Admittedly I have no evidence, but anecdotally when I started taking my health and diet seriously I feel like I gained about 10 IQ points just from fueling things better. Similarly, learning various mnemonics's and learning strategies has really helped my ability to learn more rapidly. You could argue that I'm simply capitalizing on my innate ability better -- and that would be true -- but the same argument would apply to gaining strength by training muscles. Nobody sees a muscular person thats ripped and thinks they just are lucky to be that way -- we recognize that there is both biology and effort in play, but for some reason people seem to think intelligence is pure biology. I cant prove my case, but im skeptical of that proposition.
No doubt people have different ceilings for their potential, but I feel like most people who are viewed as smart arent just leaning on natural talent, rather they've cultivated skills that get the most out of what they have
I did dance around the lifestyle point a bit, for this reason. I've never seen a study on "we took otherwise-healthy people with bad habits and got them to eat, sleep, and exercise better", but I'd have higher hopes for that boosting healthy-adult IQ than literally anything else.
And I agree on the ambiguity around 'innate ability'. One could argue that general health is 'subtractive', that bad sleep/diet/exercise are drawing down potential that would exist otherwise. But that's far less clear-cut than something like "only works in dementia patients" or "only helps with Korsakoff Syndrome". I think there's something to it - lifting heavier weights and sleeping more are not unbounded sources of improvement - but I think it's also kind of a copout. If those things operate on a continuum, and people we consider "healthy adults" can still see benefits from them, that's what we wanted to find.
No doubt people have different ceilings for their potential, but I feel like most people who are viewed as smart arent just leaning on natural talent, rather they've cultivated skills that get the most out of what they have