Updated: Dec 15, 2019
Casein is a slow-digesting dairy protein that people often take as a supplement. It releases amino acids slowly, so people often take it before bed to help with recovery and reduce muscle breakdown while they sleep. Several studies have shown it helps boost muscle growth, along with a ton of other benefits.
Milk contains two types of proteins — casein and whey. Casein is 80% of the milk protein, while whey is 20%.
Casein protein is digested slowly, while whey protein digested quickly. This is an important difference between these two popular dairy proteins.Like other animal proteins, casein is a complete protein source. That means it provides all the essential amino acids your body needs for growth and repair.
It also contains various unique proteins and bio-active compounds, some of which have health benefits .
There are two main forms:
This is the most popular form and is digested slowly.
This form is predigested and rapidly absorbed. A 33-gram (1.16-ounce) scoop of standard casein protein powder contains 24 grams of protein, 3 grams of carbs and 1 gram of fat.
It may also contain various micro-nutrients (such as calcium), but the exact composition will vary depending on the brand.
Casein Takes Much Longer to Digest Than Whey
Casein is well known as a “time-release” protein because of its slow absorption rate in the gut.
This means that it feeds your cells with amino acids at a low level over a long period of time.
It can help your cells synthesize protein, even during times when your body might normally be breaking down its own muscles to feed itself, such as when you haven’t eaten for some time.
For this reason, it’s called “anti-catabolic” and helps reduce muscle breakdown.
One study tested digestion speed by providing participants with either a casein or whey protein shake. Researchers monitored the blood amino acid content, specifically the key amino acid leucine, for seven hours after ingestion.
As you can see below, they found a quicker and larger spike from whey protein due to its rapid absorption rate. Despite a smaller initial peak, casein levels stayed more consistent over time.
Changes in muscle size are closely related to the balance between muscle protein synthesis and muscle protein breakdown, also known as the "Protein Balance" . If more proteins are generated than lost, there is a "Positive Turnover Rate" and muscle growth occurs . .
A 2012 study suggests that protein consumption before bed helps increase protein turnover . Theoretically, this should result in more muscle growth. Recently, an experimental study came out supporting this . Quite a large limitation of these studies is that total protein intake was not matched between groups, which raises the question of whether or not the greater increase in muscle size can be attributed to the timing of protein.
Basically, the evidence we currently have on this matter is not conclusive. It makes sense to consume some protein before bed for improved protein balance, but we shouldn't feel like muscle growth can't occur without it. Making sure you hit your protein targets for the day, with normal distribution, is the bigger picture [5-7].
In another study, researchers gave participants either whey or casein protein and then measured their digestion rate by analyzing circulating levels of the amino acid, leucine, over a seven-hour period.
They found that circulating levels of leucine rose 25% higher in the whey protein group, indicating faster digestion.
This means that the casein group reduced the total amount of protein burned for fuel over a seven-hour period. That means an improved net protein balance, a key factor for muscle growth and retention.
Bottom Line: This protein is anti-catabolic. It reduces protein breakdown within the body due to its slow digestion rate and sustained supply of amino acids to muscle cells.
Casein is often used at night to prevent the protein breakdown that may occur, since you go through a relatively long period without food while you sleep.
In one study, a casein protein shake before bedtime helped strength-training men increase type 2 muscle fiber size by 8.4 cm2 in the supplement group, compared to 4.8 cm 2 in the training-only group .
Lipids Health Dis. 2007; 6: 25.
Published online 2007 Sep 25. doi: 10.1186/1476-511X-6-25
Bovine milk in human nutrition – a review
Saudi J Biol Sci. 2016 Sep; 23(5): 577–583.
Published online 2015 Jun 17. doi: 10.1016/j.sjbs.2015.06.005
Milk derived bioactive peptides and their impact on human health – A review
Curr Pharm Des. 2003;9(16):1309-23.
Bioactive proteins and peptides from food sources. Applications of bioprocesses used in isolation and recovery.
J Nutr. 2003 May;133(5):1308-15.
Postprandial kinetics of dietary amino acids are the main determinant of their metabolism after soy or milk protein ingestion in humans.
Am J Physiol Endocrinol Metab. 2001 Feb;280(2):E340-8.
The digestion rate of protein is an independent regulating factor of postprandial protein retention.
First published April 29, 2015, doi: 10.3945/jn.114.208371J. Nutr. jn208371
Med Sci Sports Exerc. 2012 Aug;44(8):1560-9. doi: 10.1249/MSS.0b013e31824cc363.
Protein ingestion before sleep improves postexercise overnight recovery.
Proc Natl Acad Sci U S A. 1997 Dec 23;94(26):14930-5.
Slow and fast dietary proteins differently modulate postprandial protein accretion.