or synthetic fat used in frying, baking and other types of cooking. It is also used in food preparation and flavoring that doesn't involve heat, such as salad dressings and bread dips, and in this sense might be more accurately termed edible oil. When vanaspati is made, trans fatty acids are also produced; these increase the risk of heart disease when consumed in excess and are best avoided.