How do you know that is a “gut ecosystem collapse” as opposed to hypermotility? Or an overgrowth of the gut ecosystem? Or a problem with the intestinal lining?

Observing that if you eat/drink something specific then you get the shits is valid. Concluding that it is due to a specific mechanism is not valid unless you have something objective like a test supporting that.

It’s like if your train is late and you just conclude that it must be because the steam condenser’s gasket is leaking based on nothing. Maybe true, or maybe the conductor broke his leg, or there is a signaling failure.

With IBS-D every meal can result in a loose and unsatisfying stool output! Don't ask me how I know. GLP-1s actually make a hell of a difference for me.