There’s this kind of worldwide belief that you can only digest ~30g of protein per meal, and any additional protein is just peed out and wasted.

This is BS–Here’s why:

This so called “protein intake ceiling” derives from studies years ago that studied protein intake and urine nitrogen levels. The researchers basically observed increased Nitrogen losses in urine with increased protein intakes. From this, the researchers determined that the extra protein was wasted.

We now know that when you eat protein you don’t use it directly–ingested protein breaks down into amino acids to create their own proteins. When you eat more protein, your body can afford to lose some. The proteins being replaced are damaged or oxidized proteins. This means protein synthesis AND protein breakdown are increased.

Going back to the old studies, this is why urinary nitrogen levels are increased. It doesn’t reflect wasted proteins, it reflects an increase in the damaged or oxidized proteins that the new proteins replaced.

Also, your body does more than just use proteins to build muscle or make other proteins, it uses the nitrogen from amino acids to synthesize important non-protein molecules such as purines and pyrimidines. These are the building blocks for nucleic acids such as DNA and RNA.

Lastly, your small intestines store and absorb a ton of amino acids that your body will be ready to use when it has to. The overall protein usage from small intestine ingestion is around 9195%. This is why people get the “protein sh*ts”. Nitrogen has two pathways in the small intestines:

• Absorption

• Rectal excretion

The unabsorbed protein goes to the colon to get fermented by bacteria, then pooped out

Summary:

The idea that eating more than ~30g of protein results in wasted protein is incorrect. Your body will breakdown and find a way to use almost all the protein you eat

References:

  1. Stefferud, A., United States, U. States, & Department of Agriculture, D. of Agriculture. (1959). Food : The Yearbook of Agriculture 1959.
  2. Pannemans DL, Halliday D, Westerterp KR. Whole-body protein turnover in elderly men and women: responses to two protein intakes. Am J Clin Nutr. 1995 Jan;61(1):33-8. doi: 10.1093/ajcn/61.1.33. PMID: 7825534.
  3. L. Hambræus, Protein and Amino Acids in Human Nutrition, Reference Module in Biomedical Sciences, Elsevier, 2014, ISBN 9780128012383, https://doi.org/10.1016/B978-0-12-801238-3.00028-3.
  4. François Mariotti, Sylvain Mahé, Robert Benamouzig, Catherine Luengo, Sophie Daré, Claire Gaudichon, Daniel Tomé, Nutritional Value of [15N]-Soy Protein Isolate Assessed from Ileal Digestibility and Postprandial Protein Utilization in Humans, The Journal of Nutrition, Volume 129, Issue 11, November 1999, Pages 1992–1997, https://doi.org/10.1093/jn/129.11.1992
  5. Claire Gaudichon, Sylvain Mahé, Robert Benamouzig, Catherine Luengo, Hélène Fouillet, Sophie Daré, Marc Van Oycke, Françoise Ferrière, Jacques Rautureau, Daniel Tomé, Net Postprandial Utilization of [15N]-Labeled Milk Protein Nitrogen Is Influenced by Diet Composition in Humans, The Journal of Nutrition, Volume 129, Issue 4, April 1999, Pages 890–895, https://doi.org/10.1093/jn/129.4.890
  6. Zebrowska T, Simon O, Münchmeyer R, Bergner H. Untersuchungen zur Sezernierung endogener Aminosäuren in den Verdauungstrakt und zur Aminosäurenresorption beim Schwein [Secretion of endogenous amino acids in the gastrointestinal tract and amino acid resorption in the swine]. Arch Tierernahr. 1976 Feb;26(2):69-82. German. doi: 10.1080/17450397609423240. PMID: 962584.