Nutrition is one of the areas where normally rational people (such as the typical HN reader) go totally off the deep end and rely on superstition, hearsay, herd mentality, group think, and whimsy. In general, most people (including people who are otherwise quite rational) do exactly what advertisers and corporate sponsors want when it comes to nutrition.
There was a huge bombshell study a few years ago, documented in the book "The China Study" on the western diet (by a guy from Cornell). It should have made headlines. People should have thrown their meat and dairy into the garbage immediately... but nothing happened.
Instead, ostensibly "health conscious" people convince themselves that there are "good fats" and ask themselves if they are getting enough protein.
Most Americans are overweight and will end up with obesity, type II diabetes, heart disease, or colon cancer. Most vegetarians are overweight, pallid, and sick-looking (b/c they eat so much "good fat" -- more fat than McDonalds aficionados eat!).
What it means for a human to eat optimally is well known and has been backed up by rigorous scientific research... why hasn't it made headlines? Because in this case it's too much work to think critically... it's not easy going against the grain of an entire society built upon destructive mythology with the nutrition establishment telling us to eat according to a harmful food pyramid, etc.
So the whole "nutrition" industry is focused on convincing people that health is obtained by consuming or avoiding magic ingredients like "Omega 3s" or "turmeric" or "trans fats" or "mono unsaturated fats" or "whole grains" or "protein". All of this is at best minimally correlated or patently false.
What's the difficult truth that nobody wants to hear? Animal protein causes disease. That means no meat, fish, milk, etc. The optimal human diet is plant based and should contain 80% carbs (as much of this unrefined as possible), 10% protein, and 10% fat). Fresh fruit has a few percent fat already, so very little extra fat is needed, even if it's so-called "good fat".
If you're skeptical, read The China Study and then raid your fridge and cure your future cancer immediately :)
1) humans can survive on lots of things including meats, cardboard, etc. b/c we evolved to survive in extreme conditions/habitats. That does not mean that all things we can digest are optimal for health. Also, it is trivial for any modern human to obtain sufficient calories only from optimal foods.
2) link to book (well worth the $10, read the reviews on Amazon):