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No, you do not keep the full weight of the food inside you because you shit.

First of all: there's no way a 115 pound girl ate three 1-lb burgers and fries. Remember the BK stackers? A 1lb burger is a QUAD stack. Remember the video of the dude ordering the octuple stacker? And trying to eat it? That's 2lbs. Each burger patty is typically 1/4 pound of beef weighed before cooking. I am a 190 pound dude, and I'm pretty sure even I would have difficulty consuming 3 lbs of beef in burger form.

So, what she did was eat 3 quarter-pound burgers, plus fries and soda for a grand total of: approximately one pound of food. Hence the 115-116 weight change.

In sum: the answer is simple--your friend made a dumb mistake.

How did that fact go over everyone's heads? Nice catch Evark, there is no one that betch could have consumed that much food in one sitting.