It may sound strange but the better you can digest food, the less weight you will lose. This is because the body can tap more calories from the same amount of food than a gut with a low functionality. The same goes for the speed the food passes your gut. The faster it goes through, the less time there is for digestion and calorie extraction.
The gut flora is most important for the speed and the digestive functionality of the gut.
In bad/normal conditions in nature a slow speed and a high function is good. In the conditions we live it becomes a disadvantage.