there's one thing none of them argue: the law of conservation of mass and energy.
The good news here is that, assuming you're constantly using energy to remain alive (maintaining a constant internal temperature, beating heart, lung function, etc), you can vary your body mass by adjusting the ratio of energy taken in versus the energy spent over a given amount of time. (Obviously this is all chemical mass-energy conservation)
^-I like thinking of it that way 'cause no one can argue it, although it's so abstract that I think most people kind of think, "great, you're telling me to eat less & exercise more...thanks" lol...well it's true. :)