A single experiment, especially if it has a small sample size---but even large sample sizes can be ruined by experimental error etc.---or even a small number of such experiments which confirm each other---just aren't going to give very reliable results for a complex system.
When things are tightly controlled in a lab for a simple system (with few variables), then basic statistical methods can yield believable results. This is why the replication problem is mostly in psychology, medicine, and social sciences (I presume biology & ecology should be on the list, but they aren't so consequential maybe, except for the biology that is already in the medical category). Those disciplines work with complex systems that we have little macro-level understanding of and do not yield themselves to be controlled well in a lab. Whatever lab controls can be instituted really control very little because we are often talking about human persons who are complex biological systems themselves.
Of course the other problem is that the researchers themselves generally do not understand statistical theory. And that is a very big factor here generally. You basically have to be an actual statistician or very quantitatively/analytically skilled. There is statistical literature, for example, about the problems with p-values and statisticians trying to figure out what to do about that since nobody understands these things.
All that being said, with many iterations of experiments addressing the same question, say, done by many different research teams (say in different places and at different times, to whatever appropriate level), then we can start to see real physical connections emerge. it might still be the case that some hidden confounding variable is the ultimate arbiter of whatever relationship is observed, but the observed relationship will still be real but just mediated but unknown observables. It would be better to know the precise causal chain, but it's still pretty cool to know a real relationship still, even if it is only correlation and not causation.
It costs a lot of money to do so many repetitions. So for something that doesn't ultimately have much economic impact, there is no incentive to spend a lot of money to get some small bits of information to be cataloged away and rarely if ever accessed. Even if large sums of money are spent to answer some question about a complex system, the outcome might still be uncertainty about whether the relationship observed is real.
I have a theory about this. It’s because diet, in particular above literally everything else, is extremely core to who a person is. In fact, diet encapsulates culture to a large degree. The ethical problem of killing animals is obvious to literally every person though. So when you say you are vegan or vegetarian, it is implied that you are saying you have absolved yourself from a near universal ethical problem, and you are directly attacking the culture and identity of the person and identifying yourself as a superior human. Even if you have no tinge of that in your mind (which I think is highly unlikely for someone who is freely choosing the switch to vegetarianism based on animal welfare or optimizing personal health — making the switch by force due to health problems is different, but people will always assume the former reason). So a vegetarian is quite literally attacking another person by identifying themselves as such… at least that is how the other non-vegetarian person takes it. So it’s natural they have the strongly negative reaction. Vegetarians and especially vegans are the odd man out here. They are the unusual people bucking established human behavior of hundreds of thousands of years. Being able to reject food is a new development for the most part.
Note that I’ve been vegetarian for about 3 decades and still am.
I very much appreciate this article and strongly agree with most of it. It’s almost certainly optimal for most people to eat a plant-forward whole foods (minimally or at least freshly processed) diet, but that will not be true for everyone, especially those with rare digestive or autoimmune conditions.
I might add one point though: it could be the case that dietary needs and tolerances are strongly connected to early life experience and conditioning. For example consider that the prevalence of peanut allergies might actually be caused to some degree by people avoiding giving their children peanuts. When a human body learns that it gets what it needs from certain inputs, retraining may become impossible if not extremely difficult. And sensitivities can develop (or go away) due to life experience. I've had my dietary sensitivities change over time, personally. Of course, I'm just going off of my own experience of those sensitivities and not on researchers studying them.
So if any arbitrary human were raised on a plant-based, whole-foods diet, say incorporating ferments and seaweeds, etc. to get even B12, or at least dairy products, then they would be fine for the most part. Their body would learn how to extract the necessary nutrition.
My point is that it might not just be genetics that, for an adult, determines what their current optimal diet is. It might be the case that their optimal diet would be different if they were raised on a different diet. Of course, there are always trade-offs. In this imaginary world, it is almost certainly that a unique person would eventually come into being who has problems with the established diet and needs to begin eating animal flesh in order to establish optimal health. I would bet such an occurrence would be rare though, but of course I don’t know for sure. Having the entire culture veg reverses most of the economic concerns though. The reverse economic situation arises, where eating meat has added cost.