I think the answer is mostly two, with a little side of four. Nutrition science is very difficult to perform correctly, because of the difficulty and cost of randomized controlled trials of diets. In particular, the China study was not controlled, and it was performed via surveys (which have a reputation for being inaccurate in diet studies). Added to that is the fact that food isn't really about food. It's deeply intertwined with our sense of personal identity and culture, and that leaves you with many researchers stretching their relatively weak evidence a little bit further than is warranted. Once you get as far removed from the actual science as recommendations from the FDA or Harvard School of Public Health, there is virtually no room left for nuance, and they are already making recommendations from a public health perspective, which is not the perspective you seek.
With respect to confirmation bias, I suspect there is some of that going on. Specifically, you say, "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." There are probably not "mass gatherings of nutrition scientists" who disagree with the China Study, but there are mass gatherings of experts in medicine and various biological fields (and maybe the odd nutrition scientist) who would probably disagree with the China Study. Googling "paleo conference" I could easily find several such mass gatherings. So you aren't wrong, but you are implying more with your selection of outsider expertises than is warranted (exercise, anthropology or economics, rather than doctors, biologists, etc.).
With respect to the object-level consideration, I recommend you read Stephan Guyenet's recent series of articles on meat Is Meat Unhealthy?, which considers both positive and negative factors of eating meat. He is not a nutritional scientist (he is a neurobiologist), but he is by all appearances a careful scholar. You can read the summary first if you want to see his ultimate conclusions, which don't strongly support either the "vegan" or the carnivorous sides of the debate.
However, with respect to the China Study itself, he only touches on it in this series thusly:
The China Study
The China Study was a massive ecological study relating diet and lifestyle to chronic disease risk in China. It has been invoked by researcher and vegan diet advocate Colin Campbell to support the idea that animal foods promote cardiovascular disease and cancer, even in the small quantities that were typical of the regions studied. After having reviewed the study data, the publications based on it, and the various commentaries on it, it appears relatively clear that the China Study does not support the conclusion that meat consumption is associated with cardiovascular disease or cancer risk (17, 18, 19, 20, 21). Everyone seems to agree on that, except Campbell and certain other vegan diet advocates. I won't discuss the China Study further.
And elsewhere has criticized it on methodological grounds (e.g. here)
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.