Originally posted by: dullard
Don't listen to Zysoclaplem, he doesn't know what he is talking about here.
Your body digests a portion of the food and whatever is left is passed as waste. Of what is digested, some or all is used and some may be stored as fat.
All that matters here is conservation of mass (chemistry 101). Mass isn't created or destroyed to any significant amount for this discussion. If (Mass in) > (mass out), then you gain mass (ie you gain weight). If (Mass out) > (mass in), then you lose weight. One great way of affecting that mass balance is to eat food that you won't digest. You can keep that same "mass in" portion (ie eat a lot) and increase the "mass out" portion (ie the runs) and lose weight. Unfortunately, your body does eventually gets accustomed to that effect and compensates by not having the runs.
That is why high-fiber diets always win in the long-term for diet comparison studies. Not only can you stick to the diet (because you aren't starving yourself), but fiber isn't digested. Also, fiber tends to stick to things that you would normally digest and the fiber pulls it out before you get the chance to digest it.
Edit: doh, I should have kept reading this short thread. Vi_edit said it before me.