Or it could be both, there's a lot of feedback loops on the digestive system. Or it could be that both things are mostly unrelated, but there's a common issue which causes both unrelated (e.j. high processed sugar which make consumers crave more, separately also promotes monoculture on the guy biome).

Thing is there's very little papers identifying casual effects, only correlations. That's why there isn't an official medical advise, but just recommendations from people on the field (as in going this can't hurt). If the scientific community actually believed there was an issue proven to be caused by gut biome, we'd read papers on how to tweak probiotic cocktails and what not to prevent this.