Assume the smoothies have the same nutrient profile as a regular diet. Like if you drank runny mashed potatoes and protein powder to simulate a normal food intake.
If you do poop because of the nutrients, is there a healthy diet at which waste products are no longer left in the intestine to turn into poop?
Drag has no intention of doing this, drag is just curious about the science.
The task for dietary fibers is to give volume in the colon and give the peristaltic muscles something to do.
They have limited nutritional value.
And they are an energy source for the good microbes there too.
Gotta get those microbial micronutrients.
Why are volume and muscle movement needed?
Imagine trying to squeeze toothpaste out of a tube but it’s almost empty vs almost full.
Your gut is meant to keep things moving along… If it doesn’t, it ain’t good.
It’s the optimal for our digestive system, of course in some cases like in the hospital or nursing home people have trouble with getting the correct diet they then get laxatives to help.
That’s not actually an answer. “It’s needed because it’s optimal”, that doesn’t explain any science. Drag wants to know the science.
If you want science then start with Wikipedias articles:
And then read all the references when you want to dig deeper.