In human health and nutrition, the concept of good fats and bad fats is typically used, especially with respect to dietary risk factors for coronary artery diseases and stroke. This concept originated with the notion that there are 2 forms of cholesterol that, for the sake of simplicity, have been referred to as good and bad. The concept has now been extended to include other dietary fats that may also be associated with increases in cholesterol concentrations.
Dietary fats, including cholesterol, are transported in the blood as lipoprotein complexes. They include LDLs and HDLs. Because LDL, which transports cholesterol into tissues, is involved in the formation of atherosclerotic plaques in humans, it is considered by health professionals to be the bad form of cholesterol. This is especially the case when there is an overabundance of cholesterol that can become deposited in arteries, which results in atheromatous lesions. By contrast, HDL has the capacity to transport cholesterol away from tissues and is thus considered to be the good form of cholesterol. The amount of cholesterol each of the lipoproteins carries, relative to the total amount of cholesterol, partially determines the amount of risk that an individual may have for developing atherosclerosis, coronary artery disease, or stroke. Thus, for humans, fats containing LDL cholesterol are bad fats and fats containing HDL cholesterol are good fats.

Other than cholesterol per se, dietary fats containing specific fatty acid types are also considered to be bad or good for humans, depending on whether they have been associated with increases or decreases in total plasma cholesterol concentrations. Fats that predominantly contain saturated fatty acids are referred to as saturated fats, whereas those containing high proportions of polyunsaturated fatty acids are termed polyunsaturated fats. Saturated fats from animal and some plant sources that typically increase cholesterol concentrations are examples of bad fats. By contrast, good fats generally include polyunsaturated fats, containing omega-6 fats from oilseeds that typically lower plasma cholesterol concentrations. Polyunsaturated omega-3 fats from marine sources typically decrease inflammatory processes and blood pressure and also result in several other desirable effects; they are also considered good fats.
In general then, polyunsaturated fats are considered to be good fats. Even CLA, a type of polyunsaturated fatty acid, is considered by some to be a good fat, and researchers continue to investigate its effects. It has been proposed that CLA may help promote lean body mass or that it may even have antitumorigenic effects. By contrast, trans-fatty acids, even though they are unsaturated, are metabolized in humans in a manner similar to that for saturated fats and are generally considered to be bad fats.

Finally, monounsaturated fats, such as oleic acid, are fairly neutral with regard to their effects on cholesterol. However, in some circumstances (ie, the Mediterranean diet), they have their proponents.

The reality is that the overall risk of heart disease for humans is not as simple as this brief summary may indicate. However, suffice it to state that the good fats-bad fats concept is a handy way for humans to keep track of dietary habits and risk of heart diseases, and it can be used in discussions with health care providers. As such, it is a useful concept.