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Where does the protein we eat go?

Out of 25 g of milk protein drunk after exercise, about a third stays in the gut and liver, nearly a fifth becomes muscle within 12 hours, and the rest feeds other tissues or is burned (Trommelen, 2023). Every day, the body builds 250 to 300 g of protein, far more than it eats: it mostly recycles its own.

A meal's journey, the daily building site, then what renews itself fastest: pick a meal and move through the steps.

What happens to a protein meal?

Proteins are long chains of amino acids. The gut cuts them up and absorbs them, the liver helps itself first, then the blood distributes the rest to the muscles and other tissues. What the body does not use is burned: the nitrogen leaves as urea in the urine.

Meal:
The meal 25 g of protein Stomach Intestine not absorbed: about 1 g Liver the gut and liver keep 7.4 g 23.9 g absorbed portal vein sugar from the meal: 3.9 g in 8 h after the meal0 h The blood receives 16.5 g blood proteins: 1.5 g Muscles +4,5 g alanine, glutamine Other tissues skin, immunity, marrow... rest or burned: 10.5 g Kidneys urea in the urine sugar from kidneys blue = sugar Gold band:thickness = grams

  • humanMeal studied: milk protein drunk after a resistance workout, 36 young men, followed for 12 hours with labeled protein (Trommelen, 2023). With 20 g of casein at rest, 55% reaches the blood within 5 hours and 11% becomes muscle (Groen, 2015).
  • humanSpeed: amino acids reach the blood at their peak around 50 minutes for whey, around 90 minutes for casein and milk (18 studies, 602 people, Gorissen 2020). A steak passes into the blood more slowly than ground beef: 49% versus 61% in 6 hours (Pennings, 2013).
  • estimate“Rest or burned”: what the blood received minus what became muscle or blood proteins (calculation). The study does not separate the other tissues from what is burned.
  • estimateBalance: the study measures what reaches the blood within 12 hours (66% with 25 g, 53% with 100 g). The rest has not arrived yet: the unabsorbed part (about 4.5%, Bos 1999), what the gut and liver keep for themselves (calculation) and, with 100 g, what is still being digested. Muscle (18% and 13%) and blood proteins (6% and 4%) are part of what reached the blood.
  • humanProtein turned into sugar: after 4 eggs (23 g of labeled protein), 3.9 g of the 50.4 g of sugar made in 8 hours came from this protein, about 8% (Fromentin, 2013, and calculation). Without eating, the share of new sugar in blood sugar goes from 47% after 14 hours to 67% after 22 hours and 93% after 42 hours; it counts lactate, glycerol and amino acids together (Landau, 1996).
Good to know: this page explains what happens to protein in the body; it does not say how much to eat. The figures come mostly from studies of young men drinking milk protein: a solid meal, a woman, an older or sick person may give different values.

Frequently asked questions about protein in the body

Can the body absorb more than 30 g of protein per meal?

Yes. After exercise, 100 g of milk protein was absorbed and used for more than 12 hours; with 25 g, blood amino acids returned to the level of the no-protein group after about 5 hours (Trommelen, 2023). The idea of a ceiling came from shorter studies, stopped at 4 or 5 hours.

Does the body store protein?

Not like fat or sugar. It has no reserve made for that: all its proteins are working. Free amino acids, waiting to be used, total less than 100 g in the whole body (Calvez, 2024). What is not used is burned, and the nitrogen leaves as urea.

What happens to excess protein?

The liver removes the nitrogen from extra amino acids and turns it into urea, which the kidneys release into the urine. The rest of the molecule serves as fuel or becomes sugar. With milk, about 14.5% of the nitrogen eaten is already turned into urea within 8 hours: 95.5% is absorbed and 81% stays in the body (Bos, 1999, and calculation).

Why does the body break down its proteins to rebuild them?

To repair those that get damaged, to adapt to meals, fasting, exercise or infection, to adjust certain reactions quickly, and to keep a reserve of amino acids when we are not eating. Lifespans vary widely: less than one hour for an enzyme that acts as a switch (Svensson, 1997), 15 years for skin collagen and 117 years for cartilage collagen (Verzijl, 2000). This recycling costs about 20% of the energy spent at rest (Welle and Nair, 1990).

How much protein does the body lose each day?

Even with enough intake, about 47 mg of nitrogen per kilo per day, or about 21 g of protein for 70 kg (WHO, FAO and UNU report, 2007, and calculation): urea, stool, skin, hair. Muscle also loses about 2 g of creatinine per day, which has to be remade from amino acids (Brosnan, 2011).

See also

Egg white vs yolk protein: do you need to cook them? Where do the fats we eat go? What does creatine do and how much should you take? What happens hour by hour when you fast?

Sources

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  2. Groen BB, Horstman AM, Hamer HM, et al. Post-prandial protein handling: you are what you just ate. PLoS One, 2015;10(11):e0141582. doi:10.1371/journal.pone.0141582
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  4. Pennings B, Groen BB, van Dijk JW, et al. Minced beef is more rapidly digested and absorbed than beef steak, resulting in greater postprandial protein retention in older men. Am J Clin Nutr, 2013;98(1):121-128. doi:10.3945/ajcn.112.051201
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True or false?

Most of the protein in a meal ends up in the muscles.

False. 11 to 18% of the meal becomes muscle within 5 to 12 hours (Groen 2015, Trommelen 2023). The gut and liver keep a third to a half.

The body builds more protein each day than we eat.

True. About 250 to 300 g per day, several times the intake, thanks to the amino acids from its own broken-down proteins (Calvez, 2024).

Muscle renews its proteins faster than the liver.

False. The liver goes about 13 times faster: 0.45% per hour versus 0.035% in the same patients (van Dijk, 2019).

The steps as text

The texts shown during the animation, to read without it.

  1. A small share becomes sugar
    Once the nitrogen is removed, the liver can turn what is left of some amino acids into sugar (glucose), which it sends into the blood. But it is not much: 8 volunteers ate 4 eggs, or 23 g of protein, after a night without eating. In 8 hours, this protein gave only 3.9 g of the 50 g of sugar made by the body (Fromentin, 2013).
  2. Between meals, muscle lends its building blocks
    Without a meal, the body must keep sugar in the blood. Muscle breaks down some of its proteins and sends out mostly alanine and glutamine (Felig, 1970; Holeček, 2024). The liver makes sugar from alanine, and so do the kidneys with glutamine. This new sugar, also made from lactate and from the glycerol of fats, provides 47% of blood sugar after 14 hours without eating, and 93% after 42 hours (Landau, 1996).
  1. The stock: 10 kg of protein
    A 70 kg adult contains about 10 kg of protein (Levitt, 2016): muscles, skin, bones, blood, organs. By comparison, free amino acids, waiting to be used, total less than 100 g in the whole body (Calvez, 2024). There is no protein reserve: all of it is at work.
  2. The great recycling
    Every day, the body breaks down about 250 to 300 g of its proteins and builds as much (Calvez, 2024). With 80 g eaten, for example, it therefore builds 3 to 4 times more than it eats: most amino acids come from its own broken-down proteins.
  3. Who builds what?
    Muscles, the largest, account for only about a quarter of turnover in the fasted state (Phillips, 2020). The liver, the gut lining, the blood and the immune defenses, much smaller, renew their proteins much faster. For a good half, no one has measured organ by organ in humans.
  4. What goes out
    An adult who is neither gaining nor losing weight excretes as much nitrogen as they eat: mostly urea in the urine. Even with enough protein, they lose about 21 g of protein per day (WHO, 2007, and calculation), plus about 2 g of creatinine from the muscles (Brosnan, 2011).
  5. Gaining muscle: a few grams a day
    Muscle is made of about 75% water and 15 to 20% protein (Roberts, 2020). Gaining 1 kg of muscle in a month, for example, means keeping 150 to 200 g of protein, or 5 to 7 g per day (calculation). That is only about 2% of the 250 to 300 g built each day: muscle grows through a tiny difference between what it builds and what it breaks down.
  6. Why break down to rebuild?
    The body breaks down to repair: a damaged protein works poorly. It also breaks down to adapt to meals, fasting, exercise or infection, and to adjust quickly: an enzyme that acts as a switch usually lives less than an hour (Svensson, 1997), so simply stopping its production turns it off. Finally, its proteins serve as a reserve when we are not eating. This building site costs about 20% of the energy spent at rest (Welle and Nair, 1990).

Other texts from the animation

  • Who builds the day's 280 g? (estimate)
  • In one day, the gut lining already replaces about 40% of its proteins (calculation based on Nakshabendi, 1999). Muscle, by contrast, changes barely 1%.
  • After one week, the gut lining is almost entirely new, and the liver has replaced more than half of its proteins. That is why the gut and liver help themselves first at every meal.
  • After one month, the brain has replaced about 70% of its proteins, and albumin and antibodies more than 60%. Muscle, about 30%: it renews slowly, but it is so large that it weighs heavily in the daily balance.
  • After two months, almost everything is new except muscle, still half old. Renewing proteins does not mean replacing cells: a neuron keeps its place for a lifetime and changes its parts.
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Metabolive. "Where does the protein we eat go?" Updated November 8, 2026. https://metabolive.com/en/everyday/protein-in-the-body.