PUFAs—short for polyunsaturated fats are a type of fatty acid. The type of fat that is available to us via the foods we eat generally fall into two categories: unsaturated fat and saturated fat. For ages we were advised to avoid saturated fats and emphasize unsaturated fats (or avoid fat all together). Many cooking oils actually said "heart-healthy" on the bottle. The research is overwhelmingly clear that saturated fats are not only healthy, but that they are extremely beneficial for overall health. What is not often talked about is the downright damaging impact of the class of highly unsaturated fats known as PUFAs.
Where are PUFAs present?
PUFAs are present in nearly all foods (even vegetables). While some extremists would say to avoid high-PUFA vegetables I don't believe you need to go that far. Small amounts of unprocessed PUFA in a well balanced diet are unlikely to cause any issues. The most damaging PUFAs are the processed liquid cooking oils below:
Canola oil, Grapseed Oil, Corn Oil, Soybean Oil, Generic Vegetable Oil, Walnuts Oil, Cottonseed Oil, Sesame Oil, Peanut Oil, Margarine, Flaxseed Oil.
Why should you avoid them?
The primary issue with PUFAs is that they are highly unstable. All fats have a temperature with which they oxidize (i.e. become unstable, go rancid, become toxic). For PUFAs that temperature is very low. An easy way to remember this is that UNsaturated fats are UNstable and Saturated fats are Stable. Unstable fats are prone to oxidation. Oxidation lead to free radicals. Free radicals lead to cellular damage in your body that can manifest both internally in the form of damaged organs/glands and externally in the form of rapidly aging skin. In addition, many of these fats can become unstable before they even enter your body. During processing, many of these "seed" oils are exposed to high heat (imagine trying to extract oil from a grape seed), which can often make the oils go rancid. In addition, these oils may be exposed to high heat during travel which can also lead to a damaged and unstable product. This is one of the reasons many of these oils are shipped in dark containers and are required to be refrigerated.