  urlLink Michael Nielsen: Comment on Getting fit : "I think this is a good place to share a disturbing thought that occured to me a while ago: in order to lose weight, you have to actually excrete more mass than you take in.
I mean, unless you happen to be nuclear powered, you can't really 'burn weight off' by turning it into energy. For some reason this is so counter-intuitive that it had three of us physics grad students momentarily doubting conservation of mass... Now obviously exercize *does* work (I lost some, oh, 13 kg this summer myself. Gained a little of it back now that it's cold here in Chicago, unfortunately...) so presumably what it's doing is causing you to excrete more than you otherwise would -- the by-products of the metabolism of that fat which you weren't metabolizing.
But think of it -- those by-products have to add up to eight (or thirteen) kilograms! No wonder it takes such a long time. I wonder if exercize makes you breathe denser? I find this line of thought dovetails nicely with the 'eat more fiber' advice that nearly all diets recommend..." 
