I think about this a lot.
I do feel that it is calories in/calories out....
I am leaning that the issue is how the "out" happens. I know with pharmacokinetics of drug metabolism you can have two phenotypically similar people but due to a mutation in CYP 219 one person will be a regular metabolizer and one will be a poor metabolizer. Meaning if I gave 1 advil to both people the normal metabolizer will react as predicted and the level of advil in their blood will follow the expect pattern of going up, stabilizing and then slowly leaving.... the poor metabolizer will have blood levels where the plateau portion is significantly longer before the body starts to clear it.
Like Boston and NYC can both have the same number of cars enter at the same time but due to the way they’re structured all those cars will come out at different rates and different ways.
Tie the concept into the hormones that surround fat retention/insulin release etc.... and you can see how the same amount of calories could be just a blip for one person, but another, say with thyroid disease, it would be significant intake because of how the body processes it.
I don't even think that it means someone is all that broken, just different. I do think it means they'll (and I include myself here) will have a much harder/longer road to the same results AND that you'll never know what will work for each person to get to their personal goals because although we all have the same basic plumbing that can "generally" respond... the actual intricacies of that response is as individual as a fingerprint.
That's why I was drawn to PB... "Blueprint" is simply how to build something right? Like we might all want a house, but perhaps some of us will want 3 bedrooms and a study, while others will want 4 bedrooms and perhaps a third will want 2 bedrooms and a huge loft space.
I like coming here and reading about all the different thoughts and struggles. It is helping me experiment and come to know how my body will respond as I apply the PB for who *I* am and what fits in my overall lifestyle