In my unscientific view based on a sample size of 1, resistant starch, which really just means starch like cooked spuds, pasta or rice which is allowed to go cold before eating (can be reheated, and remember cooked food goes off quickly so store it safely) or something like inulin (banana flour is rich in it) or acetic acid (vinegar) are easy ways to boost lower gut health, by providing inputs which favour the bacteria you want. You want a lower gut rich in buterate consumers.

If in 20 years time this cool, folksy self treatment is exploded, aside from being dead of old age, I won't care because i got to eat loads of potato lutkes, arancini, pasta reheats, yummy yoghurt and kefir and vinegary dressings on salad. Dying of old age eating nice food for no provable health upside? Fine with me!