> You place a burden of evidence on him...
It isn't saidajigumi placing the burden of proof on him, it's the scientific method. Everything we know about nutrition tells us that we don't understand it enough to pull something like this off. It would be a breakthrough if it turned out we can, but the null hypothesis is that it won't work, so that's what our position should be.
If our default position is that this doesn't work, and the guy is really relying on his mix for most of his sustenance, that means we think he's going to experience adverse health effects because of his diet. Perhaps we have no evidence to suggest they'll be adverse enough to qualify his diet as "poisonous", but the assumption isn't completely baseless, as you've asserted.
Again, I have no idea who's right and who's wrong, but the two claims are certainly not equally likely.