Probably my only concession to fad-ish, anti-science-ish dietetics is "pro-biotics yoghourt" (specifically yoghourt).

I swear, when I first started having one a day, a feeling of general well-being and a measurable digestive improvement were almost immediate.

Likewise, when due to travel I stop my routine, I almost immediately feel it too.

So yeah, this is one that so far apparently the science is shaky, yet my own personal direct experience tells me it works (for me).