I would guess it's this, which is a meta-analysis of a bunch of studies: https://www.researchgate.net/publication/260840409_Human_Cognitive_Function_and_the_Obe

I would guess it's this, which is a meta-analysis of a bunch of studies: https://www.researchgate.net/publication/260840409_Human_Cognitive_Function_and_the_Obesogenic_Environment https://www.researchgate.net/publication/260840409_Human_Cog... (blue full-text pdf button in upper right)