--and that is vitamin B12, which is essential and which the human body cannot break out of non-animal sources, unlike some of our animal cousins. We simply do not have the proper stomach or intestinal length to do that.

So, while I agree that human beings are often encouraged to consume too much protein in general and too many animal products in particular, anyone who is not having any animal products in the diet--and that includes dairy products and eggs--needs to supplement with B12, preferably a methylcobalamin or adenosylcobalamin form, in case one has genetic problems with the methylation cascade: there are fewer metabolic hoops for the cobalamin to jump through when taken in these forms as opposed to the standard cyanocobalamin form usually sold (and you don't want cyanide radicals running around your body anyway).

B12 deficiency is common even among carnivores as they get older--our levels of intrinsic factor noticeably decline with age--and vegans and vegetarians really do need to supplement from the get-go.