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After class today, the topic of eating one's one fecal matter came up. There was a sharp divide as to how people thought the defecations would change. I realize this is a bit of an odd question but understanding it would help me understand how our body handles waste.

One theory was that the body would continually reuse some elements of the fecal matter such that, eventually and with no other input, nothing would be defecated.

Another theory was that there are elements in the fecal matter that the body would never make any use of, so it would continually get excreted (up until the person died, of course).

Could someone please explain the likely outcome of continually and only eating your own poop (assuming you could stay alive long enough)?

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In a healthy person, stool weight depends mainly on the quality and quantity of the diet. The mean stool weight of a normal defecation is about 320 g [1].

Dry weight of fecal matter contains [2]:

  • 30 % bacteria
  • 30 % undigested food and fiber
  • 10-20 % fat
  • 2-3% protein

Let's do the math (the only nutrients in feces are fats and proteins):

  • 20% * 320g = 64g of fats which can release 64 * 9.3 = 595 kcal of energy
  • 3% * 320 = 9.6g of proteins which can release 9.6 * 4.1 = 40 kcal of energy

That's about 650 kcal. 2,000 kcal is a rough average of what people eat in a day. But your body might need more or less than 2,000 [3].

As you can see, fecal matter can't supply enough nutrients. Continuously (re)eating feces and only feces will lead to denutrition, less feces and death.

References:

  1. Seyed Mohammad Kazem Hosseini Asl MD, Seyed Davood Hosseini MD. Determination of the Mean Daily Stool Weight, Frequency of Defecation and Bowel Transit Time: Assessment of 1000 Healthy Subjects
  2. shigeta's answer on What is the composition of human feces?
  3. HowStuffWorks.com. How many calories does a person need daily?
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    $\begingroup$ The author seems aware that the person would clearly die from lack of nutrition but is more wondering about the theoretical result. I think you should add that after continuous cycles of digestion, the fat and protein components of faeces would at some point be completely eliminated. So would the bacteria, but while passing through the intestinal environment, new bacteria would inevitably get mixed into the mass again. Thus, the result would be a mixture of undigestible fibre and bacteria. $\endgroup$
    – Armatus
    Jun 5 '14 at 20:06
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    $\begingroup$ A very thoughtful response. I guess my further question would be that, with only fecal matter as food intake, would the amount of bacteria and undigested stuff eventually (and theoretically of course) dwindle to nothing? Or would that continue to just be processed and excreted? $\endgroup$ Jun 5 '14 at 20:31
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    $\begingroup$ @AHungerArtist fibers and bacteria are not processed. Fibers will be excreted over and over again as-is. Bacteria may vary because they are multiplying in the intestine. $\endgroup$
    – Cornelius
    Jun 5 '14 at 20:34
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    $\begingroup$ Want to add a point that some animals like rabits eat their feces because a single round of digestion is not very efficient. $\endgroup$
    – WYSIWYG
    Jun 6 '14 at 9:38

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