# How much water is lost in "real" weight loss? **URL:** \ **Category:** Factual Questions\ **Created:** [October 1, 2020, 9:14pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991 "2020-10-01T21:14:40Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 1, 2020, 9:14pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/1 "2020-10-01T21:14:40Z")
I’ve tried Google, and the more specialized Reddit subs have posting requirements that border on shibboleths. I can’t get in there. I’ve done the math, and the person who goes to a fitness center boot camp for an hour a day and loses 15 pounds in a month simply can’t have gone calorie negative by 1,750 Calories a day, right? Plus, I knew a guy who went to Overeaters Anonymous and without exercising more lost about 20 pounds in a month just by eating much better. There’s no way he was even more in deficit than that. As the fat cell shrink, they must be expelling water. What percent of that 20 pounds is really water? I’m talking about “real” loss, not just overnight, and not dehydration, because people I’ve asked keep bringing up those points. If you lost 20 pounds that stays off even when you’re hydrated, and is gone a month later, how much of that is water? ---
**Author:** ![jaycat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jaycat/32/3015_2.png) [@jaycat](https://boards.straightdope.com/u/jaycat)\ **Post date:** [October 1, 2020, 9:30pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/2 "2020-10-01T21:30:12Z")
> [@Cardinal](#): > > I’ve done the math, and the person who goes to a fitness center boot camp for an hour a day and loses 15 pounds in a month simply can’t have gone calorie negative by 1,750 Calories a day, right? Why not? If your maintenance level is 3000 calories/day and you reduce your intake to 1250 calories/day. ---
**Author:** ![Fiendish\_Astronaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiendish_astronaut/32/3295_2.png) [@Fiendish\_Astronaut](https://boards.straightdope.com/u/Fiendish_Astronaut)\ **Post date:** [October 1, 2020, 9:42pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/3 "2020-10-01T21:42:39Z")
According to some Aussie scientists in this article… > If you lose 10kg of fat, precisely 8.4kg comes out through your lungs and the remaining 1.6kg turns into water. > **[This is what happens to your fat cells when you lose weight](https://www.independent.co.uk/life-style/health-and-families/weight-loss-fat-cells-what-happens-burned-scientist-explanation-carbon-dioxide-water-urine-a8258941.html)** > > Fat is converted to carbon dioxide and water, not energy ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 1, 2020, 10:19pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/4 "2020-10-01T22:19:22Z")
How is someone’s normal maintenance that high? Not their sitting around staying alive output, it isn’t. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 1, 2020, 10:20pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/5 "2020-10-01T22:20:41Z")
That’s according to the balanced reaction of the oxidation of fat molecules. I’m adding in the shrinking cells losing capacity to hold water. ---
**Author:** ![Fiendish\_Astronaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiendish_astronaut/32/3295_2.png) [@Fiendish\_Astronaut](https://boards.straightdope.com/u/Fiendish_Astronaut)\ **Post date:** [October 1, 2020, 10:30pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/6 "2020-10-01T22:30:03Z")
Is that not what the scientists are describing? (Genuinely asking this - enlighten me if it is not as it seems like an unambiguous statement to me) ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [October 1, 2020, 10:41pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/7 "2020-10-01T22:41:45Z")
> [@Cardinal](#): > > How is someone’s normal maintenance that high? [3000 calories] Not their sitting around staying alive output, it isn’t. Firstly, 3,000 calories isn’t that high. The _average_ calorie requirement for men is 2,500 a day and depends on things like your height and weight, level of activity and genetics. Secondly, why would we talk about the “sitting around staying alive” calorie requirement? Few people are that inactive. Why would we make that the baseline? ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 1, 2020, 11:06pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/8 "2020-10-01T23:06:21Z")
Because that’s their baseline. That’s what they have to take in just to stay alive. so they have to go in deficit using that number. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 1, 2020, 11:10pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/9 "2020-10-01T23:10:54Z")
No, they were disabusing people of a enormous scientific misconception, that if you burn fat you only get energy. They were pointing out that the atoms are not destroyed, only rearranged into less energetic new molecules. Actually, this comes up all the time in the AP chemistry class I teach. I guess there might be somebody out there who thinks that the actual atoms are destroyed and all you get is energy, but of course that is ridiculous. What they’re saying is that the energy is released, but the atoms exit the body in the form of water and carbon dioxide generally. I’m adding in that as the fat molecules are removed from the cells, the cells literally become smaller, therefore able to accommodate less water inside them. If you shrink the cell, I assume that water is expelled also. I just can’t find any kind of percentage that tells me realistic amounts of water loss per pound of fat weight loss in an actual human, not on a lab bench. A pound of fat in a jar doesn’t exist in cells an expand or shrink. As I thought through that, I realized that they aren’t answering my question at all. They are only talking about the atoms that existed as fat molecules. I want to know what percentage of mass left the body that was never part of the energy equation in the fat molecule at all. ---
**Author:** ![Fiendish\_Astronaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiendish_astronaut/32/3295_2.png) [@Fiendish\_Astronaut](https://boards.straightdope.com/u/Fiendish_Astronaut)\ **Post date:** [October 1, 2020, 11:19pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/10 "2020-10-01T23:19:34Z")
I see. I don’t think my Google Fu is up to that one unfortunately. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [October 2, 2020, 12:47am UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/11 "2020-10-02T00:47:58Z")
> [@Cardinal](#): > > I want to know what percentage of mass left the body that was never part of the energy equation in the fat molecule at all. Conventional wisdom suggests an energetic expenditure of 7700 calories is required to lose 1Kg body mass. Fat contains 9000 calories of energy per Kg. So assuming that all your energy expenditure comes from fat (a stretch, but lets run with it), that 7700 calories requires 0.85kgs of fat, so 0.15kgs (15%) of the weight loss is from other things, like reduced water capacity in adipose tissue. Of course, human metabolism is way more complex than the simple calculation above, but maybe it suffices … ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 2, 2020, 1:21am UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/12 "2020-10-02T01:21:47Z")
That’s much more of what I was looking for. I never heard this conventional wisdom number. I guess I’ll go with that until informed further. ---
**Author:** ![Wesley\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wesley_clark/32/20581_2.png) [@Wesley\_Clark](https://boards.straightdope.com/u/Wesley_Clark)\ **Post date:** [October 2, 2020, 1:44am UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/13 "2020-10-02T01:44:43Z")
if you’re male, heavy and active a 1750 calorie deficit is easy. assume you’re a 300 lb male. your maintenance if you are sedentary is about 3500 a day. each mile you walk burns maybe 165 calories. so if you walk an hour a day that’s about 500 calories in exercise. So if you were eating 1600 calories a day at that weight, you’d burn about 2400 calories a day in fat and muscle. a pound of muscle has roughly 400 calories in it. supposedly. Also a pound of pure lard is 454x9 calories but a pound of body fat is 3500 calories. implying a pound of fat has 389 grams of fat, 65 grams of non caloric weight. this matches the figures above of about 15% of body fat being water. ---
**Author:** ![Wallaby](https://avatars.discourse-cdn.com/v4/letter/w/dc4da7/32.png) [@Wallaby](https://boards.straightdope.com/u/Wallaby)\ **Post date:** [October 2, 2020, 3:29am UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/14 "2020-10-02T03:29:58Z")
Here’s a very entertaining Ted Talk adressing the subject. [https://www.youtube.com/watch?v=vuIlsN32WaE&ab\_channel=TEDxTalks](https://www.youtube.com/watch?v=vuIlsN32WaE&ab_channel=TEDxTalks) ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 2, 2020, 5:24am UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/15 "2020-10-02T05:24:40Z")
Well, he says that the atoms are rearranged into CO2 and water, which is what I was talking about. He doesn’t address how much mass you lose because of the constricting flesh. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 2, 2020, 3:05pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/16 "2020-10-02T15:05:12Z")
Ultimately I’m curious what the OP is trying to achieve. Yes, biology is based on chemistry which is based on physics. But using reductive physics equations to make biological predictions is silliness, not science. You maybe can produce upper and lower bounds via physics, but they’ll be so loose that there’s still more noise than signal inside those bounds. It almost sounds like the OP is angling towards concluding that the water removed from cells isn’t “real” weight loss in some sense. I hope this doesn’t come across as threadshitting; I’m genuinely curious what the OP is thinking and what he’s trying to achieve. As a chemist / chemistry teacher he’s got vastly more expertise in this area than I do. Which makes me all the more curious about the motivation. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [October 2, 2020, 3:29pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/17 "2020-10-02T15:29:53Z")
Who’s making biological predictions? A human will expel x% of non-fat product mass from the tissue during fat loss, on average. It’s not really a prediction, and it’s not silliness. I’m just wondering. I also seem to have underestimated a person’s maintenance calories, so my estimate of the water loss was high. It’s not zero, though. There’s nothing more to it. It’s a nerd board. I read the first SD book in probably 1987. I ask nerd questions. ---
**Author:** ![Kron](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kron/32/12444_2.png) [@Kron](https://boards.straightdope.com/u/Kron)\ **Post date:** [October 2, 2020, 3:54pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/18 "2020-10-02T15:54:36Z")
For what it’s worth (nothing at all), just about two years ago I did a fourteen day water-only fast. Day three put me into ketosis, and I began shrinking relatively rapidly… And was peeing MUCH more often than was normal prior to that point. All told, I lost somewhere around sixty pounds, and I was quite certain based on what was happening that my body was definitely shedding water from those depleted fat cells (and fat-soluble… Stuff, because there was an interesting bouquet until things levelled out). ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 2, 2020, 3:58pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/19 "2020-10-02T15:58:46Z")
> [@Cardinal](#): > > Who’s making biological predictions? … I apologize if I came off as hostile. That wasn’t my intent, although on re-read I can see how that’s a reasonable interpretation. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [October 2, 2020, 5:24pm UTC](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991/20 "2020-10-02T17:24:01Z")
> [@Cardinal](#): > > Because that’s their baseline. That’s what they have to take in just to stay alive. so they have to go in deficit using that number. This was in the context of you being incredulous that a person’s maintenance calories could be as high as 3000 calories a day. Well, that frequently is the maintenance calories for men somewhat taller, fatter or more muscular than the average, and with average or above levels of exertion during a typical day. If you’re now saying we should imagine a hypothetical person who _doesn’t_ have typical levels of exertion, but instead hardly moves…then sure, 3000 calories would be a very high requirement for such a person. What does that have to do with what we were talking about though? [Next page](https://boards.straightdope.com/t/how-much-water-is-lost-in-real-weight-loss/921991.md?page=2)