Casein protein

Casein accounts for 80% of the protein in cow’s milk (compared to 30% in human milk, 23% in goat’s milk, and 15% in the milk of sheep and buffalo). It is essentially the only protein in cheese (coagulated casein plus milk fat) and strained yogurt (a.k.a. Greek yogurt).

Compared to whey protein, casein is lower in essential amino acids (EAAs), notably leucine, and so has lower biological quality. Its speed of digestion and absorption may also be lower, depending on the type of casein powder you choose: micellar casein, casein hydrolysates, and caseinates.

Micellar casein is the form of casein found in milk. It digests very slowly: consuming 40 grams can maintain elevated levels of serum EAAs, notably leucine, for 6–7 hours (compared to about 4 hours for whey). This is because, under acidic conditions (as found notably in your stomach), micellar casein coagulates into a blob that is difficult for your digestive enzymes to break down. Unfortunately, slower digestion speed means less stimulation of MPS with micellar casein than with whey protein.

The two other forms of casein, caseinates and hydrolysates, are created by destroying the micellar structure of casein, allowing for faster digestion. Their digestion speed is, in fact, very similar to that of whey protein. However, whey protein, being richer in EAAs and leucine, still stimulates MPS more than do caseinates and hydrolysates during the first 3 hours after ingestion (and similarly thereafter).

Bioactive peptides

Like whey protein, casein is composed of subfractions that form bioactive peptides when digested. In casein, those subfractions are α-, β-, and κ-caseins, from which your digestive enzymes can form peptides that stimulate opioid pathways and benefit your immune and cardiovascular systems. Of those peptides, glycomacropeptide (GMP) and the β-casomorphins (BCMs) are of special interest.

GMP exists naturally in small amounts in casein powder but comes about primarily through the digestion of κ-caseins. It acts as an antimicrobial, strengthens the immune system, and benefits dental health.

BCMs, which are produced during the digestion of β-caseins, are the peptides with opioid, or morphine-like, properties. Of the various BCMs, only BCM-7 has been heavily investigated, because of associations found with higher risks of certain disorders and diseases, such as autism, cardiovascular disease, and type I diabetes. However, a comprehensive review by the European Food Safety Authority concluded that those associations were based largely on speculation and somewhat conflicting explanations, suggesting that more research into the role of BCM-7 in human health is required.

A1 vs. A2 β-casein

There are two types of β-casein protein subfraction: A1 and A2. A2 is the natural and original form of β-casein. It is the form found in the milk of humans, goats, sheep, and purebred Asian and African cattle. The A1 variant, a genetic mutation, appeared in European cattle about 5,000 years ago. Due to crossbreeding, most dairy products contain both A1 and A2 (both are present in the milk of prominent cattle breeds such as Ayrshire, Guernsey, and Holstein).

The practical difference between the two types of β-casein is that your digestive enzymes can form BCM-7 out of A1, not A2.

Although the role of BCM-7 in actual diseases is uncertain, there is consistent evidence from animal studies that consuming A1 promotes inflammation through the binding of BCM-7 to opioid receptors in the gut. Human data are scarce, but the few studies available suggest with moderate certainty that A1 is proinflammatory. In fact, some of these studies suggest that people who believe they are lactose intolerant are actually sensitive to A1 instead; they do not report symptoms of lactose intolerance when drinking milk that contains A2 only.