The most popular way of arguing that fish oil will prevent heart disease is to show that it lowers blood lipids, continuing the old approach of the American Heart Association's "heart protective diet." Unfortunately for that argument, it's now known that the triglycerides in the blood are decreased because of the fish oil's toxic effects on the liver (Hagve and Christophersen, 1988; Ritskes-Hoitinga, et al., 1998). In experiments with rats, EPA and DHA lowered blood lipids only when given to rats that had been fed, in which case the fats were incorporated into tissues, and suppressed mitochondrial respiration (Osmundsen, et al., 1998).
The belief that eating cholesterol causes heart disease was based mainly on old experiments with rabbits, and subsequent experiments have made it clear that it is oxidized cholesterol that damages the arteries (Stapran, et al., 1997). Since both fish oil and oxidized cholesterol damage rabbits' arteries, and since the lipid peroxides associated with fish oil attack a great variety of biological materials, including the LDL lipoproteins carrying cholesterol, the implications of the rabbit experiments now seem very different.