"The amount of fat stored depends on the caloric balance of the whole organism. Excess calories, regardless of whether they are in the form of fat, carbohydrate, or protein, are stored as fat.  consequently, appreciable amounts of dietary carbohydrate and some protein are converted to fat. The body can make saturated and monounsaturated fatty acids by modifying other fatty acids or by de novo synthesis from carbohydrate and protein. However, certain polyunsaturated fatty acids, such as linoleic acid, cannot be made by the body and must be supplied in the diet."

http://www.iseo.org/foodfatsoils.pdf