What is casein protein?

casein protein

What is casein protein?

Casein Protein

Casein is commonly referred to as the slow or nighttime protein because of its relatively slow digestion time compared to other options. It’s perhaps the second most popular protein after the king: whey. If whey protein is the most popular, is casein good for anything? Or, given its higher price, a waste of money? In this series, we’ll cover the research and go over the potential benefits of including casein protein in your supplement regimen.


What is casein protein?

Milk is 3.5% protein by weight. Casein and whey are the major proteins found in milk. Casein makes up about 80% of the protein in milk, while whey accounts for most of the remainder1. Proteins behave differently in several ways. One way in which we characterize proteins is digestion/absorption speed.

After consuming protein, the amino acids are released into the blood and absorbed by varying tissues around the body. About three hours after consuming whey protein, amino acid levels will return to their baselines. After consuming casein protein, it takes seven hours for amino acid levels to return to baseline levels! This longer process is why casein is known as the slow release protein2.

This slow process can theoretically lead to benefits, especially if casein is used at night, or other periods of prolonged fasting. There is, however, mixed research on the practical benefits of protein timing,  and whether daily protein consumption levels are more important.

Proteins 101

All proteins are a collection of amino acids. In total, there are 21 amino acids the body needs. These amino acids can be arranged in an almost infinite amount of ways giving each protein unique characteristics that cause them to behave differently inside the body3.

Casein Protein Quality

Not all proteins are created equal and there are a few ways we evaluate them.

Protein Efficiency Ratio

One way to measure the quality of a protein is to determine how much growth it causes. In this method, researchers feed protein to a rat. They compare the amount of weight gain to the amount of protein it consumed. Casein’s value of 2.7 is considered the standard value. Any protein exceeding this standard is considered an excellent protein. As a comparison, beef, egg, and whey exceed casein’s value while beans, peanuts, soy, and wheat fall short4.

Biological Value

Protein is unique because it contains nitrogen; an element necessary for tissue growth. This method of rating proteins generates a ratio: how much of a protein’s nitrogen was used for growth compared to how much was consumed from a food. A score of 100 means that 100% of nitrogen consumed was used for tissue growth. The biological value of casein is 77, which is higher than soy and wheat protein, but lower than, beef, milk, and whey4.

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

This tool measures a protein’s amino acid quality. It compares the number of essential amino acids the body uses from the test protein to a reference protein. A value of 1.0 is the highest (best) score. The PDCAAS of casein is 1.00, indicating it is a high-quality protein4.

The Bottom Line

Casein is a very popular way of supplementing protein. There are several possible benefits to consuming casein protein, which we’ll cover in this series.

Casein Protein Series

  • Is casein a good post-workout protein supplement?
  • Casein Protein and Sleep
  • What are the different types of casein protein? (coming soon)
  • Does casein protein increase strength? (coming soon)
  • Does casein protein increase muscle mass? (coming soon)
  • Can casein protein help with weight loss? (coming soon)
  • Does casein protein decrease body fat? (coming soon)
  • Is casein protein worth using? (coming soon)


  1. Davoodi, S. H., & Shahbazi, R. (2016). Health-Related Aspects of Milk Proteins. Iranian Journal of Pharmaceutical Research, 15(3), 573-591.
  2. Antonio, J., & Ellerbroek, A. (2017). Casein Protein Supplementation in Trained Men and Women: Morning versus Evening. International Journal of Exercise Science, 10(3), 479-486.
  3. Martos, M., & Turnbough, M. (2012, June 28). Protein Parts. Retrieved from https://askabiologist.asu.edu/venom/building-blocks-protein.
  4. Hoffman, J. R., & Falvo, M. J. (2004). Protein – Which is Best? Journal of Sports Science and Medicine