> As an example, we've often been told to eat more fibers to increase "flow", the flow increase is because the fibers are digested by bacteria and the mild toxicity of the process forces the digestive system to flush everything.

This isn't an accurate portrayal of the benefits of fiber. The laxative effects of fiber are just one of many benefits. For example, Fiber can bind to saturated fats, disabling the negative affects of them. Specifically, in terms of bacteria, many bacteria digest fiber, which in turn creates short chain fatty acids, which have many health benefits.

You're right about one thing, though: this is indeed a complex subject.