Why do polyunsaturated fatty acids lower serum cholesterol?

Replacement of saturated by polyunsaturated fatty acids in the diet may lower serum very low-density and low-density lipoprotein concentrations because the liver preferentially converts polyunsaturated fatty acids into ketone bodies instead of into very low-density lipoprotein triglycerides. Thus unlike saturated fatty acids, polyunsaturated fatty acids are transported to the tissues for oxidation without leaving a trail of lipoprotein remnants in the form of low-density lipoproteins.