When you exercise and/or are on a diet, how does the weight you lose physically leave your body? Is it through defecation, or do you actually burn it somehow?

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    You breathe in O2 and breathe out CO2. Ever wondered where that C came from? waste is of course also a way to lose a lot of weight, especially since most of your body weight is water. I found this on google for you if you want to learn more about breathing snowbrains.com/brain-post-why-do-we-breathe . It's quite literally burning, a fire also converts several things into CO2. Ever wondered why the ashes is lighter than the log you've just burned?
    – Raditz_35
    Aug 31, 2018 at 8:22
  • @Raditz_35 - I think you need to put that out there as an answer, since it does directly adress the question. Aug 31, 2018 at 15:49
  • @PoloHoleSet That sounds like too much work. I really want to spend as little time as possible on researching what humans excrete in what quantity and through which hole. Since the fact that we breathe is what I would call common knowledge, I leave it as a comment for the Op to realize they already knew that and specify the question further
    – Raditz_35
    Aug 31, 2018 at 15:53
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    @Raditz_35 - My comment was entirely selfish - I liked your comment better than what is currently out there. Aug 31, 2018 at 15:55
  • @PoleHoleSet please feel free to copy paste it as an answer if you think it is good enough for that . I don't mind
    – Raditz_35
    Aug 31, 2018 at 16:00

2 Answers 2


This is the end result of a fairly complex system of interactions in the body.

Your fat cell population is usually defined by adolescence/puberty, and remains relatively stable throughout your life. The size will vary.

When your body needs more energy from fat storage, it releases fat from the cells in the form of triglycerides. The body converts these (somewhat inefficiently) into usable energy forms (glucose). The end products from this conversion cycle are mostly carbon dioxide, water and the aforementioned energy.

Most of the CO2 is exhaled, and the same for the water, along with sweat, urine, saliva, etc. So essentially, you are spending some of the fat to drive the conversion process, the conversion process produces energy which is used by the body, and the majority of the rest is exhaled, which is how "fat" leaves the body. A little is excreted or used for cooling (sweat), and the rest is either reabsorbed or serves another purpose in the body.

  • not only fat is used to get energy, but also muscle and muscle glycogen. That is why estimation for average person, is that if you lose 1 lb of weight, you just lost 1/3lb of muscle mass Sep 22, 2018 at 19:24
  • "Most of the CO2 is exhaled" - Correct! I've seen/read some scientific tests that showcased that about 84-86% of all "burned" calories are actually exhaled, leaving only around 15% of the calories to come out as sweat etc.
    – MJB
    Sep 25, 2018 at 9:39

Energy Storage

Excess glucose is transformed into a carbohydrate, glycogen, and packed into temporary stores in muscle tissue and in your liver. As your glycogen stores get filled, your body transforms the additional excess glucose into triglycerides and stores it in fat cells that are distributed throughout your body under your skin and in a fatty sheath of tissue that hangs off your stomach called the omentum.

Energy Use (basically the reversed process of storing energy)

When cells need energy, they generally use glucose in your blood. As your glucose levels get low, your body taps your temporary glycogen stores and also extracts triglycerides from your fat cells.

Source: https://www.livestrong.com/article/420965-does-coconut-oil-make-you-fat-or-lose-weight/ (first thing I found on Google, next time please use Google before asking such a simple question)

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    This doesn't really address where the mass/weight goes or how it is "lost." Aug 31, 2018 at 15:50
  • @PoloHoleSet The fat cells are used by your body to produce energy in order to fulfill movements when e.g. running, lifting, cycling etc.
    – Suimon
    Sep 1, 2018 at 9:18
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    Are those cells converted from mass to energy via E=MC^2? Is energy extracted from them via a bio-chemical process and the cells are completely destroyed, and then other mass gets filtered out and excreted via the kidneys? Liver? Through the intestines? Sweat glands? Mixed with the exhaled air through the lungs? Saying "fat cells are used" still does not explain, even remotely, where the actual mass that is lost comes from, nor how it is "lost" from the body. Sep 4, 2018 at 14:58
  • It's not my question, but that is what OP is asking. If you're not going to answer the question that is being asked, why bother? Sep 7, 2018 at 15:47

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