Yeah, I thought I knew what you were saying regarding your 'mass' reference, and I just having a little fun. However, I really didn't understand how you were describing mass at all, I really thought you were talking about body mass.
If I actually understand you right, when you say mass, you are really talking about dietary intake?
To restate your last 2 sentences...
The amount of 'dietary intake' (What you described as 'mass' or 'calories') has to be equal to the amount that leaves your body.
It doesn't get converted to energy.
This is absolutely false, the body actually gets ~67% of your dietary intake. The remaining 3rd is used by your colon, liver and kidneys for example for proper function.
Of the 67%, the body actually uses it as energy, storage or simply passes it through the digestive tract.
Hope that helps!