Casein and whey are both proteins found in milk. They both contain all the essential amino acids, although they have slightly different amino acid profiles. Notably, whey protein has a higher leucine content (about 11-12% versus casein’s 9%).^[3]

Because of its chemical structure, casein does not combine well with acids. When it comes into contact with stomach acid, it becomes gel-like in consistency, and this coagulated substance is the reason that casein is digested so slowly. Whey protein is absorbed more quickly, releasing amino acids into the system within an hour.^[13][4] These differences can affect the way we use these protein supplements. For example, taking a whey protein supplement after training makes sense, as the body can immediately use the amino acids for muscle growth and recovery. However, casein is a better choice prior to sleeping, since it gives the body a continuous supply of amino acids while fasting for (hopefully) 8 hours while sleeping.^[12]

Some people prefer the texture of whey protein as a drink, since it mixes more easily with water. Both proteins are good for muscle growth and recovery, and both can increase satiety.^[2][6][8]

Micellar casein, the type most often found in supplements, is casein in its natural form. The micelles are small spherical structures with an outer layer that prevents casein from dissolving in acid and water. The micellar structure is the reason that casein forms a gel in stomach acid, making it digest more slowly. To reap the benefits of a sustained, slow release of amino acids, micellar casein is the best option.

Casein hydrolysate (also called hydrolyzed casein or casein peptides), on the other hand, is processed so that the micelles are already broken down and is a form of predigested casein. Casein hydrolysate has a similar absorption time to whey protein and so is not useful as a presleep protein supplement.^[6][7]

Taking 30 grams to 40 grams of casein 30 minutes prior to sleeping has been shown to have small but significant positive effects. It can improve muscle recovery, aid muscle protein synthesis (MPS), and may even increase metabolism.^[2][11][12]

Casein supplementation in the evening could also cause a decrease in appetite the following day, which could be a useful tool for people trying to lose weight.^[5][2]

A presleep bolus of protein (especially a slow-digesting protein like casein) can improve both body composition and athletic performance.^[2] While these benefits are worth noting, it is important not to overestimate their significance. Daily calorie intake, total daily protein intake, and appropriate training are far more important in terms of body composition and performance goals.

Around 80% of milk protein is casein, and around 30% of that is beta casein. There are several types of beta casein, with A1 and A2 being the most common. Beta casein, like all proteins, consists of a long chain of amino acids. A1 beta casein was noted as a natural genetic change in European cattle when a histidine molecule was replaced with a proline molecule in this amino acid chain. The ingestion of A1 beta casein (but not A2) leads to the formation of beta-casomorphin-7 (BCM7), a peptide that has been associated with several health concerns. In vitro and animal studies have shown an association between the BCM7 peptide and type 1 diabetes, allergies, and cardiovascular disease. However, there is no evidence showing a causative link between the ingestion of A1 beta casein and any chronic health condition in humans.

Some human studies have shown greater gastrointestinal distress with A1-containing milk than with A2-containing milk, possibly due to an inflammatory response to the BCM7 peptide. This could be helpful to people with symptoms of lactose intolerance, because it is possible that A2-containing milk could cause fewer symptoms than mixed A1/A2 milk. However, all milk products still contain whey protein, so any symptoms caused by whey protein will not be improved by A2 milk.^[14][15][16]