In high school we did an experiment where we caught different types of food on fire, and using them as fuel burning under a beaker, measured the temperature change of the water in the beaker.

Later I learned that cow-dung was used for fuel in some of the poorer places on earth.

I always assumed there must be some calories left in stool; and that this seeming-dogma about 'calories in calories out' was completely ignoring what could be a huge amount of calories getting shat out (or not).