Part of the whole "diets don't work" is this stupid argument but be prepared, because apparently the other half is "yoyo dieting makes you end up gaining because you pile on the pounds as soon as you resume 'eating normally' and have destroyed your metabolism".  So be prepared to explain  " of course you shouldn't be doing weight watchers and then going back to being a cake graveyard, you should actually learn to feed yourself like a reasonable person forever. Yes the body may be slightly more efficient, "slower" if you must, at fewer calories but that's just because it's working in the manner it should. Edit: quotes, commas for clarity