Thanks for pointing out the journal error, that has been corrected. Also big thanks for the working link.
The "experiment" with the human subject in the ISMEJ article was stupid, which was why I didn't mention it. Everything I'm saying is based on the mouse experiments.
I do think your interpretation of these experiments is way too restricted. In a frequentist sense, everything you're saying is reasonable, since we don't know how well various results generalize (mouse to human, intravenous to oral, etc...). But in a Bayesian sense, this is pretty good evidence. I guessed that polymyxin would reduce body fat (regardless of how it's administered) just based on reading the ISMEJ article, which never mentioned antibiotics. That means the first article alone was enough to promote the hypothesis out of entropy. I then found the second article by searching for papers discussing polymyxin and obesity, and the result was what I expected (large drop in fat after polymyxin administration), so that's a big evidential boost in favor.
Neither of those gives evidence for the mouse result generalizing to humans. However, we do know that gram-negative bacteria are pretty ubiquitous and do trigger an immune response in humans similar to that in mice, so based on the physical systems we should expect a similar response to antibiotics.
There was a special issue of Nature focusing on human microbiota a few months back which seems to have a lot more relevant research with humans, but I haven't had time to go through them in depth yet (which is why this was a poorly-researched comment rather than a full discussion post).
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.