If a banana is said to have 100kcal and you and I eat it are we going to get:
1. all of those calories?
2. the same amount of energy or it varies person to person?
In: Biology
Depends on how hungry you are and your metabolism as well as if you already ate and how much.
So if you are starving and you eat a banana with nothing else in your digestive system and assuming you are healthy young individual you will extract around 80-90% of the calories (for some people it will be more, for some it will be less) because nothing is 100% efficient.
Other factors include if you already ate too much because then the food is rushed through your system to make room that is already in your stomach (making you go to bathroom immediately after overeating).
If you are at your daily caloritic limit (body can only process a limited amount) so average 2000-3000 but for people who work extremely physicaly demanding jobs or they work in extreme cold where they burn more calories it can be 4000-5000. Again all of this depends on the individual, their age, size, health etc.
We are all different and operate differently just like our digestive tract.
TL;DR On average our digestion is 80-90% efficient, the rest just passes through.
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