We don’t normally think of vegetables as being oily or fatty, so where do vegetable oils come from? Mostly, they come from the seeds of various plants (usually cotton, corn, soy sunflower, and safflower) which often require quite a bit of processing in order to extract any oil. Possibly the most important part of living a Paleo lifestyle is to eat real, whole foods with little to no processing, and vegetable oils just don’t qualify.
Paleo and PUFAs
The processing of the oils isn’t the only concern. The seeds used to manufacture them are chock full of polyunsaturated fats (PUFA) which are highly unstable. Exposing them to heat, light, and even air can cause them to oxidize, and that’s when it really gets dangerous. Consuming oxidized fat leads to oxidized cholesterol in our blood and that’s the stuff that is likely to latch onto the insides of our arteries, causing inflammation and plaque build-up.
Omega-6 vs. Omega-3
Not all PUFAs are bad though. While omega-6 fats are inflammatory, omega-3 fats have anti-inflammatory properties and have actually been shown to confer a number of health benefits. Vegetable oils are dangerous because they contain far more omega-6 than omega-3 fats. Ideally, we should be consuming at least as many omega-3 as omega-6 fats in order to counteract the inflammation caused by the omega-6 fats. Consuming foods cooked in rancid vegetable oils makes it almost impossible to achieve a good ratio.
Mark Sisson has even more reasons for avoiding vegetable oils.