Sometimes there are situations in science where something that would seem pretty obvious and quite relevant gets generally ignored.
The point you are raising seems to me to be one of those things.
Occasionally one does see a study where, for example, nitrogen content of the stool is measured and compared to nitrogen intake, so that efficiency of absorption of a given protein can be evaluated.
But by far the most commonly, nutritional and fat-loss studies simply look at food intake and do not substract out the entirely-measurable caloric content of the stool.
Because of the relatively rarity or perhaps lack of studies looking at this next point, I can’t prove my expectation on this one.
But, when I consider cases of fairly sedentary or only quite moderately active people who consume a very high number of calories per day and do not gain fat from it, and I consider the fact that there is no evidence whatsoever that these people are producing more body heat than other people, I have to figure that a large part of perhaps all of their “excess” (relative to normal) caloric intake is simply not absorbed.
A person who actually burned say, 6000 calories per day while weighing only say 160 lb and who did not do much physical activity would inevitably be throwing off a lot of body heat: twice as much or more as the average person in that situation.
The simplest explanation is that they are not burning the extra 3000-3500 calories: rather, they are failing to absorb it. This might be due to gut flora outcompeting the body for absorption of nutrients: in other words, bacteria manage to take in a high percentage or even a majority of the macronutrients.
As you point out it would be entirely doable to measure the caloric content of the stool to prove the point, but I can’t point to where that has been done. Which doesn’t prove it’s never been done, but it certainly isn’t routine in nutritional and fat-loss studies.
On your specific question, I’d have to say as a personal guess that the absorption varies according to the individual.
A more informal evaluation is stool volume. Fiber can of course provide volume without necessarily there being any nutrients that were not absorbed, but other than fiber, bile and compounds excreted via the bile (which do not add up to much), and shed intestinal cells, as I understand it the entire dry mass of the stool is bacteria or dead bacteria.
So if for example consuming only low-or-no-fiber MRP’s and/or milk and/or eggs, etc. for the day and still having a quite substantial stool volume, I think it would be correct to say that this would be strong evidence or perhaps actual proof of a substantial amount of the nutrients not being absorbed.
Gas is also evidence of this. It is produced by bacterial metabolism rather than our own metabolism.