As far as gut bacteria go, we are their world and they want to keep us alive and kicking.
There is a whole branch of science starting to look at the symbiotic relationships involved and the types of signalling mechanisms between the bacteria and our own cells. I read a while ago a paper titled something like:
"Who does the fat belong to?" it was about the signalling mechanisms involved in fat storage and it appears that gut flora play a significant role there, so if they tell us to store fat, then is it ours or theirs???????
Found it, it was Hyperlipid:
Hyperlipid: Search results for fiaf fat storage