My basic sanity check for any sort of experiment purporting to show a new mechanism responsible for obesity, is "under this mechanism, does it make sense for lots of people to be obese now in America, but hardly anyone a hundred years ago in America, or today in countries like Japan where people have high access to resources but eat less?"
If a mechanism for obesity leaves you confused by the patterns of obesity that occur in the real world, then it's probably better not to afford it much likelihood.
I'm not sure what your response is supposed to be saying to the grandparent. Wouldn't this make total sense if gut flora changed in the united states over the past 100 years? especially if you consider that period includes the introduction and widespread use of antibiotics as well as diet changes, chemical effects that are known to change gut flora. Because gut flora is acquired from the mother, it makes sense that different ethnic groups in different parts of the world would have different compositions also. Gut flora in various societies doesn't seem to have been studied very much (I'm a lazy googler and only found one study that was tangential) but I wouldn't be surprised if different nations had different gut flora.
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.