Tried that -- usually the comeback contains (in random order): tough genetics, mean metabolism, starvation mode, some other condition, the thyroid etc. -- you'll probably start stabbing yourself to end this shit before you reach the shores of sanity in discussion like that. What I found works better: "OK, this is actually really easy: It's an 'energy in - energy out' system that needs to be in balance. If you figure out how to cultivate mass without putting extra energy in you should already be on your way to Stockholm and claim your Nobel prize." From that point on you can balance every argument with an equally stupid counter-position: "... but starvation mode" - "unlimited energy for the world!"  "... but genetics" - "space ships with fat propulsion!!" Heh.