At this point in time, there is not likely a more controversial diet than the Paleo diet. Advocates tout how much better the vast majority of people who undertake it feel as well as the numerous health improvements, primarily autoimmune diseases that go in to remission, that people see under a Paleo diet. Critics argue that there wasn't a single Paleo diet, per se, and that grains, legumes and dairy have been a major part of a healthy diet for many years. The question then becomes, "So who is right?" Therein lies the problem, the argument between the advocates and critics really isn't about who is right, it's about whether or not the science supports the Paleo diet or not. The problem here, in my opinion, is that those criticizing the diet set the bar at a level that won't likely be met for some time, but that doesn't mean the supporters of Paleo are wrong.
I've been in many discussions with people who basically believe the Paleo diet is poppycock. I've argued with trolls on the internet, I've had physicians tell me it's complete BS, and I've read Paleofantasy in utter surprise that a higher level scientist would make several errors in logic in a published piece. I will spare you the arguments on straw men such as the Paleo diet is low carb or that it's basically a bacon orgy. I will focus on the basic assumption that the Paleo diet views grains, legumes, and dairy as foods that are likely to be problematic for more people than they are healthful.
The first error in logic that comes up in a typical argument is that the onus of proof is on the person making the claim. This is a typical argument from a person who is a contrarian and just likes to argue. If I say I don't believe those 3 groups of food are healthy for people to consume and you say that I'm wrong, it seems to me that both of us has made a claim that needs backing up. At this point, both should provide their reasoning behind their claim, this is how science works.
This isn't typically how it goes, however, and when they ask for evidence they go straight for the gold standard. They say you must provide a randomized controlled clinical trial(RCT) in order for them to take your claim seriously, which is setting the bar higher than they should because, as far as I am aware, there exists no such study that backs up either side of the argument. They then point to a few epidemiological studies or studies comparing whole grain consumption to refined grain consumption to show that grains are perfectly fine to consume. None of them actually show this because epidemiological studies can't show causation and comparing whole grain consumption to refined grain consumption shows nothing about whether grains are a healthy food choice, only that whole grains are better than refined grains.
This brings up a couple of important points about applying logic. First, just because there is no evidence that something exists doesn't mean it doesn't, especially if it hasn't been studied in depth. At some point I'm sure there was scant evidence that the Earth was round or that bloodletting was a stupid idea, but both ended up being true anyway. There was likely anecdotal evidence for both long before there was an RCT, that's how science works, observation followed by testing. Nevertheless, I'm sure the countless people who died or whose illness was prolonged from bloodletting would have loved to know that there was an argument to be made against it. About 10% of our entire medical system is based on RCTs, the rest is based on anecdotal evidence, epidemiological studies, and other forms of evidence that would not suffice critics of the Paleo diet unless they were using it to argue against the diet, but who have likely benefited from medical care that was based on that inferior data.
Another point about logic is that it should be applied universally. Many critics of Paleo like to point out that just because something wasn't eaten in Paleolithic times doesn't mean that it's bad for you, which is actually true. In the same sense, just because we have been eating something for 10,000 or even 50,000 years and it hasn't killed us before we've reproduced doesn't mean it's good for us, only that it doesn't affect reproduction and therefore won't place selective pressure on our species. This means it won't thin out the herd and force a change in our species, it may simply become a nuisance that people have to deal with as they get older and damage accumulates. Arthritis doesn't kill you, but I imagine anyone who has lived with it for decades would be happy if they didn't have to live the rest of their lives with it.
My final point is about arbitrary points in time. It is not logical to assume that anything that happened prior to the RCT is BS or that anything we are eating now that doesn't cause an immediate cardiac arrest is healthy. There weren't RCTs in ancient Greece when modern humans began to realize that the Earth wasn't flat, but it turned out to be true anyway. Most people aren't interested in finding out exactly what the science
says, most are interested in what is most likely the truth and this sometimes requires going outside of what the science specifically says and extrapolating from other areas of science to fill in the holes. In the same way that not all foods that we eat now are healthy, not all new foods are bad just because we didn't eat them at some arbitrary point in time. We are constantly changing and some new foods that weren't available in the Paleolithic such as olive oil and broccoli have turned out to be quite healthy for us.
So why do I follow a Paleo diet? I don't really follow a Paleo diet, I follow a Paleo template. I limit grains, legumes and dairy because I feel there is enough evidence that those foods contain proteins that are not digested well by us that make their way in to our colon and mess with our microbiome. This is likely a dose dependent issue and genetic variability as well as epigenetic factors likely play a role so certain foods may be perfectly fine for some people and not so fine for others. Until there is a way to tell for sure, I will limit those foods, but I still eat them on occasion. You can hedge your bets by eating a diet high in vegetable matter, moderate in meat, and high in fermentable fibers that the healthy bacteria in your gut use to sustain themselves. This should, in theory, also allow you to get away with eating things that are not necessarily healthy for your microbiome as it bolsters the good guys so that the bad guys don't take hold. At least that is what I have come up with based on the evidence, if any new evidence comes to light I will certainly take it in to consideration.