What you're saying is completely accurate, but I'd add that it's all relative. Are you falling towards the ground, or is the ground falling towards you? For instance malnutrition lowers IQ, in both directions. There is an inverse correlation between IQ and BMI, but what's most interesting is that that correlation has maintained just as strong even as obesity rates skyrocketed, which is suggestive that there's probably something causal, in some direction, somewhere in there.
And so in modern times if it turns out that eating less than most people apparently want to contributes to IQ, are you doing something good by eating less, or are they doing something bad by eating more? I think it's basically the same thing, just looked at in different ways.
Or are smarter people better able to regulate their food intake? (Either innately, or because society gives them other privileges which makes them less likely to overeat)
I would say, that on the whole, this has to do with habituated impulse control and self-restraint.
Classical writers speak of this as well, things like how inordinate and undisciplined appetites (not just for food, mind you; sex, too, and undue acquisitiveness of all sorts, for instance) darken the mind. What is inordinate and undisciplined is not proportioned or directed by reason. So, such character traits are rooted in fidelity to reason which means that not only do they avoid the aforementioned darkening of the mind by moderation of appetite, but the very character strength of being able to do so enables rational existence in other things.
Innate intelligence doesn't secure discipline. Indeed, it gives the person a bigger footgun and allows for more elaborate rationalizations of vice.
Which then begs the question, what is IQ actually measuring - something more like innate intelligence, or a fairly big slice of learned, habituated test-taking ability?
Regardless of what underlying trait it's actually measuring, the habituation factory is a big component of its supposed bias - that is, has your background taught you the kind of problem-solving habits that will help you to post the best possible score?
> Which then begs the question, what is IQ actually measuring - something more like innate intelligence, or a fairly big slice of learned, habituated test-taking ability?
This question was asked and answered many decades ago in sociology. Researchers moved onto more interesting topics and fields. IQ tests measure g factor (https://en.wikipedia.org/wiki/G_factor_(psychometrics)).
> "It is a variable that summarizes positive correlations among different cognitive tasks, reflecting the assertion that an individual's performance on one type of cognitive task tends to be comparable to that person's performance on other kinds of cognitive tasks."
In other words people who are good at these tests are also good at real world tasks. Meaning IQ measures much more than one's ability to pass an IQ test. There are of course many examples of poorly structured IQ tests (including people re-taking the same IQ test and doing better at it the second time around). However a well-structured IQ test presenting novel questions (absent popular culture references and trivia) provides a very good approximation of the g factor in almost all cases. This means high IQ is highly correlated with things like income, unemployment, crime, homelessness, addiction, divorce, and many other objectively measurable life outcomes.
There is room for a philosophical debate about what g factor is, but it is beyond contest at this stage that g factor is real, and IQ almost always does a very good job of measuring it.
> which is suggestive that there's probably something causal, in some direction, somewhere in there.
Perhaps suggestive, but far from conclusive (I know you know this too). To me, it is suggestive that there is likely some other factor that may explain the relationship better, but then again, I am wrong more often than right, so what do I know? ;)
For example, compare that to growing wealth inequality, and I wouldn't be surprised if that is a potential factor. Less income = less access to care, less access to healthier food options, perhaps less time to for self-care, etc., and if wealth/career potential is gatekept by academic achievement, economic utility, or intelligence, then I can see the two, intelligence and BMI, being correlated, but not directly causal. Though, no study would give people large sums of money to improve their lives, so I doubt we will know for certain.