Casein is the main group of proteins present in milk and dairy, accounting for about 80% of total protein. Its unique structure and physicochemical properties give it important physiological roles and are attracting growing interest for its potential benefits for human health. Let’s summarize the knowledge on the composition, physiological roles and potential benefits.
Composition and structure of casein
Casein makes up around 80% of the total protein in cow’s milk. The remaining 20% are mainly soluble whey proteins, such as β-lactoglobulin, α-lactalbumin, bovine serum albumin and immunoglobulins. This high proportion of casein gives milk and dairy products their characteristic nutritional and functional properties.
Casein is not a single protein, but rather a family of phosphoproteins (1). The four main types of casein present in cow’s milk are:
- α S1-casein: The most abundant (40-50% of total casein). It has a primary structure rich in proline and phosphoserine residues, giving it a strong calcium-binding capacity.
- α S2-casein: Representing around 20-25% of total casein, it is also rich in phosphoserine and interacts strongly with calcium.
- β-casein: Constituting around 25-35% of total casein. Genetic variants of β-casein, in particular forms A1 and A2, have been the subject of specific research.
- κ-casein: Representing around 10-15% of total casein. κ-casein plays a crucial role in stabilizing casein micelles.
These different caseins are characterized by their low solubility at the pH of milk (around 4.6), which leads to their aggregation into complex spherical structures called casein micelles. It is the colloidal calcium phosphate that helps caseins bind together, by acting like a bridge linking the casein proteins together and stabilizing the micelle structure in suspension in the milk. Colloidal calcium phosphate is essential to maintain the integrity of this structure and the functionality of the micelles.
Physiological roles
As a source of high biological quality protein, casein fulfils a number of essential physiological roles (1):
- Source of essential amino acids: Casein contains all the essential amino acids needed for growth, repair and maintenance of body tissue. Its slow, gradual digestion ensures sustained release of amino acids into the bloodstream.
- Mineral transport: The digestion of caseins releases smaller protein fragments called casein Phospho-peptides that binds to, and transport, minerals such as calcium and phosphorus, potentially improving their intestinal absorption (2) due to preventing the formation of insoluble calcium phosphates that are harder to absorb.
- Satiety-enhancing effects: Due to its slower digestion compared with whey proteins, casein may contribute to a prolonged feeling of satiety, which could be beneficial in weight management (3).
- Role in milk coagulation: κ-casein plays a crucial role in stabilizing casein micelles. The action of chymosin, an enzyme present in rennet, cleaves κ-casein, destabilizing the micelles and leading to milk coagulation, a fundamental step in the manufacture of cheese and other fermented dairy products (2).
- Production of bioactive peptides: The enzymatic digestion of casein releases various bioactive peptides with potential biological activities, such as antihypertensive, antimicrobial or immunomodulatory effects.
Potential health effects
Although research is still ongoing, several studies suggest potential health benefits from casein consumption:
- Support for muscle growth and recovery: Due to its slow release of amino acids, casein is often consumed after exercise or before bedtime to promote muscle protein synthesis and recovery during the night. Studies have shown its effectiveness, particularly when compared with rapidly digested proteins, in maintaining muscle mass over the long term (4,5).
- Weight management: Casein’s ability to induce satiety could be beneficial in weight management strategies by helping to reduce overall calorie intake (3).
- Bone health: The presence of calcium and the casein phospho-peptides that promote its absorption could contribute to bone health and the prevention of osteoporosis (7).
- Potential antihypertensive effects: Certain casein-derived peptides have demonstrated in vitro and in vivo angiotensin-converting enzyme (ACE) inhibitory properties, suggesting a potential antihypertensive effect (8).
- Potential immunomodulatory effects: Bioactive peptides derived from casein have shown antimicrobial activity against certain pathogenic bacteria and could influence immune function. Caseinomacropeptide derived from κ-casein has been studied for its prebiotic properties and its potential role in protecting against infections (6).
Considerations and controversies
This focus wouldn’t be complete if we did not note the existence of certain controversies concerning the consumption of casein by certain individuals:
- Cow’s milk protein allergy: Casein is one of the main allergens in cow’s milk.
- β-Casein A1 variant : Some studies have suggested a link between the consumption of milk containing mainly β-casein A1 variant and gastrointestinal disorders. However, these first data need further research to be confirmed (9,10).
Casein is a complex family of proteins that is abundant in milk, playing essential physiological roles as a source of amino acids, a transporter of minerals and a precursor of bioactive peptides. Its slow digestion properties and potential benefits for muscle growth, satiety and potentially bone and cardiovascular health make it a nutrient of interest. However, further research is needed to fully elucidate the mechanisms of action and long-term effects of casein consumption on human health.
