Does everybody process food with the same efficiency? Suppose I took a group of people, cleansed their colons then fed them an identical diet for a few days. If I collected their stools and burned them in a calorimeter, would everyone have extracted the same amount of energy from identical food? Do some people need to eat more or less simply because their digestive system is in/efficient?
No, I don’t think you’d get exactly the same caloric value out of the collected stools. Different people’s digestive tracts have somewhat different amounts of motility, which means the food stays in the nutrient-absorbing small intestine for different amounts of time. It stands to reason, therefore, that there would be different amounts of nutrient absorption.
Most of the differences among healthy individuals are pretty small, although some disease states can make for big differences, so you might find that your data set has a pretty small standard deviation. It would be almost impossible for any given set of people to have the exact same numbers.