[pknight]

I've come across plenty of evidence in the literature but there are lot of variables that can prevent any positive effects from taking place. If you're taking probiotic supplements in isolation and the rest of your diet is suboptimal in terms of bacteria-friendly nutrients, it's unlikely that you'll get results. It could be the case that many people are throwing money away if they're taking probiotic supplements without addressing their diet and lifestyle.

There's so much variety between people, one formulation for one person might work well for one person but have negative effects on another even if they have a supportive diet and lifestyle. People might simply have to trial multiple formulas that have the right mix of strains before they find one that works for them.

Prebiotic foods seem to have more consistent results on gut health, according to some studies.

(not medical/health advice disclaimer)