Actually, there are top of the line athletes with a vegan diet. It's not very popular, but that is probably because being vegan is not popular. The B12 source

Actually, there are top of the line athletes with a vegan diet. It's not very popular, but that is probably because being vegan is not popular. The B12 source argument has recently been solved as well. Most animals source their B12 from micro-organisms in the soil. If humans stop washing their food excessively, this would fix the B12 intake problem.