My hypothesis is that probiotics also need to be paired with the right diet. Bacteria needs to be fed, and needs to compete with the existing gut flora. If conditions aren't right, then they may never get a foothold.
Yeah and honestly I don't think there is a "right diet", because that might be highly dependent on the person, his/her food history, medical past, antibiotics, genetics, physical activity, the "initial gut bacteria" at birth, breast fed or not, etc.
Of course one can expect the usual "less sugar and fat, more macro nutrient", but unless there's proof and proper cause-effect relationships, it's going to be complex to draw conclusions. I'm not really sure that sugar causes a bad balance in the gut bacteria.
Not to mention all the chemicals in food and the environment that makes it almost impossible to track what could work and what would not.
