An evidence-free fad diet? We're looking at dozens (maybe hundreds) of research papers published over the past 40 years. Most of them are not based on conjectures but on clinical and dietary surveys looking at long-term health effects. Perhaps you think this is irrelevant because of selection bias, however, if a study found the diet had any relatively negative health effects, I'm pretty sure it would also get published. Of course, this can always turn into an unsolvable "faith in research" argument.
"Boring" diets, yes, because these long-term studies are necessary in order to convincingly support the idea that a diet is healthy, which necessarily implies that the diet is pretty old and probably won't hit the news all that much.
Finally, and as an edit, don't worry: I haven't gotten sucked into the diet that I'm using as an example. I don't follow or endorse it, in part because it relies too much on unsustainable fisheries.
> We're looking at dozens (maybe hundreds) of research papers published over the past 40 years.
And how many nutrition researchers and students do you think there are? How many do you think choose to do a study about a popular diet? Do you think everyone who fails to reject the null hypothesis with whatever mediocre p-value criteria their supervising professor decided on publishes their paper anyway? Do you think most journals would accept such a paper?
> however, if a study found the diet had any relatively negative health effects
That's the thing. Almost any semi-reasonable diet isn't going to kill you or make you horribly sick. Every dietary recommendation you find online will probably keep you alive and healthy. Then, by the nature of having lots of scientific papers and the way publishing works, you can find as many papers as you like claiming that your favorite dietary recommendation is slightly better than the other ones.
"Boring" diets, yes, because these long-term studies are necessary in order to convincingly support the idea that a diet is healthy, which necessarily implies that the diet is pretty old and probably won't hit the news all that much.
Finally, and as an edit, don't worry: I haven't gotten sucked into the diet that I'm using as an example. I don't follow or endorse it, in part because it relies too much on unsustainable fisheries.