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One of the most common, least questioned pieces of dietary advice is the Variety Hypothesis: that a more widely varied diet is better than a less varied diet. I think that this is false; most people's diets are on the margin too varied.
There's a low amount of variety necessary to ensure all nutrients are represented, after which adding more dietary variety is mostly negative. Institutional sources consistently overstate the importance of a varied diet, because this prevents failures of dietary advice from being too legible; if you tell someone to eat a varied diet, they can't blame you if they're diagnosed with a deficiency.
There are two reasons to be wary of variety. The first is that the more different foods you have, the less optimization you can put into each one. A top-50 list of best foods is going to be less good, on average, than a top-20 list. The second reason is that food cravings are learned, and excessive variety interferes with learning.
People have something in their minds, sometimes consciously accessible and sometimes not, which learns to distinguish subtly different variations of hunger, and learns to match those variations to specific foods which alleviate those specific hungers. This is how people are able to crave protein when they need protein, salt when they need salt, and so on.
If every meal you eat tastes different, you can't instinctively learn the mapping between foods and nutrition, and can't predict which foods will hit the spot. If you need and crave protein, and wind up eating something that doesn't have protein in it, that's bad.
If the dominant flavor of a food is spice, then as far as your sense of taste is concerned, its nutritional content is a mystery. If it's a spice that imitates a nutrient, like MSG or aspartame, then instead of a mystery it's a lie. Learning how to crave correctly is much harder now than it was in the past. This is further exacerbated by eating quickly, so that you don't get the experience of feeling a food's effects and seeing that food on your plate at the same time.
I'm not sure how to empirically measure what the optimum amount of variety is, but I notice I have builtin instincts which seem to seek it when I have fewer than 10 or so different meal-types in my habits, and to forget/discard meal-types when I have more than that; if this parameter is evolved, this seems like a reasonable guess for how varied diets should be.
Yeah, I came to a similar conclusion after looking at this question from Metaculus. I might have steered to far in the opposite direction, though. I have currently two meals in my rotation. At the very least one of them is "complete food" (So I worry less about nutrition and more about unlearning how to plan meals/cook).
This post is a container for my short-form writing. See this post for meta-level discussion about shortform.