Short answer: as soon as you eat it.
Okay so think of it this way: your body is built out of molecules which function like little Lego building blocks. Your body uses those Lego bricks puts together these bricks in different combinations to build the various things it needs to build. Digestion is the process of turning food into those little building blocks so they can be used when and where they are needed.
In terms of contributing to you mass, the building blocks are adding mass as soon as you eat them. Whether they are already assembled into something or are still waiting to be used, their mass is the same. So, once you have pulled the Lego pieces out of the rest of the stuff you can’t absorb (thanks intestinal tract!) you have that mass.
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