As a thought experiment, consider the case of Angus Barbiery, who allegedly lost almost 200kg in about a year by not eating at all, save for necessary nutrients provided as supplements.

My question is, where did those 200 kilograms go.

And please, don't suggest that it was "converted to energy". 200 kilograms of mass would be approximately equivalent to 1.8*1016 joules. For comparison, Czech nuclear power-plant Temelín generates approximately 4.3*1016 joules of energy.

So how does the mass exit the body after the glucose is metabolized?

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    $\begingroup$ I suggest you consult a text book to find out the chemical products of the metabolism of glucose (or fat). Then blush and withdraw your question. $\endgroup$ – David May 13 '18 at 21:45
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    $\begingroup$ Closely related/possible duplicate of: Breathing faster to lose more weight $\endgroup$ – fileunderwater May 13 '18 at 21:51
  • $\begingroup$ To burn fat it needs to be aerobically. Cardiovascular exercise is critical...steady state heart rate 65 to 85% of max heart rate using large muscles. After 10 to 15 minutes, THEN one is burning fat. So holding onto that rate of exercise for another 30 minutes means major burning of actual body fat calories. Figure out how much you would be able to lose per day walking for an hour. Don't be discouraged, you are also changing your body to BURN fat more readily like a race car versus a volvo. People who are more muscle than fat NEED calories regularly. Fasting is just stupid and stupider. $\endgroup$ – stormy May 13 '18 at 22:22
  • $\begingroup$ Nobody except you is suggesting that the human body is a nuclear reactor, so the strident tone in your question is unfortunate. And what is presented is a calculation, not a thought experiment. (You owe Schrödinger an apology, I think — or at least his cat.) Energy and matter may be interchangable, but before this was discovered there was a law of conservation of matter based on general everyday experience of how one form of matter is converted into another. $\endgroup$ – David May 14 '18 at 9:10
  • $\begingroup$ @David I was googling this before asking and a great amount of people have written things like "he converted the fat to energy". The thought experiment goes before the calculation, hence new paragraph. $\endgroup$ – Tomáš Zato - Reinstate Monica May 14 '18 at 14:19

as CO2 and water

in respiration sugar (or fat) is combined with oxygen to produce energy(ATP and heat), water, and CO2.

enter image description here

fats are converted to Acetyl Co-A just like glucose is, the rest of the metabolic pathway is exactly the same.

enter image description here

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    $\begingroup$ Well, you can indulge those who are too lazy to do the required research before asking a question, or you teach them how to find out things for themselves. However I suspect the poster hasn't even the chemistry to understand your first diagram, nevermind the bizarre construct that follows. $\endgroup$ – David May 13 '18 at 23:09
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    $\begingroup$ Not everyone see the need to mock laymen who are lacking a key piece of information but obviously put some though into their question. $\endgroup$ – John May 13 '18 at 23:15
  • $\begingroup$ Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It is not for laymen, although the poster is certainly not a layman, but clearly has an education in physics. Regardless, he should follow the guidelines about posting: "Have you thoroughly searched for an answer before asking your question?". I just suggested that he blush when he realized that metabolism involves chemistry rather than nuclear physics, and a calculation is not a thought experiment. $\endgroup$ – David May 14 '18 at 8:49
  • $\begingroup$ Let me explain my criticism of your answer.Here is a question about metabolism from a physics student who has forgotten his school biology basics; that glucose is metabolized to carbon dioxide and water. You provide a diagram with terms like "Glycolysis", "Krebs Cycle","Chemiosmosis" — only comprehensible to someone who is already familar with biochemistry. You also use logically inconsistent representations with arrows between an explanation (aa group removed), a compound (urea) and a milieu (urine). Your answer is at the wrong level, and for more detail a link to a text would be far better. $\endgroup$ – David May 14 '18 at 9:02
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    $\begingroup$ I think it would be more truthful and interesting to add "and heat and ketones" to your bolded answer of CO2 and water. It's not all just CO2 and water. There are other metabolites excreted in urine as well. It's not a lot every day, but over time, it adds up. $\endgroup$ – anongoodnurse Aug 17 '18 at 6:04

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