Because the evidence pointed so strongly to oxidized oils as the major culprit in atherosclerosis, scientists assumed that polyunsaturated oils were being oxidized within the bloodstream.

But new evidence indicates that, in most cases, the oils are already oxidized when they are eaten. That is, when they are on your plate and in your salad bowl.

When exposed to air, polyunsaturated oils quickly oxidize, especially if heated.

These oils include corn, soybean, peanut, safflower, sunflower, and even canola oil, which contain both omega-3 and omega-6 polyunsaturated oils.

While omega-3 oils are healthy, they should not be used in cooking or exposed to room air. This is why I always tell people to keep their omega-3 oils in the refrigerator — it reduces oxidation.

In nature, such oils contain varying amounts of vitamin E (tocopherols and tocotrienols), which help prevent oxidation.

But when vegetable oils are processed, the vitamin E is removed and never replaced, making the oils very susceptible to oxidation.