I don't know and I don't really know how to explain this scientifically, but maybe it goes off body mass or something. Or perhaps how the chemicals are distributed throughout the body. If say you haven't much muscle, flesh and fat on you, maybe its effects are too concentrated and damage nerves etc. You could perhaps just drop the question into conversation:
"And what happens if I fall under the recommended weight?" But he might not know the answer either.

Sorry if this doesn't make sense, I know it's badly worded. But I hope it helps you, if you can decipher it...