According to this:
http://www.jarvm.com/articles/Vol12Iss1/Vol12Iss1Shoveller.pdf
it does.
Saying, L-Carnitine increases energy expenditure in overweight, but not lean cats...
PDF looks legit, not even a "pay-to-play" or anything.
After cats are spayed/neutered their estrogen/testosterone levels drop and because of this they have difficulty controlling their appetite and will fell more hungry then what they are. The energy they require from food also goes down, yet they want to eat more, and so they gain weight...