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Popular health advice suggests that one shouldn't consume >30g of protein at a time (without a gap of ~3 hours, or otherwise depending on the type of protein), because only so much can be digested at a time.

This seems wrong to me? Shouldn't the protein just form a backlog? Where does the protein go? And if this were true, couldn't people just lose weight by consuming all their meals in one go?

I was under the impression that poo doesn't contain any macronutrients (apart from some occassional indigestion, or fats not absorbed on a high-fiber diet), just fibre and water. Perhaps it has to do something with protein synthesis, and the amino acids get excreted in urine?

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    $\begingroup$ Trivially, feces contains cells and cell waste, which necessarily have proteins. You may want to narrow your title and question to discuss the persistence of dietary protein in the human GI tract. $\endgroup$
    – acvill
    Jul 30 at 17:13
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    $\begingroup$ I wouldn't start with popular health advice as a basis for understanding anything. $\endgroup$
    – Bryan Krause
    Jul 30 at 19:02
  • $\begingroup$ perhaps try some before your exercise sessions and see if your gain weight? were you thinking of starting a line of poop protein supplements for gyms? $\endgroup$ Jul 31 at 8:30

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Human stool, on average is about 25% solids and 75% water. Up to 54% of the dried solids consist of bacterial biomass, of which protein comprises up to 50% (1). This is pretty consistent with other research that stool contains up to 25% protein (dry weight) (2), although there is still debate over how much of that is attributable to undigested protein and how much is bacterial biomass.

Basically all dietary protein is absorbed in the duodenum and proximal jejunum (3), so, pretty much just the first half of the small intestine. And, since the food you eat only spends a finite amount of time in this region of the gut, protein uptake is likely limited by transporter density and also by the kinetics of gastric and pancreatic enzymes that break down large dietary proteins into small, absorbable peptides and amino acids.

Lastly, when your body does uptake more protein than it needs (or can utilize) at a given time, it doesn't just backlog (there is no storage form of protein in humans). Instead, it's catabolized and used to generate glucose or ketone bodies, which can eventually be stored as glycogen or fat. These processed also generate a significant amount of ammonium waste, which is converted to urea in the liver and excreted in the urine, putting extra strain on the kidneys.

References:

  1. Rose C, Parker A, Jefferson B, Cartmell E. The Characterization of Feces and Urine: A Review of the Literature to Inform Advanced Treatment Technology. Crit Rev Environ Sci Technol. 2015 Sep 2;45(17):1827-1879. doi: 10.1080/10643389.2014.1000761.

  2. Volk T, Rummel JD. Mass balances for a biological life support system simulation model. Adv Space Res. 1987;7(4):141-8. doi: 10.1016/0273-1177(87)90045-7.

  3. APUS: An Introduction to Nutrition 1st Edition (Byerley); Chapter 5.4: Protein Digestion, Absorption and Metabolism. https://med.libretexts.org/Courses/American_Public_University/APUS%3A_An_Introduction_to_Nutrition_(Byerley)

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  • $\begingroup$ Interesting. So where does the bacterial biomass protein come from? Does bacteria convert fiber into protein? $\endgroup$ Aug 9 at 14:37

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