So I know that it’s not from tears or sweat because they are composed of mainly water and sodium, it’s not from going poo or pee as poo and pee are made of the parts of food that supply your body with no nutrients and your body also doesn’t “burn body fat” because there is a theory that no matter is ever destroyed so it’s none of those but another thing is it’s not your hair, ear wax, finger nails or anything of that sort because people can lose over 17 pounds in 3 months but you can’t build up that amount of hair , ear wax or whatever else in three months so how does body fat leave your body

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    $\begingroup$ there is a theory that no matter is ever destroyed - what theory is that? Do you have a reference? $\endgroup$
    – Arsak
    Oct 5, 2018 at 20:07
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    $\begingroup$ @Nathan welcome to Biology SE. I see that you are new contributor, so i highly recomend you to take a tour in help center. There you can find what we expect from an question. For now your question is full of oppinion based claims, with unclear poin of what you exacly want to know. Please feel free to edit your answer or according to rules of Biology SE it can be close. $\endgroup$
    – L.Diago
    Oct 5, 2018 at 20:25
  • $\begingroup$ Where does wood (or gasoline &c) go when you burn it? $\endgroup$
    – jamesqf
    Oct 6, 2018 at 16:46

1 Answer 1


Many of the statements in your question are false, including your ideas about waste products.

Ultimately, fats, just like carbohydrates, end up as water and carbon dioxide if they are used in cellular respiration for energy. The reaction path is different, but the ultimate reaction products are the same as you would get from burning fat in a fire: reaction with atmospheric oxygen oxidizes the reactants to carbon dioxide and water.

This is the reverse process of photosynthesis, where carbon dioxide and water react to form sugars, with oxygen as a byproduct.

The mass leaves the body as exhaled carbon dioxide and as metabolic water through the urine, vapor in exhaled air, etc.


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