I’m trying to say this without sounding like a conceited jack@&$ but failing so I’ll just do it. Based on what seem to be objective markers of cognitive performance over the last quarter century this hasn’t been an issue. Neither has it been an issue for my kids who have been vegetarian since birth when benchmarked vs peers and they are easily at the top edge of their grade cohorts of reasonably similar socioeconomic status. Maybe my kids or I would have unified quantum field theory and gravity by now if we ate meat. I don’t know but it seems unlikely. Again though, FWIW we do our best without being zealots to eat varied foods (ie not just spaghetti and Mac/cheese), limit processed food, drink 99.9 percent water vs sugared drinks, etc…. And we are not vegan so eggs and other dairy provide some balance.