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Food goes in, excrements come out. This seems obvious.
But it seems like the mass of the excrements is less than the one of the food.

Most people I asked answered: "There's a difference because it's transformed into energy, like heat...". I think that's bull-excrements. There's still the law of conservation of mass: our body does no nuclear reactions, or does it?

Example: Someone's on a "high protein / low carb" diet, he may ingest like 1kg of food, he isn't really productive on the toilet (it's difficult to find any fibers in those diets), he loses 500g in a day... My question is: where does the 1.5kg of matter go?

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Whatever the diet, the food intake contains macronutrients: carbohydrate, fat and protein. When they are metabolised all of these molecules will end up, for the most part, as carbon dioxide, water, and ammonia, unless they are incorporated into components of the body.

These waste products are, of course, lost via breathing, and via the urine. That's where the "missing mass" goes.

A related question is here.

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Thank you, the fact that we aren't carrying around a nuclear reactor is reassuring for me. –  Roman Jan 17 '13 at 13:25
    
Yup, faeces is only the matter that wasn't taken up by the digestive system - plus some stuff excreted via bile by the liver. –  Armatus Jan 17 '13 at 21:33
    
If the total mass of everything released (as carbon dioxide, urine, faeces, etc.) was equal to the mass of the intake then animals wouldn't function. Your friend is wrong, Roman, in that the conversion of mass to energy does not account for the energy difference but it does occur albeit it at very low levels. It is this conversion of mass to energy that provides the energy needed for the body to function with the energy eventually being lost as heat. –  Jack Aidley Jun 18 '13 at 14:09
    
Metabolic heat comes ultimately from thermodynamically favorable chemical reactions---breaking chemical bonds whose energy of formation is high, creating chemical bonds whose energy of formation is lower. Technically, yes, the mass of one glucose molecule is ever so slightly more than the mass of six carbon dioxide molecules and six water molecules, the difference corresponding exactly to the energy released when the glucose is catabolized, but we usually don't think of this as mass-energy conversion; we think of it as releasing energy stored in chemical bonds. –  Zack Mar 26 at 1:53
    
(ugh, months later I notice that I should have said "the mass of one glucose molecule and six oxygen molecules...") –  Zack 2 days ago

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