Well, if you consider that evidence is now pretty supportive of the idea that man actually cooked meat WAY earlier than originally thought, it wouldn't be that hard to eat 'paleo'
Also, i am not sure on this, but I think I remember reading about Fitch years ago (maybe after he had a fight in TKO, an old Canadian promotion?) showing up to a gym with nothing but his car and everything he owned in it and sleeping on the floor at night. Maybe i have the wrong person though, as I said, it was a long time ago.
And yeah, some people stop eating meat and feel (or even think they feel, placebo is a strong effect, it should NEVER be underrated) better, some are athletes, a handful are fairly high level. Other athletes start from nothing and begin to make enough money to eat MORE meat, and start performing much better.
A few years ago there was a great bit of reporting on Cuban bodybuilders, very neat look at a group who was dirt poor, and the difference between those who could afford expensive meat and those who could not. Very interesting and it had a lot of tie-ins to different peasant populations (now and also in the past) and their eating habits, like how chinese soldiers in ancient times had a diet that included meat in every meal, something the peasants of the nation could never have (except for fishermen, but I think it was more about red meat).
Also, have no doubt that someone out there gets the entirety of their calories from a single food source, like bananas. Somewhere out there, there is at least one person doing or surviving on pretty much everything and anything. There are a lot of batty people.
I've yet to come across a single good argument against the idea that the best course you can take regarding nutrition is to eat a wide variety of quality foods. I've also listened to people try and raise hell about genetically modified foods who actually thought wheat and corn occured the way they currently are in nature, and didn't realize that apple trees are possibly the most succesfully cloned organism of all time (and also not native to North America, I lived by an apple orchard as a kid
).
There's a good chance the louder someone is yelling about nutrition, the crazier they are, and the more they are pushing a product or lifestyle, the more they just want either acceptance or your money.