Vegan + vitamin B12 supplement is recommended by Dr. Greger. Herbivore humans can thrive too. My grandma thrive on plant based diet, rarely ate meat (very small). She passed away at 102. 

There are physicians out there recommending a vegan diet: http://www.pcrm.org/

Somebody shared this: How Not to Die - Michael Greger, M.D.  
https://www.youtube.com/watch?v=aSgNkhgVtks&list=LLaKrtj0u_XhJPVXH4SuTEqA&index=7