There was that RCT showing that creatine supplementation boosted the IQs of only vegetarians.
While looking for the RCT you’re referencing, I instead found this one from 2023 which claims to be the largest to date and which states “Vegetarians did not benefit more from creatine than omnivores.” (They tested 123 people altogether over 6 weeks; these RCTs tend to be small.)
To summarize, we can say that the evidence from research into the effects of creatine supplementation on brain creatine content of vegetarians and omnivores suggests that vegetarianism does not affect brain creatine content very much, if at all, when compared to omnivores. However, there seems to be little doubt that vegans do not intake sufficient (if any) exogenous creatine to ensure the levels necessary for maintaining optimal cognitive output.
While looking for the RCT you’re referencing, I instead found this one from 2023 which claims to be the largest to date and which states “Vegetarians did not benefit more from creatine than omnivores.” (They tested 123 people altogether over 6 weeks; these RCTs tend to be small.)
A systematic review from 2024 states: