how much does bad diet affect IQ? is it just to do with childhood development or can having a bad diet as an adult deteriorate IQ too? which things should you make sure you're not deficient in to prevent/ slow down deterioration?

I heard that malnourished kids in Third World countries do much better in school if given lots of vitamins, which suggests that it’s not just developmental but can respond to supplementation now.

I’m pretty sure that it’s not a big factor in First World adults, mostly because I feel like I would have heard about it if it was, but I can’t remember seeing any specific proof.