Eat a high-fat meal or a can of Pringles with Orlistat, then you'll have an idea of what happens if your digestive system fails to extract calories from what you eat. Trust me, you'll know.

No, our digestive tracts are extremely good at extracting any and all usable calories from our food.

100% of the time, when someone makes this claim, they are mis-estimating either their intake or expenditure, or not tracking for a long enough time period to escape the error bars inherent in daily hydration variance.