Published nutritional calorific values are calculated with a correction for "availability", on the basis that some combustible material is excretedNow you mention it I recall that the CV for fibre was changed recently, after they discovered that it is more digestible than previously thought.
Fitness is another secondary effect. It is noticeable that soldiers need to eat less as their training proceeds: some courses begin with 6000 calories or more per day, and the instruction to eat everything at every meal break, but digestive efficiency improves as the lads train their musclesThat's interesting. Some background: I have a very long history of unwittingly overtraining (the consequences have now put an end to my walking and cycling days). I had often wondered why other fitter cyclists used to keep telling me that I was eating too much when I wasn't gaining weight. I was doing huge volumes of exercise and eating huge quantities, but achieving fitness levels little better than someone doing no exercise at all. In 2009 my metabolism was averaging 3400kcal/day, and even after six years of detraining it has only dropped to 2800 now. As I mentioned above, I cease to function if I reduce to 2500/day.