Speaking to Tony Wright about this in view of his proposition that we evolved our large brains through eating tropical fruit he explained to me that although our origins are symbiotic i.e. with fruit trees (see http://www.beyond-belief.org.uk/node/7 and http://www.beyond-belief.org.uk/node/8) we are not that symbiotic organism any more. Our system is not frugivorous any more...we are not the same species we were..we are now running on an ancient mammalian system that precedes our frugivorous phase. So, in short, dues to changes in our brains and digestive systems we can no longer assimilate and run healthily on a predominantly frugivourous diet.
Tony gave me this link where there is a bit of a summary regarding brain glucose and other means of fuel http://evolvinghealthscience.blogspot.co.uk/2012/12/why-you-can-all-stop-saying-meat-eating.html while remarking that "no consideration is given in the article of increasingly specialised fruit eating in non seasonal tropics regarding energy requirement and gut/brain theory given our obviously delicate tropical physiology.... Also of interest is the structure of neural tissue, which is much less protein rich than muscle i.e .protein cannot have been a limiting factor" as we see here http://faculty.washington.edu/
Interestingly something I have noticed in my own life is that in situations of exalted expanded states of consciousness my metabolism has changed in a marked way and suddenly I have craved lots of tropical fruit and can metabolise it, it as if my brain is just burning it up. On return to a more mundane or 'normal' state of awareness this effect drifts away.
This is an interesting topic and connects us deeply to the ongoing story of our species.