1) Whats her normal temperature? Is it superhigh because all the food is being burned off for heat? Unlikely because of her lack of muscle mass, but whatever.
2) Does she spend all day pooping? Because at 8000 calories and nowhere on her body for it to be stored, the food would just have to go through and come out the other end.
3) When she does poop, is it fully digested and in proper poop form/composition, or is it still mostly food because her body doesn't actually digest what she's putting into it?
The calories obviously have to go somewhere. She doesn't have a lot of muscle mass so (with my lack of medical knowledge) I don't know how she could have a high metabolism. Meaning, the food isn't being used for energy or heat production. So where would all the calories go, if they're not being used or stored? Out the other end, probably not very well digested, would be my assumption.
So why isn't food being digested? Maybe thats a good place to start? Maybe she needs some Betaine