The problem here is people equate calories in/calories out with the equation "calories eaten - (calories burned from activity + basal metabolism) = weight loss"

The problem here is people equate calories in/calories out with the equation "calories eaten - (calories burned from activity + basal metabolism) = weight loss". While energy in/out dictating weight loss is basically true on a biochemical level, that equation is false because your biochemistry undergoes massive changes as a result of what you eat, and gut bacterial metabolism also depends on the substrate provided.