First of all, Vitamins A, D, E, and K are
fat-soluble, which means they can only be digested, absorbed, and transported in conjunction with fats. In other words, you're far more likely to absorb vitamins from plants if they are eaten in the presence of natural fats. As ridiculous as that may sounds to us, consider that eating vegetables with butter and/or cream allows your body to absorb more of the bioavailable fat-soluble vitamins.
Animals also store their fat-soluble vitamins in their own fat. Dairy is a liquified transfer of fat-soluble vitamins (and other nutrients) from mother to infant. "Low fat" milk is missing natural vitamins, which is why the dairy industry must dump tons of vitamins into highly-processed low fat milk. To suggest that "fat" is empty is simply playing on people's irrational fears of fat. Eating natural animal fat has never been proven to be harmful, and people have survived off of fat-derived nutrients for thousands of years.
Dr. John McDougall (cited in that link) promotes a vegetarian diet — something that relatively very few cultures have used for nourishment over the course of human history. The only vegetarian cultures that were ever able to avoid malnutrition were ones that accidentally ate the bugs, dirt and larvae on the unwashed leaves of their meals. When those individuals moved to civilized cities, they soon became malnourished when their plant-based food was cleaned before eating. This is why all modern vegetarians require modern supplementation — much of those supplements are typically synthetic and aren't properly absorbed by the body. Children in particular require lots of fat to grow and develop. Vegetarian children especially don't get the fat and nutrition they need to grow properly.
For the most part, vegetarians tend to be undernourished because most people's stomachs are incapable of deriving much of the nutrition from plants. The human stomach most closely resembles that of a dog — which is overwhelmingly carnivorous. Whereas most ruminants (such as cows and deer) have four stomachs, don't produce hydrochloric acid and have very long intestines compared to humans.
A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again. The process of rechewing the cud to further break down plant matter and stimulate digestion is called "ruminating".
Source:
http://en.wikipedia.org/wiki/Ruminant
So, any theory that humans can unlock all of the nutrients that raw plants contain is false. To unlock those nutrients, one needs to pre-digest all of those plants — typically with lacto-fermentation or at least partially by using an acidic medium,
such as oil/vinegar, to help absorb some of the nutrients in plants. Few vegetarians seem to fully understand this. Most people think you can just eat plants and be nourished, but that isn't really so for most humans. Dr. John McDougall claims that people really don't need all that much fat-derived nourishment, and can easily find it with basic supplementation. Perhaps that's true from a survival standpoint, but anyone who eats a serving of liver or heart (or liverwurst) more than once or twice a week would find themselves with much more nourishment than any plant could produce — even if you could absorb all of the nutrients that plants have to offer. Perhaps this is why some primitive cultures resorted to cannibalism. Supposedly cannibals thrived on the energy provided from the human organ meats in their diet and often targeted the organs of fishermen, who had far more nourishment in their bodies than other people.
But, it's not surprising that many people see an improvement in their health after switching to vegetarianism. Transitioning from a Standard American Diet to just about anything will yield an improvement. You could probably switch from a Standard American Diet to a high quality dog food and also see an improvement in your health.
As for the post about oils... it's true that most modern cooking oils are terrible for you. Most refined oils have gone through a high-heat refining process that causes all sorts of free radicals to be created even before you buy them. They are basically toxic oils in a bottle. In reality, most oils are rancid by the time they are bottled and have been deodorized by their manufacturer to mask the horrendous stink that would otherwise be apparent from the refining process.
Some people believe that unrefined vegetable oils are ok, but the second you heat an unrefined vegetable oil, it starts to release harmful toxins. Olive oil may be ok on salads, but it will release some toxins when you cook with it at higher temperatures — though it's still way better than a refined vegetable oil. So, the post about oils is generally correct in that most oils in a bottle are pretty bad for you.
However, natural animal fat is the least toxic way to cook. Here's a brief, and oversimplified, explanation as to why it's better to cook with lard than oil:
http://youtu.be/6yJan2RZ9d8
If you're going to eat French Fries,
once in awhile, it's best to cook them in Beef Tallow than a modern oil. Lard, tallow and butter have been demonized by the edible oil industry/lobby, but they are clearly healthier to cook with from a stability standpoint, and natural animal fats are essential for absorbing Vitamins A, D, E and K.
, evidence does show that refined Omega-6 oils are linked to cancer. It's a batshit insane oil to be using for heating or cooking, nevermind deep frying. That includes canola which still has too much Omega-6 in it. Oils don't have to be esterified into trans-fats for their Omega-6/Omega-3 content to turn harmful.