Could it be possible that the sugar in the soda was being process by the gut microbes and their byproducts were blocking the absorption of the excess calories? By ceasing the sugar consumption, the byproducts were no longer present and the calories were then consumed by the body?
Matter is neither being created nor destroyed, you are dealing with a bio reactor that has inputs and outputs and the bio reactor doesn't process everything with 100% efficiency.
The sugar in soda is already pretty simple, so what would gut flora actually do to it? What possible byproducts could there be from microbes processing fructose? If the sugars were entirely processed by the gut flora then that would generate a ton of heat and gas in the gut that would be uncomfortable at best or most likely just kill you.
Does gut flora influence how nutrients get absorbed? Almost certainly. Is there a behavioral and/or metabolic component to weight loss? Again, almost certainly.
But if you measure how many calories you were eating at maintenance and then eat 500 less per day, you will 100% lose weight. Will you lose a pound a week, the amount predicted by the simple caloric model of weight? You might lose a little less, but you will lose weight for sure.
