# What goes in must come out? \[A question about poo\] **URL:** \ **Category:** Factual Questions\ **Created:** [April 12, 2019, 5:19pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447 "2019-04-12T17:19:12Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Walken\_After\_Midnight](https://avatars.discourse-cdn.com/v4/letter/w/b487fb/32.png) [@Walken\_After\_Midnight](https://boards.straightdope.com/u/Walken_After_Midnight)\ **Post date:** [April 12, 2019, 5:19pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/1 "2019-04-12T17:19:12Z")
A person typically eats around three meals a day. According to the internet, a normal frequency of pooing is anywhere from three times a day to three times a week. It is clear that there is a disparity in volume between what goes into and what goes out of the body, at least via the primary entrance and exit points. What proportion of the matter/substance that we eat is ultimately expelled from the body in the form of poo? And what happens to the rest? ---
**Author:** ![senoy](https://avatars.discourse-cdn.com/v4/letter/s/dbc845/32.png) [@senoy](https://boards.straightdope.com/u/senoy)\ **Post date:** [April 12, 2019, 5:46pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/2 "2019-04-12T17:46:14Z")
You breathe it and pee it out. Oxygen in, Carbon Dioxide out, right? That carbon is coming from your food. Some of your food gets sweated out. A lot get turned into ammonia and water and peed out. I don’t have a percentage of each. ---
**Author:** ![Walken\_After\_Midnight](https://avatars.discourse-cdn.com/v4/letter/w/b487fb/32.png) [@Walken\_After\_Midnight](https://boards.straightdope.com/u/Walken_After_Midnight)\ **Post date:** [April 12, 2019, 6:06pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/3 "2019-04-12T18:06:31Z")
> [@senoy](#): > > You breathe it and pee it out. > > Oxygen in, Carbon Dioxide out, right? That carbon is coming from your food. Some of your food gets sweated out. A lot get turned into ammonia and water and peed out. > > I don’t have a percentage of each. There must be also be some variable quantity of food that is expelled as fart. Is it right that trees, and other plants, are essentially made from air - their carbon component, anyway - and they get water and other nutrients from the soil? What proportion of a tree/plant, minus the water, is made of carbon and what proportion is “other nutrients”? What do we do with the oxygen once we’ve used it? Does it all go back out of the mouth as carbon dioxide? ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 12, 2019, 6:14pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/4 "2019-04-12T18:14:42Z")
> [@Walken\_After\_Midnight](#): > > It is clear that there is a disparity in volume between what goes into and what goes out of the body, at least via the primary entrance and exit points. I am only addressing this part of your question while the others may address the rest. Fruits and vegetables are around 90% water. Cite : [http://www.berkeleywellness.com/healthy-eating/food/article/how-much-water-your-food](http://www.berkeleywellness.com/healthy-eating/food/article/how-much-water-your-food) Meat and poultry are around 60% water Cite : [https://www.fsis.usda.gov/wps/wcm/connect/42a903e2-451d-40ea-897a-22dc74ef6e1c/Water\_in\_Meats.pdf?MOD=AJPERES](https://www.fsis.usda.gov/wps/wcm/connect/42a903e2-451d-40ea-897a-22dc74ef6e1c/Water_in_Meats.pdf?MOD=AJPERES) Normal poo is about 75% water and if you go only once a few days, you are probably constipated and there is even less water in your poo. Cite : [Feces | Human Digestion, Waste Disposal, Excretion | Britannica](https://www.britannica.com/science/feces) So the apparent discrepancy in volume you are noticing is from the water in the equation. ---
**Author:** ![senoy](https://avatars.discourse-cdn.com/v4/letter/s/dbc845/32.png) [@senoy](https://boards.straightdope.com/u/senoy)\ **Post date:** [April 12, 2019, 6:15pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/5 "2019-04-12T18:15:45Z")
[quote=“Walken\_After\_Midnight, post:3, topic:832447”] There must be also be some variable quantity of food that is expelled as fart. [QUOTE] There’s other miscellany. Dead skin cells, hair, tears, blood, spit. > [@](#): > > Is it right that trees, and other plants, are essentially made from air - their carbon component, anyway - and they get water and other nutrients from the soil? What proportion of a tree/plant, minus the water, is made of carbon and what proportion is “other nutrients”? Yes, plants are largely air and water. > [@](#): > > What do we do with the oxygen once we’ve used it? Does it all go back out of the mouth as carbon dioxide? Pretty much although there is lots of stuff going on. I guess some of it could become water. I’d have to look at the actual chemical process. Basically, you breathe in and Oxygen enters your bloodstream. That oxygen is carried to your cells. Your cells then take that oxygen and in a process called cellular respiration combine it with sugars to produce water and carbon dioxide. The carbon dioxide then jumps back into your blood where it is taken to the lungs and expelled. The basic chemical reaction is C6H12O6 (simple sugar) + 6 02 (oxygen) -\> 6 C02 (carbon dioxide) + 6 H20 (water) + heat ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [April 12, 2019, 6:19pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/6 "2019-04-12T18:19:36Z")
> [@Walken\_After\_Midnight](#): > > … > > What do we do with the oxygen once we’ve used it? Does it all go back out of the mouth as carbon dioxide? In very simplified form, it goes back out as carbon dioxide and water from the breakdown of sugars, etc. For the more complete answer, lookup cellular metabolism or the Krebs Cycle. ---
**Author:** ![Jasmine](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jasmine/32/2964_2.png) [@Jasmine](https://boards.straightdope.com/u/Jasmine)\ **Post date:** [April 12, 2019, 6:21pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/7 "2019-04-12T18:21:24Z")
[\*\* How much of your food do you actually absorb?\*\*](http://janeshealthykitchen.com/food-combining/) > [@](#): > > Research says humans assimilate between 10% and 80% of the food we eat, with the average somewhere between 40 and 60 percent. The undigested food is excreted, recycled back into the Earth. > > Well, that’s a very large range. How much YOU assimilate fluctuates constantly and is pretty hard to measure. The primary factors are: how well you chew your food, the nutrient-density of the food, the type of food (i.e. starches, proteins, fats), whether it is raw or cooked, the level of life force in the food, enzymes present, your intestinal flora, the health of your organs, your emotional state, stress level, and the amount of toxins, empty calories, or useless material to be eliminated. > > Other important factors are your metabolism rate, exercise and activity level, the quantity of food you eat, your age, the time of day, your internal body temperature, and the temperature of the food consumed. ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [April 12, 2019, 6:24pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/8 "2019-04-12T18:24:18Z")
Plants are mostly cellulose, and cellulose is C[sub]6[/sub]H[sub]10[/sub]O[sub]5[/sub]. The atomic weights of C, H and O are 12, 1 and 16 respectively. So the carbon content of cellulose is 44% by weight. However, the oxygen in the cellulose also comes from carbon dioxide, so the total percentage of dry plant matter derived from air is almost 94%! ---
**Author:** ![Beckdawrek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beckdawrek/32/3647_2.png) [@Beckdawrek](https://boards.straightdope.com/u/Beckdawrek)\ **Post date:** [April 12, 2019, 6:55pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/9 "2019-04-12T18:55:07Z")
So, fat persons are just full of shit? That just seems rude. ---
**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:** [April 12, 2019, 7:41pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/10 "2019-04-12T19:41:42Z")
Yeah, a lot of people don’t realize that the primary excretory organ in the human body is not the colon nor the kidneys, but the lungs. **Ignotus** , don’t forget that some of the oxygen in cellulose came from water, not from carbon dioxide. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [April 12, 2019, 8:06pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/11 "2019-04-12T20:06:16Z")
> [@senoy](#): > > You breathe it and pee it out. > > Oxygen in, Carbon Dioxide out, right? That carbon is coming from your food. Some of your food gets sweated out. A lot get turned into ammonia and water and peed out. > > I don’t have a percentage of each. Also, a fair amount of water is exhaled-- I think more than is generally sweated out in moderate climates, but can’t find any good sources quickly. ---
**Author:** ![Mr.Bill](https://avatars.discourse-cdn.com/v4/letter/m/5f9b8f/32.png) [@Mr.Bill](https://boards.straightdope.com/u/Mr.Bill)\ **Post date:** [April 12, 2019, 8:14pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/12 "2019-04-12T20:14:39Z")
> [@Jasmine](#): > > The primary factors are: … the level of life force in the food … What on God’s green earth does that mean? ---
**Author:** ![Walken\_After\_Midnight](https://avatars.discourse-cdn.com/v4/letter/w/b487fb/32.png) [@Walken\_After\_Midnight](https://boards.straightdope.com/u/Walken_After_Midnight)\ **Post date:** [April 12, 2019, 9:00pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/13 "2019-04-12T21:00:48Z")
> [@Jasmine](#): > > > [@](#): > > > > Research says humans assimilate between 10% and 80% of the food we eat, with the average somewhere between 40 and 60 percent. Many animals [eat their own poo](https://www.livescience.com/59601-why-do-animals-eat-poop.html) in order to absorb further nutrients and “humans are sort of abnormal in not doing it”, according to a primate curator. > [@Ignotus](#): > > Plants are mostly cellulose, and cellulose is C[sub]6[/sub]H[sub]10[/sub]O[sub]5[/sub]. The atomic weights of C, H and O are 12, 1 and 16 respectively. So the carbon content of cellulose is 44% by weight. > > However, the oxygen in the cellulose also comes from carbon dioxide, so the total percentage of dry plant matter derived from air is almost 94%! Does the hydrogen in the cellulose come from water via the roots? What is the remaining 6% of dry plant matter, and does it come from absorption through the roots? ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 12, 2019, 9:39pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/14 "2019-04-12T21:39:08Z")
> [@Chronos](#): > > **Ignotus** , don’t forget that some of the oxygen in cellulose came from water, not from carbon dioxide. That’s not supported by experiments. > [@](#): > > One of the first uses of an isotopic tracer in biology was in the determination of the source of the oxygen produced by photosynthesis. Using a heavy isotope of oxygen, 18O, to label one of the two reactants in the photosynthetic process, Samuel Ruben and collegues at UC Berkeley were able to determine that the oxygen gas came not from carbon dioxide, but from water. Cite : [http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles%20of%20Life2e/Animated%20Tutorials/pol2e\_at\_0604\_The\_Source\_of\_the\_Oxygen\_Produced\_by\_Photosynthesis/pol2e\_at\_0604\_The\_Source\_of\_the\_Oxygen\_Produced\_by\_Photosynthesis\_scr.html](http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles%20of%20Life2e/Animated%20Tutorials/pol2e_at_0604_The_Source_of_the_Oxygen_Produced_by_Photosynthesis/pol2e_at_0604_The_Source_of_the_Oxygen_Produced_by_Photosynthesis_scr.html) ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 12, 2019, 10:08pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/15 "2019-04-12T22:08:55Z")
> [@Chronos](#): > > Yeah, a lot of people don’t realize that the primary excretory organ in the human body is not the colon nor the kidneys, but the lungs… True. An average human exhales about 2.3 lbs of CO2 per day. Cite : [Redirect Notice](https://www.google.com/url?sa=i&source=web&cd=&ved=2ahUKEwjH2_qQxMvhAhWqJzQIHXnEBWcQzPwBegQIARAC&url=https%3A%2F%2Fslate.com%2Fnews-and-politics%2F2009%2F08%2Fare-you-heating-the-planet-when-you-breathe.html&psig=AOvVaw0ZeeO8OND5HrspiAD047JD&ust=1555192087984993) This is the equivalent to burning about 1.6 lb of sugar (glucose to be more accurate). ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 12, 2019, 10:18pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/16 "2019-04-12T22:18:14Z")
> [@am77494](#): > > That’s not supported by experiments. Actually … The experiment you cite showed that the oxygen released by photosynthesis came from carbon dioxide. Since photosynthesis consumes water, and the experiment shows oxygen from water isn’t released, it must be part of the sugars produced, which is the statement you attempted to refute. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 12, 2019, 10:54pm UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/17 "2019-04-12T22:54:06Z")
> [@naita](#): > > Actually … > The experiment you cite showed that the oxygen released by photosynthesis came from carbon dioxide. Since photosynthesis consumes water, and the experiment shows oxygen from water isn’t released, it must be part of the sugars produced, which is the statement you attempted to refute. Please give it another read and check if this correct. I did read it again but maybe I am not reading it correctly. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 13, 2019, 12:01am UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/18 "2019-04-13T00:01:47Z")
> [@am77494](#): > > Please give it another read and check if this correct. I did read it again but maybe I am not reading it correctly. The cite neither proves nor disproves the claim. None of the tagged CO2 was emitted as O2, so all of it must have gone to the plant’s structure. The tagged H2O was emitted as oxygen. However, it doesn’t say that \*all \*of it was emitted. Some may have gone to plant structure as well. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [April 13, 2019, 1:01am UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/19 "2019-04-13T01:01:02Z")
> [@Dr.Strangelove](#): > > The cite neither proves nor disproves the claim. None of the tagged CO2 was emitted as O2, so all of it must have gone to the plant’s structure. The tagged H2O was emitted as oxygen. However, it doesn’t say that \*all \*of it was emitted. Some may have gone to plant structure as well. The article was presented as an early work to demonstrate that the oxygen emitted during photosynthesis comes from splitting water. The oxygen atoms that come with CO2 stay in the system. This is called the Calvin cycle. The Calvin cycle is named after Melvin Calvin, who won a Nobel Prize in Chemistry in 1961. See the graphic from Khan Academy : > **[The Calvin cycle (article) | Photosynthesis | Khan Academy](https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-calvin-cycle-reactions/a/calvin-cycle)** > > Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere. Notice how the oxygen emitted comes from water. So all the oxygen in cellulose comes from CO2 and not from water unless I am understanding the Calvin cycle incorrectly. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [April 13, 2019, 1:27am UTC](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447/20 "2019-04-13T01:27:52Z")
> [@am77494](#): > > So all the oxygen in cellulose comes from CO2 and not from water unless I am understanding the Calvin cycle incorrectly. The Calvin cycle is the light-independent part. But what about the light-dependent part? What gets added to NADP[sup]+[/sup]+ADP to get NADPH+ATP? Is it just energy+hydrogen, or does some of the oxygen make it over as well? The diagram doesn’t say. [Next page](https://boards.straightdope.com/t/what-goes-in-must-come-out-a-question-about-poo/832447.md?page=2)