Serious Eats - seriouseats.com

If you want to maintain that ability to function with a first rate intelligence, then I suggest you keep your meaty eats around. The seeming lack of intelligence often observed in our meat-rejecting friends is not all from the "herbal supplements" that often make up part of their diet. Unfortunately for those who feel a meat-free life is more enlightened existence, their diet lacks vital amino acids and complex proteins vital to the proper production of brain cells and neurotransmitters. I try to follow the lead of my omnivorous friend Ecopimp, who lives closer to the earth than any of us city people would be comfortable with, who knows the patterns of life and death and humanity's place within that chain (yes we are animals, and thus part of the food chain too, entitled to our needs but not to gluttony or waste). We should work to improve the footprint of all we eat, connect with the growers and farmers who make up our local food chain, and to mitigate the damage of our demands with efforts for the greater good. And why not have the full brain functions of a properly nourished body to help propel these efforts at improving the world. That seems like the only humane and responsible approach for all its citizens.

http://en.allexperts.com/q/Nutrition-Dieting-939/Vit-B-complex-proteins.htm