When people talk about fat as a cause of heart disease, they usually mean saturated fat, which includes most types of animal fat. There is a large number of saturated fatty acids (SFA) which vary according to the number of carbon atoms that make up the chain. Only three of them, with chains of medium length (called lauric, myristic, and palmitic acids) actually raise cholesterol; those with shorter or longer chains have no effect. The other kind of fat that has been shown to have a bad effect on risk factors for atherosclerosis is hydrogenated vegetable oil.

Good Fats and Bad Fats

Fats Bad for the Heart	Fats Good for the Heart
Saturated fats (SFA)	Poly-unsaturated fats (PUFA)
Trans fats	Mono-unsaturated fats (MUFA)

The two other types of fat, polyunsaturated and monounsaturated fats, most of which are of vegetable origin, are also important. From a health point of view, these are good fats, while the saturated fats are the bad fats, because of their different effects on lipids and blood pressure, as shown in the Table below.

The vegetable oil PUFA (poly-unsaturated fats) are also referred to as omega-6 fatty acids, the most important of which is called linoleic acid. Their chief beneficial effect is that they lower LDL (bad) cholesterol, but they may also lower HDL (good) cholesterol. They also inhibit blood clotting.

Fish oil PUFA are the omega-3 fatty acids, which inhibit the clotting process.

Monounsaturated fats (MUFA) are found in some vegetable oils and nuts. People who eat a Mediterranean diet rich in olive oil (which contains the MUFA oleic acid) have low cholesterols and little heart disease. As shown in the Table, MUFA have the most beneficial effects on the blood lipid profile, since they lower LDL cholesterol without adversely affecting HDL (PUFA tend to lower HDL).

Effects of Different Types of Fat on Blood Pressure and Blood Lipids

	LDL cholesterol	HDL cholesterol	Triglycerides	Blood Pressure
Saturated Fat	Increase	No change	Increase	Increase
Hydrogenated oil	Increase	Decrease	Increase	Increase
MUFA	Decrease	No change	No change	Decrease
PUFA	Decrease	Decrease	Decrease	Decrease

The traditional view is that a diet that is likely to provide the best protection against coronary heart disease is one that has a high quantity of polyunsaturated fats and is low in saturated fats, or as it's sometimes stated, is a diet with a high P/S ratio.