Protein Absorption and Bioavailability

The Role of Protein in The Human Body

Proteins are nitrogenous compounds that serve a number of different roles in the human body.  In addition to serving as the primary structural element of your body’s tissues -- particularly muscle tissue -- proteins also play an important part in the synthesis of numerous hormones, enzymes, and neurotransmitters. ⁽¹⁾ 

In order to utilize the proteins that come from your diet, your body has to break them down into their most basic form -- amino acids (AAs). AAs are known as the building blocks of protein and they’re ultimately what you get after all is said and done.⁽²⁾

While they may have a number of important jobs, amino acids are perhaps best known for their close involvement in the physiological process known as Muscle Protein Synthesis (MPS).  it's through MPS that your body repairs and rebuilds muscle tissues. 

Your muscles are made up of proteins, which are constantly going through a state of flux. The physical stressors of everyday life -- from an intense workout to a leisurely walk -- break down some of these proteins, which, in turn, must be repaired by your body.

In response, your body uses amino acids from your diet to synthesize new muscle proteins that will replace old, broken-down ones.  Research shows that amino acids are direct regulators of MPS -- the more amino acids your body has available, the higher your levels of protein synthesis will be.⁽³⁾

An adequate supply of dietary protein is essential to maintaining the overall health of your muscles.  Without enough amino acids in your diet, your body won’t be able to adequately repair and replace damaged muscle proteins, which, over time, eventually leads to muscle loss.  

Protein Digestion

The process through which proteins are converted into amino acids starts in your stomach, where the digestive enzyme pepsin begins breaking down the peptide bonds that keep each protein molecule bound together.⁽⁴⁾

After the bonds are fully dissolved, you’re left with what are known as polypeptides, which are chains of amino acids connected to one another.⁽⁵⁾ From there, these polypeptides travel to your small intestine to be further digested. 

Once in the small intestine, additional digestive enzymes (trypsin, carboxypeptidase, and chymotrypsin) go to work breaking down the polypeptides into single amino acids.⁽⁶⁾

Understanding Protein Absorption

Protein absorption ultimately occurs in the small intestine where amino acids are eventually absorbed by the cells lining the small intestine.⁽⁷⁾

After pancreatic enzymes break down the polypeptides into single amino acids, they’re finally small enough to pass through your intestinal wall and enter into your bloodstream, where they can be transported all throughout your body.  

Protein Absorption Timing

The time it takes for protein -- and ultimately amino acids -- to be absorbed varies significantly from protein source to protein source.⁽⁸⁾ Some proteins are absorbed rapidly in the gastrointestinal tract, while it takes much longer for other types of polypeptides to be broken down and utilized by your body.

Take two different kinds of dairy-based proteins for example.  Boirie et al. examined the differences in absorption timing between whey and casein protein -- the 2 types of protein found in milk -- in a 1997 study. 

The researchers ultimately found that participants’ amino acid concentrations became elevated rapidly with whey protein, rising within 100 minutes after consumption and returning to baseline within 300 minutes.⁽⁹⁾

Conversely, casein protein was absorbed at a much slower rate, with participants’ amino acid concentrations remaining elevated well after 300 minutes post-meal.  

That’s because the three different regions of the small intestine (first part: duodenum, second part: jejunum, third part: ileum) absorb different protein types at different rates. 

For instance, whey hydrolysate, soy hydrolysate, whey isolate, and whey concentrate have been shown to be absorbed in the first part of the small intestine, hence they are absorbed the fastest.  Egg protein, on the other hand, takes longer to be absorbed, eventually being soaked up in the jejunum.⁽¹⁰⁾

Ultimately, if protein hasn’t been digested and absorbed by the time it leaves the ileum and enters the colon, it won’t be further absorbed into the bloodstream.  The leftover remnants are then used by colon bacteria to produces waste (stool and gas).

Time for Absorption of Different Protein

• Whey Isolate: 60-90 min
• Whey Concentrate: 2-3 hours
• Casein: 3-4 hours
• Soy: 3-4 hours
• Egg: 3-4 hours

What Affects A Protein's Rate of Absorption?

A protein’s amino acid composition affects the rate of absorption, with some types of amino acids being absorbed more easily than others.   

On top of that, research shows that the length of the protein’s amino acid chain also affects absorption, with long-chain peptides taking significantly longer to be broken down and absorbed in comparison to short-chain peptides.⁽¹¹⁾

As you can see in the chart below, each type of protein has a different rate of absorption.  Some types of protein, like whey, have a relatively high rate of absorption, while other proteins, like egg protein, are only absorbed in small amounts every hour.  

Can You Increase Protein Absorption?

Digestive enzymes have been routinely added to whey protein supplements because whey naturally contains a complex sugar (lactose) that many people have a difficult time digesting. If you lack the enzyme lactase, consuming whey without added enzymes can produce bloating, loose stools and gas. 

As opposed to whey concentrate, whey isolate has the lactose removed during the purification process.  Most manufacturers do not add digestive enzymes to their formulations of isolate or hydrolysate because of this.

However, an added benefit of including proteolytic enzymes in whey isolate and concentrate is that it increases the rate and quantity of absorption.  In fact, it’s been shown to increase absorption by as much as 3 times. 

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Grass-fed whey isolate + concentrate for maximal MPS with digestive enzymes for improved digestion and absorption

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Protein Bioavailability (Assessing Protein Quality After Absorption)

How much of the protein actually gets used by your body in the formation of new tissue after digestion occurs also varies from protein to protein, with some proteins being of higher quality than others.  Scientists use a number of different methods to measure the quality of a protein, however, one of the most common is bioavailability.  

A protein’s bioavailability is based on its biological value, which is a measurement of how efficiently your body uses a specific type of dietary protein.  It compares how much nitrogen your body absorbs from the particular type of protein to how much actually gets used in the formation of new tissue.

The scale ranges from 0 - 100% for whole foods, however, some refined foods, like whey protein, exceed this range.⁽¹²⁾

The following list includes the bioavailability of a number of different protein sources.  A high biological value is associated with a rich and well-balanced amino acid profile, while a low value denotes a lower quality protein. 

In general, animal-based proteins typically have higher biological values in comparison to plant-based sources, which in many cases, don’t contain all 9 essential amino acids.⁽¹³⁾

• References
  1. “Protein – Which is Best?" Hoffman, J.R., Falvo, M.J. Journal of Sports Science & Medicine. Sep. 2004.
  2. “Dietary Protein and Nitrogen Utilization" Tome, D., Bos, C. The Journal of Nutrition. Jul. 2000.
  3. “Contemporary Issues in Protein Requirements and Consumption for Resistance Trained Athletes" Wilson, J., Wilson, G.J. Journal of the International Society of Sports Nutrition. Jun. 2006.
  4. “protein digestion and amino acid and peptide absorption" Silk, D.B., Grimble, G.K.,Rees, R. G. Proceedings of the Nutrition Society. Feb. 1985.
  5. “Digestion and Absorption of Dietary Protein" Erickson, R.H., Kim, Y.S. Annual Review of Medicine. Feb. 1990.
  6. “Protein digestion and absorption in human small intestine" Chung, Y.C., Kim, Y.S., Shadchehr, A., Garrido, A., Macgregor, I.L., Sleisenger, M.H. Gastroenterology. Jun. 1979.
  7. “Protein digestion and absorption in human small intestine" Chung, Y.C., Kim, Y.S., Shadchehr, A., Garrido, A., Macgregor, I.L., Sleisenger, M.H. Gastroenterology. Jun. 1979.
  8. “The rate of protein digestion affects protein gain differently during aging in humans" Dangin, M., Guillet, C., Garcia-Rodenas, C., Gachon, P., Bouteloup-Demange, C., Reiffers-Magnani, K., Fauquant, J., Ballevere, O., Beaufrere, B. Journal of Physiology. Mar. 2003.
  9. “Slow and fast dietary proteins differently modulate postprandial protein accretion" Biorie, Y., Dangin, M., Gachon, P., Vasson, M.P., Maubois, J.L., Beaufrere, B. PNAS. Apr. 1997.
  10. “Effect of Peptide Chain Length on Absorption of Egg Protein Hydrolysates in the Normal Human Jejunum " Grimble, G.K., Rees, R.G., Keohane, P.P., Cartwright, T., Desreumaux, M., Silk, D.B. GASTROENTEROLOGY. Jul. 1986.
  11. “Effect of Peptide Chain Length on Absorption of Egg Protein Hydrolysates in the Normal Human Jejunum " Grimble, G.K., Rees, R.G., Keohane, P.P., Cartwright, T., Desreumaux, M., Silk, D.B. GASTROENTEROLOGY. Jul. 1986.
  12. “Protein – Which is Best?" Hoffman, J.R., Falvo, M.J. Journal of Sports Science & Medicine. Sep. 2004.
  13. “Protein – Which is Best?" Hoffman, J.R., Falvo, M.J. Journal of Sports Science & Medicine. Sep. 2004.