There are four major types of fat: saturated, polyunsaturated, monosaturated, and trans fat. The first three occur naturally. Trans fat is an artificial version of saturated fat. Although all four types of fat have the same number of calories per serving (120 per tablespoon or 9 per gram), each type can affect your body differently. Saturated and trans fats cause aging of the arteries and the immune system. Polyunsaturated fats do not seem to have any substantial aging effects per se. Monounsaturated fats appear to actually make you younger, helping to boost the levels of high-density lipoprotein (HDL) or "healthy" cholesterol in your blood.
More Answers from Michael Roizen, MD