[Llamamoe]

It really isn't that simple. Diet has a huge influence on which of your gut microbes flourish and even their gene expression, but as an adult, its effect on its overall composition are absolutely minimal - it can't kill any species off, and introducing new ones is likewise unlikely. Even probiotics have been shown to have limited effect.

Your advice would be sound if we didn't already have an epidemic of gut microbiomes compromised by antibiotics and poor diets over several generations.