Liquid stoll is a sign of lack of fiber, i used to have that when i ate only meat and eggs, as soon as i started eating 200g of broccoli and 200g of cauliflower a day it stopped happening! I think it's soluable fiber that fixes it! It also happens when i drink large amount of coffe and water and don't eat any solid food, all the water flushes out of my system very quickly! Since i do Intermittent fasting most days i have to keep this in check or I have a bad morning!

Also, have you heard about that research that correlates certain types of gut microbes with increased risk of diabetes and high cholesterol? It seems that some people have gut bateria that allows them to absorb more then others, those that absorb more tend to be fatter and those that absorb less tend to be slimer! It actually makes sense from a evolutionary point of view, having high absorption gut bacteria would allow people in the old times to absorb a lot more and be more adapted to the environment, but nowadays with all the food going around it just makes them fatasses!