It depends on the climate.  Inuit lived on a diet composed almost entirely of meat for thousands of years.  It was a more sustainable choice than farming.  (Though if the climate keeps warming, that may change.)  
What do a small number of people (40,000) living close to the Arctic circle have to do with sustainability of the methods used to feed 300 million Americans?
The question is "Can sustainable agriculture feed the world," is it not?