Briefly, both of the apes closest to humans by genetic constitution (about 98% identical), chimps and gorillas, are hindgut fermenters.  In chimps and gorillas, the hindgut, or colon, comprises about 52 percent of the total gut volume.  It houses microbes that ferment fiber, converting it to fatty acids that supply up to 65% of the animal’s energy requirements.  In contrast, in humans the hindgut comprises only about 17 percent of total gut volume, and has relatively small microbial population.  At most, microbial fermentation in the hindgut can provide about 10% of human energy requirements.

…it appears that caloric absorption from a completely raw food vegan diet may be as low as 46%.  Given the data I showed above, this means that I would have to eat more than 20 pounds of raw vegetation daily to absorb enough calories to maintain my current body mass and activity levels.