I know very well a registered dietitian who deeply knows her stuff. She's explained quite a lot to me, and given me considerable knowledge (it helps that my field is chemistry, and while biochem is different than what I do it's not completely alien).
Unfortunately I can't say much about nutrition in one single post. Like so many things, it's a really complex and rich science and to really know something about it would take years of education on the subject. As you may imagine, everything comes with lots of exceptions and qualifiers. My recommendation if you really want to learn this stuff is to talk to a registered dietitian (any quack can call themself a neutritionist, but RD is a protected title), or read recent academic textbooks (NOT popular books) on nutrition.
About the subject matter in the main post, from my knowledge, meat and heavily processed foods tend to be more in the territory of things that are worse to eat a lot of. I can pretty certainly say it's true that many people would do better to eat considerably less of that stuff than they do.
Speaking from my area of expertise now, I'll say that it's pretty silly from a thermodynamics standpoint to eat meat. You can do fine as a vegetarian, and raising plants to eat is significantly more energy efficient than raising plants to feed animals to eat. Animal raising leads to a considerable amount of energy waste, as well as material pollution. I don't have the numbers on hand, but it's enough to be a significant factor. So what do I do? I don't eat meat very often.
Read recent academic textbooks
Any recommendations?
There's a book called The China Study. It's written by the "Professor Emeritus of Nutritional Biochemistry at Cornell University, and his son Thomas M. Campbell II, a physician". Based on what I know about the words "professor" and "emeritus" and "cornell", I assume this is written by an authority in the field of nutrition.
When it was published in 2005 it recommended clearly crazy stuff: by minimizing or eliminating the consumption of animal based foods as well as refined/processed foods (e.g. adopt a "whole food plant-based diet"), you could greatly reduce your risk of diseases of affluence like heart disease, diabetes, some cancers, etc. The book follows his 60+ year career through cancer experiments on animals, conducting a pretty large epidemiological study (the China-Cornell-Oxford study), and then discusses some important clinical trials that support his recommendations. He also surveys some nutrition literature that corroborates his research.
Some other experts vocally support him; further, his recommendations don't seem to be a radical departure from either public health recommendations or prior research in the field. The FDA MyPlate, and also the UK's health initiatives ("5 fruits/vegetables a day"), as well as the Harvard School of Public Health's recommendations and others all seem to be moving in his general direction, although seemingly filtered by politics (e.g. telling Americans to stop eating meat entirely seems like political suicide, so baby steps in the direction seem more expedient; but I'm conjecturing this).
The book is widely dismissed as vegan propaganda, but the author says he's not advocating a vegan diet and in fact criticizes vegan diets as only minimally healthier than the "standard American diet". He also conducted experiments which subjected animals to carcinogens, which is not a very vegan thing to do. He does not admit to being vegan. He even observes that the evidence says restricting animal based calories to under 10% of total calories offers almost all of the health benefits as restricting them to 0%, but says as a practical matter this is much harder to stick to (e.g. you may only eat a 3mm slice of chocolate cake is much harder than simply saying no to chocolate cake). He also admits he had a bias when he entered the field of nutrition, but a bias in favor of attempting to justify the use of dairy to cure malnutrition (he came from a family of dairy farmers). He said when he discovered that his research did not support his dairy bias he abandoned his dairy bias (and would later shut down his diary farm).
Anyway, the China Study is widely criticized, but not by people in his field? I've been watching for several years now (I adopted the diet myself in 2010), and all of the negative critiques tend to fall into (a) critiques from non-experts, (b) critiques from experts in unrelated fields, (c) health experts who agree that his recommendations have merit, but that they're impractical for the general public to follow.
(C is worthwhile, but this is a problem for public health authorities to worry about. I'm much more interested in what any sufficiently motivated individual can elect to do to maximize their health)
So, this is the part that I find most surprising. There are lots of people who are PhDs of exercise, anthropology, or economics who criticize his recommendations, but I have a hard time finding a mass gathering of nutrition scientists coming out of the woodwork to shoot down his recommendations.
What should I believe? Here are things I've considered.
1. Science is crap. Don't believe expert predictions about the natural world.
2. No no, just nutrition science is crap. Don't believe any expert predictions about nutrition.
3. Nutrition science isn't crap, but the Campbells are rogue and the community of nutrition scientists have better things to do than debunk pop culture books.
4. Nutrition scientists **are** criticizing him in droves, I just don't come across them because I have confirmation bias blinders on.
5. "Nutrition scientist" is a made up discipline, and I've been tricked!
I'm more or less at a loss on how to make progress on these points. Am I missing something crucial?
What's the LW take on this? Why isn't this good enough to inform your dietary choices? Assuming you don't plan to become an expert in the field of nutrition yourself, what's a better way to inform your dietary choices?
EDIT: I would just like to thank everyone who responded. I've tried to discuss this in many forums, both IRL and on the internet and it's almost always a disaster unlike here on LW. Your measured, insightful responses are an enormous relief. You've given me a lot of food (ha!) for thought.