Have you ever heard of any danger in polyunsaturated vegetable oils from the way they are refined?
Mainly that they are already rancid [and bad for you] before they leave the factory?
What do you think?
-- Jade (last name withheld)
Conventional refined oils undergo lengthy processes involving heat and chemical solvents.
In the case of polyunsaturated oils, this is mostly done to make them suitable for high-heat cooking (i.e.: deep frying) as they are easily degraded by heat in their unrefined state.
Several nutrition extremists online -- none of whom appear to have any nutrition credentials -- claim refined polyunsaturated oils are "cancer-causing."
Let's return to reality.
The issue with refined oils isn't so much that they are already rancid -- at some point in the processing, ascorbic acid (vitamin C) or tocopherols (vitamin E) are added to lengthen their shelf life -- but that they are nutritionally inferior to unrefined oils due to high amounts of phytonutrients and antioxidants being lost in the refining process.
This is why, when it comes to polyunsaturated fats, the recommendations are mainly to consume whole foods containing these heart-healthy oils (i.e.: nuts, seeds, fatty fish) as opposed to the oils themselves.
Unrefined polyunsaturated oils consumed at room temperature (i.e.: drizzling walnut oil over a salad) contain more antioxidants and phytonutrients than refined oils, but are still somewheat nutritionally inferior to whole foods.
Some Internet health extremists (the type who believe nutrition experts are part of a giant "conspiracy") go as far as saying that polyunsaturated fats are inherently unhealthy and unnecessary for human health. Quite a laughable claim, since Omega-3 and Omega-6 fatty acids (which we must get through diet since our our bodies can not produce them) are both polyunsaturated fats.
So while an unopened bottle of corn oil is not cancer-causing and rancid, there are better ways of adding polyunsaturated fats to your diet.