The best Engineer-type answer would be "it depends". And with this answer, one can buy time to cook up an answer to "on what?"

Well.. assuming that there is no JEE-type catch here, and also assuming that you can actually eat the whole apple (core, seeds and all), shouldn't the reading just be the sum of individual readings?

Of course.. your center of gravity would shift a bit (up or down, depending on your anatomy). And if your machine is sensitive enough, the value of 'g' in "m*g" would change slightly. In such a case, the reading would also be different from the sum of individual readings, as well from the reading when you are holding the apple (depending on the height at which you are holding it).

Have I missed something else?
 

Thats a good point - i didn't think of that. I was more wondering about the act of digestion - as the apple gets digested, will the reading change? And if so, will it only go down or will it also go up momentarily (for instance if your blood vessels absorb more oxygen while digestion)...
 

Oh! I didn't think about the biology part at all!

Well, shouldn't we keep the Oxygen out of it? You would be breathing of course. And in that case, the reading would just keep fluctuating quite a bit, regardless of what happens during the digestion process. The weight of air exhaled and inhaled (the two would be different in real life) would certainly have a greater effect than the extra bit of Oxygen absorbed (if at all there is any extra Oxygen absorbed).

We have to keep it a closed 'system', or else we don't have a physics problem at hand (no system definition, no mechanics).
 

"if" the weighing machine is calibrated correctly the weight should be sum of the two :>