What is polyunsaturated fat?

Definition: What "saturated" fats are saturated with is hydrogen atoms. "Unsaturated" fats do not have hydrogen atoms at every available bonding site on the molecule. Polyunsaturated fats have more than one of these bonds which lack hydrogen. Oils with primarily polyunsaturated fats are liquid even in the refrigerator. The two main types of polyunsaturated fats are omega-3 fats and omega-6 fats, which are essential fatty acids, meaning the body cannot manufacture them.
Although polyunsaturated fats are generally thought of as healthy, there are at least two possible exceptions to this statement. One is that there is some evidence that polyunsaturated fat, in excess, can contribute to cancer risk. It is thought that the reason for this may be that polyunsaturated fats turn rancid more quickly than other fats. Therefore, proper storage of oils with high amounts of polyunsaturated fats is recommended. Store in a cool, dark place before opening, and in the refrigerator after opening.
The other potential issue with polyunsaturated fats is that of the two main types of polyunsaturated fat, the amount of omega-6 fat that we tend to consume has increased dramatically in recent decades, while in general the amount of omega-3 fat we consume is lower. Some experts feel that this imbalance is contributing to inflammation in our bodies, and increasing the risk of such chronic diseases as arthritis, diabetes, and heart disease.

Oils high in polyunsaturated oil include:
Corn oil
Soy oil
Regular safflower and sunflower oils (i.e., not "high oleic" oil),
Cottonseed oil
Flax seed oil (high in omega-3 fats)

http://lowcarbdiets.about.com/od/glossary/g/polyunsaturated.htm

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A type of animal and vegetable fat, the molecules of which consist of long carbon chains with several (poly-) double bonds. It is less likely to be converted into cholesterol in the body.

Oils with primarily polyunsaturated fats are liquid even in the refrigerator. The two main types of polyunsaturated fats are omega-3 fats and omega-6 fats, which are essential fatty acids, meaning the body cannot manufacture them.

Foods high in polyunsaturated fat include a number of vegetable oils, including soybean oil, corn oil and safflower oil, as well as fatty fish such as salmon, mackerel, herring and trout. Other sources include some nuts and seeds such as walnuts and sunflower seeds.