# When we work off weight, where does it go? **URL:** \ **Category:** Factual Questions\ **Created:** [March 8, 2010, 5:44pm UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741 "2010-03-08T17:44:26Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 9, 2010, 9:26pm UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/21 "2010-03-09T21:26:07Z")
> [@](#): > > For the same reason, if I beef up your body heat by, i dunno, putting a heater next to you, you’ll weigh X. If I transport you to the walk-in freezer, you’ll still weigh X. Strictly speaking, you would weigh more when hot than when cold. Relativity really does work that way. It’s just that the amount of difference would be so minuscule that there’d be no hope of actually measuring it. ---
**Author:** ![silk1976](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@silk1976](https://boards.straightdope.com/u/silk1976)\ **Post date:** [March 9, 2010, 9:43pm UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/22 "2010-03-09T21:43:40Z")
See, I knew that increased CO2 levels, and therefore climate change, was man-made! 😛 ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [March 9, 2010, 10:27pm UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/23 "2010-03-09T22:27:01Z")
> [@Chessic\_Sense](#): > > Sure, energy is _equivalent_ to MC^2. That doesn’t mean that your body heat _is_ MC^2. I have 5 Washingtons in my wallet right now, $5. That doesn’t mean I can close and reopen my wallet to find a Lincoln in there. > > For the same reason, if I beef up your body heat by, i dunno, putting a heater next to you, you’ll weigh X. If I transport you to the walk-in freezer, you’ll still weigh X. All forms of energy have mass. A compressed spring has more mass than a relaxed one. A block sitting on a table has more mass than one sitting on the floor. A hot rock has more mass than a cold one. The technology to measure such tiny differences does not exist for these macroscopic examples. But potential energy-mass (from the strong force) is measurable in atomic nuclei, which have masses less than the sum of the masses of the same number of free protons and neutrons. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [March 9, 2010, 10:29pm UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/24 "2010-03-09T22:29:44Z")
> [@friedo](#): > > The majority of the mass that makes up fat molecules is excreted through the respiratory system as CO[sub]2[/sub]. Most of the rest is water. In other words, it disappears into thin air. 😃 ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [March 10, 2010, 9:18am UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/25 "2010-03-10T09:18:48Z")
E=mc2 does not mean that energy _has_ mass, it means that, under some circumstances, energy can be converted into mass (e.g., when a gamma ray photon produces an electron-positron pair) and vice versa (e.g., in a nuclear reactor). There may be a vanishingly tiny amount of relativistic mass lost when the chemical bonds of the fat molecules are broken, but this is utterly negligible compared to the weight of the atoms involved. Any measurable weight loss achieved by exercise come from the carbon, hydrogen and oxygen in the fat being excreted as CO2 and water. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [March 10, 2010, 9:36am UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/26 "2010-03-10T09:36:04Z")
I’m confused. When I tried to look this up, I got stuff about fatty acids turning into keytone bodies in a reaction that was more efficient than using glucose. I tried to figure out what keytone bodies turn into (since the brain can use that as fuel), and I could find nothing. Is that the point where it is split into nitrogen, carbon dioxide, and water? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [March 10, 2010, 10:29am UTC](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741/27 "2010-03-10T10:29:22Z")
> [@BigT](#): > > I’m confused. When I tried to look this up, I got stuff about fatty acids turning into keytone bodies in a reaction that was more efficient than using glucose. I tried to figure out what keytone bodies turn into (since the brain can use that as fuel), and I could find nothing. Is that the point where it is split into nitrogen, carbon dioxide, and water? Ketone bodies are converted into acetyl-CoA. Acetyl-CoA enters the TCA cycle and is converted into CO2. [Previous page](https://boards.straightdope.com/t/when-we-work-off-weight-where-does-it-go/531741.md?page=1)